a: b
      touch $@
b:
      touch $@
# A common chain of build steps
%-GENERATED.c: %-generate
      touch $@
%.o: %.c
      touch $@
%.so: %-GENERATED.o
      touch $@
xxx-GENERATED.o: CFLAGS += adsf
# Imitates .d files created with "gcc -MMD". Does not exist on the initial build
ifneq ($(wildcard xxx.so),)
xxx-GENERATED.o: xxx-GENERATED.c
endif

$ rm -rf a b xxx-GENERATED.c xxx-GENERATED.o xxx.so
  [start from a clean slate]
$ touch xxx-generate xxx.h
  [Files that would be available in a project exist; xxx-generate is some tool]
  [that would generate xxx-GENERATED.c                                        ]
$ touch a
  ["a" exists but the file "b" it depends on does not]
$ make a xxx.so
  touch b
  touch a
  touch xxx-GENERATED.c
  touch xxx-GENERATED.o
  touch xxx.so
  rm xxx-GENERATED.c
  [It built everything, but then deleted xxx-GENERATED.c]
$ make a xxx.so
  remake: 'a' is up to date.
  touch xxx-GENERATED.c
  touch xxx-GENERATED.o
  touch xxx.so
  [It knew to not rebuild "a", but the missing xxx-GENERATED.c caused it to]
  [re-build stuff                                                          ]

$ rm -rf a b xxx-GENERATED.c xxx-GENERATED.o xxx.so
$ touch xxx-generate xxx.h
$ touch a
$ make a xxx.so
  remake: 'a' is up to date.
  touch xxx-GENERATED.c
  touch xxx-GENERATED.o
  touch xxx.so
  [It didn't bother rebuilding "a" even though its prerequisites "b" doesn't]
  [exist. But it didn't delete the xxx-GENERATED.c at least                 ]
$ make a xxx.so
  remake: 'a' is up to date.
  remake: 'xxx.so' is up to date.
  [It knew to not rebuild anything. Great.]

___dummy___: file1 file2

export PATH=/usr/lib/ccache:$PATH

export CCACHE_DIR=$HOME/.ccache

A Starting Point for Investigation: /sys/power/state and /sys/power/disk These two system files exist to allow debugging of hibernation, and thus control the exact state used directly. Writing specific values to the state file controls the exact sleep mode used and disk controls the specific hibernation mode¹. This is extremely handy as an entry point to understand how these systems work, since we can just follow what happens when they are written to.

Show and Store Functions These two files are defined using the power_attr macro: kernel/power/power.h:80

#define power_attr(_name) \
static struct kobj_attribute _name##_attr =     \
    .attr   =               \
        .name = __stringify(_name), \
        .mode = 0644,           \
     ,                  \
    .show   = _name##_show,         \
    .store  = _name##_store,        \
 

show is called on reads and store on writes. state_show is a little boring for our purposes, as it just prints all the available sleep states. kernel/power/main.c:657

/*
 * state - control system sleep states.
 *
 * show() returns available sleep state labels, which may be "mem", "standby",
 * "freeze" and "disk" (hibernation).
 * See Documentation/admin-guide/pm/sleep-states.rst for a description of
 * what they mean.
 *
 * store() accepts one of those strings, translates it into the proper
 * enumerated value, and initiates a suspend transition.
 */
static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr,
			  char *buf)
 
	char *s = buf;
#ifdef CONFIG_SUSPEND
	suspend_state_t i;
	for (i = PM_SUSPEND_MIN; i < PM_SUSPEND_MAX; i++)
		if (pm_states[i])
			s += sprintf(s,"%s ", pm_states[i]);
#endif
	if (hibernation_available())
		s += sprintf(s, "disk ");
	if (s != buf)
		/* convert the last space to a newline */
		*(s-1) = '\n';
	return (s - buf);
 

state_store, however, provides our entry point. If the string disk is written to the state file, it calls hibernate(). This is our entry point. kernel/power/main.c:715

static ssize_t state_store(struct kobject *kobj, struct kobj_attribute *attr,
			   const char *buf, size_t n)
 
	suspend_state_t state;
	int error;
	error = pm_autosleep_lock();
	if (error)
		return error;
	if (pm_autosleep_state() > PM_SUSPEND_ON)  
		error = -EBUSY;
		goto out;
	 
	state = decode_state(buf, n);
	if (state < PM_SUSPEND_MAX)  
		if (state == PM_SUSPEND_MEM)
			state = mem_sleep_current;
		error = pm_suspend(state);
	  else if (state == PM_SUSPEND_MAX)  
		error = hibernate();
	  else  
		error = -EINVAL;
	 
 out:
	pm_autosleep_unlock();
	return error ? error : n;
 

kernel/power/main.c:688

static suspend_state_t decode_state(const char *buf, size_t n)
 
#ifdef CONFIG_SUSPEND
	suspend_state_t state;
#endif
	char *p;
	int len;
	p = memchr(buf, '\n', n);
	len = p ? p - buf : n;
	/* Check hibernation first. */
	if (len == 4 && str_has_prefix(buf, "disk"))
		return PM_SUSPEND_MAX;
#ifdef CONFIG_SUSPEND
	for (state = PM_SUSPEND_MIN; state < PM_SUSPEND_MAX; state++)  
		const char *label = pm_states[state];
		if (label && len == strlen(label) && !strncmp(buf, label, len))
			return state;
	 
#endif
	return PM_SUSPEND_ON;
 

Could we have figured this out just via function names? Sure, but this way we know for sure that nothing else is happening before this function is called.

Autosleep Our first detour is into the autosleep system. When checking the state above, you may notice that the kernel grabs the pm_autosleep_lock before checking the current state. autosleep is a mechanism originally from Android that sends the entire system to either suspend or hibernate whenever it is not actively working on anything. This is not enabled for most desktop configurations, since it s primarily for mobile systems and inverts the standard suspend and hibernate interactions. This system is implemented as a workqueue² that checks the current number of wakeup events, processes and drivers that need to run³, and if there aren t any, then the system is put into the autosleep state, typically suspend. However, it could be hibernate if configured that way via /sys/power/autosleep in a similar manner to using /sys/power/state to manually enable hibernation. kernel/power/main.c:841

static ssize_t autosleep_store(struct kobject *kobj,
			       struct kobj_attribute *attr,
			       const char *buf, size_t n)
 
	suspend_state_t state = decode_state(buf, n);
	int error;
	if (state == PM_SUSPEND_ON
	    && strcmp(buf, "off") && strcmp(buf, "off\n"))
		return -EINVAL;
	if (state == PM_SUSPEND_MEM)
		state = mem_sleep_current;
	error = pm_autosleep_set_state(state);
	return error ? error : n;
 
power_attr(autosleep);
#endif /* CONFIG_PM_AUTOSLEEP */

kernel/power/autosleep.c:24

static DEFINE_MUTEX(autosleep_lock);
static struct wakeup_source *autosleep_ws;
static void try_to_suspend(struct work_struct *work)
 
	unsigned int initial_count, final_count;
	if (!pm_get_wakeup_count(&initial_count, true))
		goto out;
	mutex_lock(&autosleep_lock);
	if (!pm_save_wakeup_count(initial_count)  
		system_state != SYSTEM_RUNNING)  
		mutex_unlock(&autosleep_lock);
		goto out;
	 
	if (autosleep_state == PM_SUSPEND_ON)  
		mutex_unlock(&autosleep_lock);
		return;
	 
	if (autosleep_state >= PM_SUSPEND_MAX)
		hibernate();
	else
		pm_suspend(autosleep_state);
	mutex_unlock(&autosleep_lock);
	if (!pm_get_wakeup_count(&final_count, false))
		goto out;
	/*
	 * If the wakeup occurred for an unknown reason, wait to prevent the
	 * system from trying to suspend and waking up in a tight loop.
	 */
	if (final_count == initial_count)
		schedule_timeout_uninterruptible(HZ / 2);
 out:
	queue_up_suspend_work();
 
static DECLARE_WORK(suspend_work, try_to_suspend);
void queue_up_suspend_work(void)
 
	if (autosleep_state > PM_SUSPEND_ON)
		queue_work(autosleep_wq, &suspend_work);
 

The Steps of Hibernation

Hibernation Kernel Config It s important to note that most of the hibernate-specific functions below do nothing unless you ve defined CONFIG_HIBERNATION in your Kconfig⁴. As an example, hibernate itself is defined as the following if CONFIG_HIBERNATE is not set. include/linux/suspend.h:407

static inline int hibernate(void)   return -ENOSYS;  

Check if Hibernation is Available We begin by confirming that we actually can perform hibernation, via the hibernation_available function. kernel/power/hibernate.c:742

if (!hibernation_available())  
	pm_pr_dbg("Hibernation not available.\n");
	return -EPERM;
 

kernel/power/hibernate.c:92

bool hibernation_available(void)
 
	return nohibernate == 0 &&
		!security_locked_down(LOCKDOWN_HIBERNATION) &&
		!secretmem_active() && !cxl_mem_active();
 

nohibernate is controlled by the kernel command line, it s set via either nohibernate or hibernate=no. security_locked_down is a hook for Linux Security Modules to prevent hibernation. This is used to prevent hibernating to an unencrypted storage device, as specified in the manual page kernel_lockdown(7). Interestingly, either level of lockdown, integrity or confidentiality, locks down hibernation because with the ability to hibernate you can extract bascially anything from memory and even reboot into a modified kernel image. secretmem_active checks whether there is any active use of memfd_secret, and if so it prevents hibernation. memfd_secret returns a file descriptor that can be mapped into a process but is specifically unmapped from the kernel s memory space. Hibernating with memory that not even the kernel is supposed to access would expose that memory to whoever could access the hibernation image. This particular feature of secret memory was apparently controversial, though not as controversial as performance concerns around fragmentation when unmapping kernel memory (which did not end up being a real problem). cxl_mem_active just checks whether any CXL memory is active. A full explanation is provided in the commit introducing this check but there s also a shortened explanation from cxl_mem_probe that sets the relevant flag when initializing a CXL memory device. drivers/cxl/mem.c:186

* The kernel may be operating out of CXL memory on this device,
* there is no spec defined way to determine whether this device
* preserves contents over suspend, and there is no simple way
* to arrange for the suspend image to avoid CXL memory which
* would setup a circular dependency between PCI resume and save
* state restoration.

Check Compression The next check is for whether compression support is enabled, and if so whether the requested algorithm is enabled. kernel/power/hibernate.c:747

/*
 * Query for the compression algorithm support if compression is enabled.
 */
if (!nocompress)  
	strscpy(hib_comp_algo, hibernate_compressor, sizeof(hib_comp_algo));
	if (crypto_has_comp(hib_comp_algo, 0, 0) != 1)  
		pr_err("%s compression is not available\n", hib_comp_algo);
		return -EOPNOTSUPP;
	 
 

The nocompress flag is set via the hibernate command line parameter, setting hibernate=nocompress. If compression is enabled, then hibernate_compressor is copied to hib_comp_algo. This synchronizes the current requested compression setting (hibernate_compressor) with the current compression setting (hib_comp_algo). Both values are character arrays of size CRYPTO_MAX_ALG_NAME (128 in this kernel). kernel/power/hibernate.c:50

static char hibernate_compressor[CRYPTO_MAX_ALG_NAME] = CONFIG_HIBERNATION_DEF_COMP;
/*
 * Compression/decompression algorithm to be used while saving/loading
 * image to/from disk. This would later be used in 'kernel/power/swap.c'
 * to allocate comp streams.
 */
char hib_comp_algo[CRYPTO_MAX_ALG_NAME];

hibernate_compressor defaults to lzo if that algorithm is enabled, otherwise to lz4 if enabled⁵. It can be overwritten using the hibernate.compressor setting to either lzo or lz4. kernel/power/Kconfig:95

choice
	prompt "Default compressor"
	default HIBERNATION_COMP_LZO
	depends on HIBERNATION
config HIBERNATION_COMP_LZO
	bool "lzo"
	depends on CRYPTO_LZO
config HIBERNATION_COMP_LZ4
	bool "lz4"
	depends on CRYPTO_LZ4
endchoice
config HIBERNATION_DEF_COMP
	string
	default "lzo" if HIBERNATION_COMP_LZO
	default "lz4" if HIBERNATION_COMP_LZ4
	help
	  Default compressor to be used for hibernation.

kernel/power/hibernate.c:1425

static const char * const comp_alg_enabled[] =  
#if IS_ENABLED(CONFIG_CRYPTO_LZO)
	COMPRESSION_ALGO_LZO,
#endif
#if IS_ENABLED(CONFIG_CRYPTO_LZ4)
	COMPRESSION_ALGO_LZ4,
#endif
 ;
static int hibernate_compressor_param_set(const char *compressor,
		const struct kernel_param *kp)
 
	unsigned int sleep_flags;
	int index, ret;
	sleep_flags = lock_system_sleep();
	index = sysfs_match_string(comp_alg_enabled, compressor);
	if (index >= 0)  
		ret = param_set_copystring(comp_alg_enabled[index], kp);
		if (!ret)
			strscpy(hib_comp_algo, comp_alg_enabled[index],
				sizeof(hib_comp_algo));
	  else  
		ret = index;
	 
	unlock_system_sleep(sleep_flags);
	if (ret)
		pr_debug("Cannot set specified compressor %s\n",
			 compressor);
	return ret;
 
static const struct kernel_param_ops hibernate_compressor_param_ops =  
	.set    = hibernate_compressor_param_set,
	.get    = param_get_string,
 ;
static struct kparam_string hibernate_compressor_param_string =  
	.maxlen = sizeof(hibernate_compressor),
	.string = hibernate_compressor,
 ;

We then check whether the requested algorithm is supported via crypto_has_comp. If not, we bail out of the whole operation with EOPNOTSUPP. As part of crypto_has_comp we perform any needed initialization of the algorithm, loading kernel modules and running initialization code as needed⁶.

Grab Locks The next step is to grab the sleep and hibernation locks via lock_system_sleep and hibernate_acquire. kernel/power/hibernate.c:758

sleep_flags = lock_system_sleep();
/* The snapshot device should not be opened while we're running */
if (!hibernate_acquire())  
	error = -EBUSY;
	goto Unlock;
 

First, lock_system_sleep marks the current thread as not freezable, which will be important later⁷. It then grabs the system_transistion_mutex, which locks taking snapshots or modifying how they are taken, resuming from a hibernation image, entering any suspend state, or rebooting.

The GFP Mask The kernel also issues a warning if the gfp mask is changed via either pm_restore_gfp_mask or pm_restrict_gfp_mask without holding the system_transistion_mutex. GFP flags tell the kernel how it is permitted to handle a request for memory. include/linux/gfp_types.h:12

 * GFP flags are commonly used throughout Linux to indicate how memory
 * should be allocated.  The GFP acronym stands for get_free_pages(),
 * the underlying memory allocation function.  Not every GFP flag is
 * supported by every function which may allocate memory.

In the case of hibernation specifically we care about the IO and FS flags, which are reclaim operators, ways the system is permitted to attempt to free up memory in order to satisfy a specific request for memory. include/linux/gfp_types.h:176

 * Reclaim modifiers
 * -----------------
 * Please note that all the following flags are only applicable to sleepable
 * allocations (e.g. %GFP_NOWAIT and %GFP_ATOMIC will ignore them).
 *
 * %__GFP_IO can start physical IO.
 *
 * %__GFP_FS can call down to the low-level FS. Clearing the flag avoids the
 * allocator recursing into the filesystem which might already be holding
 * locks.

gfp_allowed_mask sets which flags are permitted to be set at the current time. As the comment below outlines, preventing these flags from being set avoids situations where the kernel needs to do I/O to allocate memory (e.g. read/writing swap⁸) but the devices it needs to read/write to/from are not currently available. kernel/power/main.c:24

/*
 * The following functions are used by the suspend/hibernate code to temporarily
 * change gfp_allowed_mask in order to avoid using I/O during memory allocations
 * while devices are suspended.  To avoid races with the suspend/hibernate code,
 * they should always be called with system_transition_mutex held
 * (gfp_allowed_mask also should only be modified with system_transition_mutex
 * held, unless the suspend/hibernate code is guaranteed not to run in parallel
 * with that modification).
 */
static gfp_t saved_gfp_mask;
void pm_restore_gfp_mask(void)
 
	WARN_ON(!mutex_is_locked(&system_transition_mutex));
	if (saved_gfp_mask)  
		gfp_allowed_mask = saved_gfp_mask;
		saved_gfp_mask = 0;
	 
 
void pm_restrict_gfp_mask(void)
 
	WARN_ON(!mutex_is_locked(&system_transition_mutex));
	WARN_ON(saved_gfp_mask);
	saved_gfp_mask = gfp_allowed_mask;
	gfp_allowed_mask &= ~(__GFP_IO   __GFP_FS);
 

Sleep Flags After grabbing the system_transition_mutex the kernel then returns and captures the previous state of the threads flags in sleep_flags. This is used later to remove PF_NOFREEZE if it wasn t previously set on the current thread. kernel/power/main.c:52

unsigned int lock_system_sleep(void)
 
	unsigned int flags = current->flags;
	current->flags  = PF_NOFREEZE;
	mutex_lock(&system_transition_mutex);
	return flags;
 
EXPORT_SYMBOL_GPL(lock_system_sleep);

include/linux/sched.h:1633

#define PF_NOFREEZE		0x00008000	/* This thread should not be frozen */

Then we grab the hibernate-specific semaphore to ensure no one can open a snapshot or resume from it while we perform hibernation. Additionally this lock is used to prevent hibernate_quiet_exec, which is used by the nvdimm driver to active its firmware with all processes and devices frozen, ensuring it is the only thing running at that time⁹. kernel/power/hibernate.c:82

bool hibernate_acquire(void)
 
	return atomic_add_unless(&hibernate_atomic, -1, 0);
 

Prepare Console The kernel next calls pm_prepare_console. This function only does anything if CONFIG_VT_CONSOLE_SLEEP has been set. This prepares the virtual terminal for a suspend state, switching away to a console used only for the suspend state if needed. kernel/power/console.c:130

void pm_prepare_console(void)
 
	if (!pm_vt_switch())
		return;
	orig_fgconsole = vt_move_to_console(SUSPEND_CONSOLE, 1);
	if (orig_fgconsole < 0)
		return;
	orig_kmsg = vt_kmsg_redirect(SUSPEND_CONSOLE);
	return;
 

The first thing is to check whether we actually need to switch the VT kernel/power/console.c:94

/*
 * There are three cases when a VT switch on suspend/resume are required:
 *   1) no driver has indicated a requirement one way or another, so preserve
 *      the old behavior
 *   2) console suspend is disabled, we want to see debug messages across
 *      suspend/resume
 *   3) any registered driver indicates it needs a VT switch
 *
 * If none of these conditions is present, meaning we have at least one driver
 * that doesn't need the switch, and none that do, we can avoid it to make
 * resume look a little prettier (and suspend too, but that's usually hidden,
 * e.g. when closing the lid on a laptop).
 */
static bool pm_vt_switch(void)
 
	struct pm_vt_switch *entry;
	bool ret = true;
	mutex_lock(&vt_switch_mutex);
	if (list_empty(&pm_vt_switch_list))
		goto out;
	if (!console_suspend_enabled)
		goto out;
	list_for_each_entry(entry, &pm_vt_switch_list, head)  
		if (entry->required)
			goto out;
	 
	ret = false;
out:
	mutex_unlock(&vt_switch_mutex);
	return ret;
 

There is an explanation of the conditions under which a switch is performed in the comment above the function, but we ll also walk through the steps here. Firstly we grab the vt_switch_mutex to ensure nothing will modify the list while we re looking at it. We then examine the pm_vt_switch_list. This list is used to indicate the drivers that require a switch during suspend. They register this requirement, or the lack thereof, via pm_vt_switch_required. kernel/power/console.c:31

/**
 * pm_vt_switch_required - indicate VT switch at suspend requirements
 * @dev: device
 * @required: if true, caller needs VT switch at suspend/resume time
 *
 * The different console drivers may or may not require VT switches across
 * suspend/resume, depending on how they handle restoring video state and
 * what may be running.
 *
 * Drivers can indicate support for switchless suspend/resume, which can
 * save time and flicker, by using this routine and passing 'false' as
 * the argument.  If any loaded driver needs VT switching, or the
 * no_console_suspend argument has been passed on the command line, VT
 * switches will occur.
 */
void pm_vt_switch_required(struct device *dev, bool required)

Next, we check console_suspend_enabled. This is set to false by the kernel parameter no_console_suspend, but defaults to true. Finally, if there are any entries in the pm_vt_switch_list, then we check to see if any of them require a VT switch. Only if none of these conditions apply, then we return false. If a VT switch is in fact required, then we move first the currently active virtual terminal/console¹⁰ (vt_move_to_console) and then the current location of kernel messages (vt_kmsg_redirect) to the SUSPEND_CONSOLE. The SUSPEND_CONSOLE is the last entry in the list of possible consoles, and appears to just be a black hole to throw away messages. kernel/power/console.c:16

#define SUSPEND_CONSOLE	(MAX_NR_CONSOLES-1)

Interestingly, these are separate functions because you can use TIOCL_SETKMSGREDIRECT (an ioctl¹¹) to send kernel messages to a specific virtual terminal, but by default its the same as the currently active console. The locations of the previously active console and the previous kernel messages location are stored in orig_fgconsole and orig_kmsg, to restore the state of the console and kernel messages after the machine wakes up again. Interestingly, this means orig_fgconsole also ends up storing any errors, so has to be checked to ensure it s not less than zero before we try to do anything with the kernel messages on both suspend and resume. drivers/tty/vt/vt_ioctl.c:1268

/* Perform a kernel triggered VT switch for suspend/resume */
static int disable_vt_switch;
int vt_move_to_console(unsigned int vt, int alloc)
 
	int prev;
	console_lock();
	/* Graphics mode - up to X */
	if (disable_vt_switch)  
		console_unlock();
		return 0;
	 
	prev = fg_console;
	if (alloc && vc_allocate(vt))  
		/* we can't have a free VC for now. Too bad,
		 * we don't want to mess the screen for now. */
		console_unlock();
		return -ENOSPC;
	 
	if (set_console(vt))  
		/*
		 * We're unable to switch to the SUSPEND_CONSOLE.
		 * Let the calling function know so it can decide
		 * what to do.
		 */
		console_unlock();
		return -EIO;
	 
	console_unlock();
	if (vt_waitactive(vt + 1))  
		pr_debug("Suspend: Can't switch VCs.");
		return -EINTR;
	 
	return prev;
 

Unlike most other locking functions we ve seen so far, console_lock needs to be careful to ensure nothing else is panicking and needs to dump to the console before grabbing the semaphore for the console and setting a couple flags.

Panics Panics are tracked via an atomic integer set to the id of the processor currently panicking. kernel/printk/printk.c:2649

/**
 * console_lock - block the console subsystem from printing
 *
 * Acquires a lock which guarantees that no consoles will
 * be in or enter their write() callback.
 *
 * Can sleep, returns nothing.
 */
void console_lock(void)
 
	might_sleep();
	/* On panic, the console_lock must be left to the panic cpu. */
	while (other_cpu_in_panic())
		msleep(1000);
	down_console_sem();
	console_locked = 1;
	console_may_schedule = 1;
 
EXPORT_SYMBOL(console_lock);

kernel/printk/printk.c:362

/*
 * Return true if a panic is in progress on a remote CPU.
 *
 * On true, the local CPU should immediately release any printing resources
 * that may be needed by the panic CPU.
 */
bool other_cpu_in_panic(void)
 
	return (panic_in_progress() && !this_cpu_in_panic());
 

kernel/printk/printk.c:345

static bool panic_in_progress(void)
 
	return unlikely(atomic_read(&panic_cpu) != PANIC_CPU_INVALID);
 

kernel/printk/printk.c:350

/* Return true if a panic is in progress on the current CPU. */
bool this_cpu_in_panic(void)
 
	/*
	 * We can use raw_smp_processor_id() here because it is impossible for
	 * the task to be migrated to the panic_cpu, or away from it. If
	 * panic_cpu has already been set, and we're not currently executing on
	 * that CPU, then we never will be.
	 */
	return unlikely(atomic_read(&panic_cpu) == raw_smp_processor_id());
 

console_locked is a debug value, used to indicate that the lock should be held, and our first indication that this whole virtual terminal system is more complex than might initially be expected. kernel/printk/printk.c:373

/*
 * This is used for debugging the mess that is the VT code by
 * keeping track if we have the console semaphore held. It's
 * definitely not the perfect debug tool (we don't know if _WE_
 * hold it and are racing, but it helps tracking those weird code
 * paths in the console code where we end up in places I want
 * locked without the console semaphore held).
 */
static int console_locked;

console_may_schedule is used to see if we are permitted to sleep and schedule other work while we hold this lock. As we ll see later, the virtual terminal subsystem is not re-entrant, so there s all sorts of hacks in here to ensure we don t leave important code sections that can t be safely resumed.

Disable VT Switch As the comment below lays out, when another program is handling graphical display anyway, there s no need to do any of this, so the kernel provides a switch to turn the whole thing off. Interestingly, this appears to only be used by three drivers, so the specific hardware support required must not be particularly common.

drivers/gpu/drm/omapdrm/dss
drivers/video/fbdev/geode
drivers/video/fbdev/omap2

drivers/tty/vt/vt_ioctl.c:1308

/*
 * Normally during a suspend, we allocate a new console and switch to it.
 * When we resume, we switch back to the original console.  This switch
 * can be slow, so on systems where the framebuffer can handle restoration
 * of video registers anyways, there's little point in doing the console
 * switch.  This function allows you to disable it by passing it '0'.
 */
void pm_set_vt_switch(int do_switch)
 
	console_lock();
	disable_vt_switch = !do_switch;
	console_unlock();
 
EXPORT_SYMBOL(pm_set_vt_switch);

The rest of the vt_switch_console function is pretty normal, however, simply allocating space if needed to create the requested virtual terminal and then setting the current virtual terminal via set_console.

Virtual Terminal Set Console With set_console, we begin (as if we haven t been already) to enter the madness that is the virtual terminal subsystem. As mentioned previously, modifications to its state must be made very carefully, as other stuff happening at the same time could create complete messes. All this to say, calling set_console does not actually perform any work to change the state of the current console. Instead it indicates what changes it wants and then schedules that work. drivers/tty/vt/vt.c:3153

int set_console(int nr)
 
	struct vc_data *vc = vc_cons[fg_console].d;
	if (!vc_cons_allocated(nr)   vt_dont_switch  
		(vc->vt_mode.mode == VT_AUTO && vc->vc_mode == KD_GRAPHICS))  
		/*
		 * Console switch will fail in console_callback() or
		 * change_console() so there is no point scheduling
		 * the callback
		 *
		 * Existing set_console() users don't check the return
		 * value so this shouldn't break anything
		 */
		return -EINVAL;
	 
	want_console = nr;
	schedule_console_callback();
	return 0;
 

The check for vc->vc_mode == KD_GRAPHICS is where most end-user graphical desktops will bail out of this change, as they re in graphics mode and don t need to switch away to the suspend console. vt_dont_switch is a flag used by the ioctls¹¹ VT_LOCKSWITCH and VT_UNLOCKSWITCH to prevent the system from switching virtual terminal devices when the user has explicitly locked it. VT_AUTO is a flag indicating that automatic virtual terminal switching is enabled¹², and thus deliberate switching to a suspend terminal is not required. However, if you do run your machine from a virtual terminal, then we indicate to the system that we want to change to the requested virtual terminal via the want_console variable and schedule a callback via schedule_console_callback. drivers/tty/vt/vt.c:315

void schedule_console_callback(void)
 
	schedule_work(&console_work);
 

console_work is a workqueue² that will execute the given task asynchronously.

Console Callback drivers/tty/vt/vt.c:3109

/*
 * This is the console switching callback.
 *
 * Doing console switching in a process context allows
 * us to do the switches asynchronously (needed when we want
 * to switch due to a keyboard interrupt).  Synchronization
 * with other console code and prevention of re-entrancy is
 * ensured with console_lock.
 */
static void console_callback(struct work_struct *ignored)
 
	console_lock();
	if (want_console >= 0)  
		if (want_console != fg_console &&
		    vc_cons_allocated(want_console))  
			hide_cursor(vc_cons[fg_console].d);
			change_console(vc_cons[want_console].d);
			/* we only changed when the console had already
			   been allocated - a new console is not created
			   in an interrupt routine */
		 
		want_console = -1;
	 
...

console_callback first looks to see if there is a console change wanted via want_console and then changes to it if it s not the current console and has been allocated already. We do first remove any cursor state with hide_cursor. drivers/tty/vt/vt.c:841

static void hide_cursor(struct vc_data *vc)
 
	if (vc_is_sel(vc))
		clear_selection();
	vc->vc_sw->con_cursor(vc, false);
	hide_softcursor(vc);
 

A full dive into the tty driver is a task for another time, but this should give a general sense of how this system interacts with hibernation.

Notify Power Management Call Chain kernel/power/hibernate.c:767

pm_notifier_call_chain_robust(PM_HIBERNATION_PREPARE, PM_POST_HIBERNATION)

This will call a chain of power management callbacks, passing first PM_HIBERNATION_PREPARE and then PM_POST_HIBERNATION on startup or on error with another callback. kernel/power/main.c:98

int pm_notifier_call_chain_robust(unsigned long val_up, unsigned long val_down)
 
	int ret;
	ret = blocking_notifier_call_chain_robust(&pm_chain_head, val_up, val_down, NULL);
	return notifier_to_errno(ret);
 

The power management notifier is a blocking notifier chain, which means it has the following properties. include/linux/notifier.h:23

 *	Blocking notifier chains: Chain callbacks run in process context.
 *		Callouts are allowed to block.

The callback chain is a linked list with each entry containing a priority and a function to call. The function technically takes in a data value, but it is always NULL for the power management chain. include/linux/notifier.h:49

struct notifier_block;
typedef	int (*notifier_fn_t)(struct notifier_block *nb,
			unsigned long action, void *data);
struct notifier_block  
	notifier_fn_t notifier_call;
	struct notifier_block __rcu *next;
	int priority;
 ;

The head of the linked list is protected by a read-write semaphore. include/linux/notifier.h:65

struct blocking_notifier_head  
	struct rw_semaphore rwsem;
	struct notifier_block __rcu *head;
 ;

Because it is prioritized, appending to the list requires walking it until an item with lower¹³ priority is found to insert the current item before. kernel/notifier.c:252

/*
 *	Blocking notifier chain routines.  All access to the chain is
 *	synchronized by an rwsem.
 */
static int __blocking_notifier_chain_register(struct blocking_notifier_head *nh,
					      struct notifier_block *n,
					      bool unique_priority)
 
	int ret;
	/*
	 * This code gets used during boot-up, when task switching is
	 * not yet working and interrupts must remain disabled.  At
	 * such times we must not call down_write().
	 */
	if (unlikely(system_state == SYSTEM_BOOTING))
		return notifier_chain_register(&nh->head, n, unique_priority);
	down_write(&nh->rwsem);
	ret = notifier_chain_register(&nh->head, n, unique_priority);
	up_write(&nh->rwsem);
	return ret;
 

kernel/notifier.c:20

/*
 *	Notifier chain core routines.  The exported routines below
 *	are layered on top of these, with appropriate locking added.
 */
static int notifier_chain_register(struct notifier_block **nl,
				   struct notifier_block *n,
				   bool unique_priority)
 
	while ((*nl) != NULL)  
		if (unlikely((*nl) == n))  
			WARN(1, "notifier callback %ps already registered",
			     n->notifier_call);
			return -EEXIST;
		 
		if (n->priority > (*nl)->priority)
			break;
		if (n->priority == (*nl)->priority && unique_priority)
			return -EBUSY;
		nl = &((*nl)->next);
	 
	n->next = *nl;
	rcu_assign_pointer(*nl, n);
	trace_notifier_register((void *)n->notifier_call);
	return 0;
 

Each callback can return one of a series of options. include/linux/notifier.h:18

#define NOTIFY_DONE		0x0000		/* Don't care */
#define NOTIFY_OK		0x0001		/* Suits me */
#define NOTIFY_STOP_MASK	0x8000		/* Don't call further */
#define NOTIFY_BAD		(NOTIFY_STOP_MASK 0x0002)
						/* Bad/Veto action */

When notifying the chain, if a function returns STOP or BAD then the previous parts of the chain are called again with PM_POST_HIBERNATION¹⁴ and an error is returned. kernel/notifier.c:107

/**
 * notifier_call_chain_robust - Inform the registered notifiers about an event
 *                              and rollback on error.
 * @nl:		Pointer to head of the blocking notifier chain
 * @val_up:	Value passed unmodified to the notifier function
 * @val_down:	Value passed unmodified to the notifier function when recovering
 *              from an error on @val_up
 * @v:		Pointer passed unmodified to the notifier function
 *
 * NOTE:	It is important the @nl chain doesn't change between the two
 *		invocations of notifier_call_chain() such that we visit the
 *		exact same notifier callbacks; this rules out any RCU usage.
 *
 * Return:	the return value of the @val_up call.
 */
static int notifier_call_chain_robust(struct notifier_block **nl,
				     unsigned long val_up, unsigned long val_down,
				     void *v)
 
	int ret, nr = 0;
	ret = notifier_call_chain(nl, val_up, v, -1, &nr);
	if (ret & NOTIFY_STOP_MASK)
		notifier_call_chain(nl, val_down, v, nr-1, NULL);
	return ret;
 

Each of these callbacks tends to be quite driver-specific, so we ll cease discussion of this here.

Sync Filesystems The next step is to ensure all filesystems have been synchronized to disk. This is performed via a simple helper function that times how long the full synchronize operation, ksys_sync takes. kernel/power/main.c:69

void ksys_sync_helper(void)
 
	ktime_t start;
	long elapsed_msecs;
	start = ktime_get();
	ksys_sync();
	elapsed_msecs = ktime_to_ms(ktime_sub(ktime_get(), start));
	pr_info("Filesystems sync: %ld.%03ld seconds\n",
		elapsed_msecs / MSEC_PER_SEC, elapsed_msecs % MSEC_PER_SEC);
 
EXPORT_SYMBOL_GPL(ksys_sync_helper);

ksys_sync wakes and instructs a set of flusher threads to write out every filesystem, first their inodes¹⁵, then the full filesystem, and then finally all block devices, to ensure all pages are written out to disk. fs/sync.c:87

/*
 * Sync everything. We start by waking flusher threads so that most of
 * writeback runs on all devices in parallel. Then we sync all inodes reliably
 * which effectively also waits for all flusher threads to finish doing
 * writeback. At this point all data is on disk so metadata should be stable
 * and we tell filesystems to sync their metadata via ->sync_fs() calls.
 * Finally, we writeout all block devices because some filesystems (e.g. ext2)
 * just write metadata (such as inodes or bitmaps) to block device page cache
 * and do not sync it on their own in ->sync_fs().
 */
void ksys_sync(void)
 
	int nowait = 0, wait = 1;
	wakeup_flusher_threads(WB_REASON_SYNC);
	iterate_supers(sync_inodes_one_sb, NULL);
	iterate_supers(sync_fs_one_sb, &nowait);
	iterate_supers(sync_fs_one_sb, &wait);
	sync_bdevs(false);
	sync_bdevs(true);
	if (unlikely(laptop_mode))
		laptop_sync_completion();
 

It follows an interesting pattern of using iterate_supers to run both sync_inodes_one_sb and then sync_fs_one_sb on each known filesystem¹⁶. It also calls both sync_fs_one_sb and sync_bdevs twice, first without waiting for any operations to complete and then again waiting for completion¹⁷. When laptop_mode is enabled the system runs additional filesystem synchronization operations after the specified delay without any writes. mm/page-writeback.c:111

/*
 * Flag that puts the machine in "laptop mode". Doubles as a timeout in jiffies:
 * a full sync is triggered after this time elapses without any disk activity.
 */
int laptop_mode;
EXPORT_SYMBOL(laptop_mode);

However, when running a filesystem synchronization operation, the system will add an additional timer to schedule more writes after the laptop_mode delay. We don t want the state of the system to change at all while performing hibernation, so we cancel those timers. mm/page-writeback.c:2198

/*
 * We're in laptop mode and we've just synced. The sync's writes will have
 * caused another writeback to be scheduled by laptop_io_completion.
 * Nothing needs to be written back anymore, so we unschedule the writeback.
 */
void laptop_sync_completion(void)
 
	struct backing_dev_info *bdi;
	rcu_read_lock();
	list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
		del_timer(&bdi->laptop_mode_wb_timer);
	rcu_read_unlock();
 

As a side note, the ksys_sync function is simply called when the system call sync is used. fs/sync.c:111

SYSCALL_DEFINE0(sync)
 
	ksys_sync();
	return 0;
 

The End of Preparation With that the system has finished preparations for hibernation. This is a somewhat arbitrary cutoff, but next the system will begin a full freeze of userspace to then dump memory out to an image and finally to perform hibernation. All this will be covered in future articles!

1. Hibernation modes are outside of scope for this article, see the previous article for a high-level description of the different types of hibernation.
2. Workqueues are a mechanism for running asynchronous tasks. A full description of them is a task for another time, but the kernel documentation on them is available here: https://www.kernel.org/doc/html/v6.9/core-api/workqueue.html ²
3. This is a bit of an oversimplification, but since this isn t the main focus of this article this description has been kept to a higher level.
4. Kconfig is Linux s build configuration system that sets many different macros to enable/disable various features.
5. Kconfig defaults to the first default found
6. Including checking whether the algorithm is larval? Which appears to indicate that it requires additional setup, but is an interesting choice of name for such a state.
7. Specifically when we get to process freezing, which we ll get to in the next article in this series.
8. Swap space is outside the scope of this article, but in short it is a buffer on disk that the kernel uses to store memory not current in use to free up space for other things. See Swap Management for more details.
9. The code for this is lengthy and tangential, thus it has not been included here. If you re curious about the details of this, see kernel/power/hibernate.c:858 for the details of hibernate_quiet_exec, and drivers/nvdimm/core.c:451 for how it is used in nvdimm.
10. Annoyingly this code appears to use the terms console and virtual terminal interchangeably.
11. ioctls are special device-specific I/O operations that permit performing actions outside of the standard file interactions of read/write/seek/etc. ²
12. I m not entirely clear on how this flag works, this subsystem is particularly complex.
13. In this case a higher number is higher priority.
14. Or whatever the caller passes as val_down, but in this case we re specifically looking at how this is used in hibernation.
15. An inode refers to a particular file or directory within the filesystem. See Wikipedia for more details.
16. Each active filesystem is registed with the kernel through a structure known as a superblock, which contains references to all the inodes contained within the filesystem, as well as function pointers to perform the various required operations, like sync.
17. I m including minimal code in this section, as I m not looking to deep dive into the filesystem code at this time.

The backbone: Matrix I chose to use Matrix as the place where I integrate everything. Despite there being some heavy (and justified) criticism on the protocol itself, it serves me well for what I need right now. Obviously, I don t like the fact that I have to provide Matrix and all of its accompanying bridges a VPS with 4GB of RAM and 3 vCPUs, but I think that that ship has sailed, unfortunately. In an ideal world, I would be using XMPP and dedicating only a fraction of the resources I m using today to have a full chat system. And since I have been running my personal XMPP server for more than a decade now, I did try to find a solution that would allow me to keep using it, but unfortunately the protocol became almost a hobbyist thing, so there s that.

A few disclaimers I self-host everything, including my Matrix server. Much of what I did won t work if you don t self-host Matrix, so keep that in mind. This won t be a post teaching you how to deploy the services. My intention is to describe what I use and for what purpose. Also, as much as I try to use Debian packages for everything I do, I opted to deploy all services using a community-maintained Ansible playbook which is very well written and organized: matrix-docker-ansible-deploy. Last but not least, as I said above, you will likely need a machine with a good amount of RAM, CPU and storage, especially if you deploy Synapse as your Matrix homeserver (which is what I recommend if you plan to use the bridges I ll mention). My current VPS has 4GB of RAM, 3 vCPUs and 80GB of storage (of which I m currently using approximately 55GB).

Problem #1: my Matrix client(s) There are a lot of clients that can talk the Matrix protocol, but most of them are either web clients or GUI programs. I live on the terminal, more specifically inside Emacs, so I settled for the amazing ement.el Emacs mode. It works surprisingly well, but unfortunately doesn t support end-to-end encryption out of the box; for that, you have to hook it up with pantalaimon. Unfortunately, the project seems abandoned and therefore I don t recommend you to use it. I don t use it myself. When I have to reply some E2E encrypted message from another user, I go to my web browser and use my self-hosted Element client. It s a nuisance, but one that I m willing to accept because of security concerns. If you re into web clients and don t want to use Element (because it is heavy), you can try Cinny. It s lightweight and supports a decent set of features. If you re a terminal lover but don t use Emacs, you may want to try gomuks or iamb.

Problem #2: IRC bridging There are basically two types of IRC bridges for Matrix:

• The regular and most used matrix-appservice-irc. This bridge takes Matrix to IRC (think of IRC users with the [m] suffix appended to their nicknames), and is what the matrix.org and other big homeservers (including matrix.debian.social) use. It s a complex service which allows thousands of Matrix users to connect to IRC networks, but that unfortunately has complex problems and is only worth using if you intend to host a community server.
• A bouncer-like bridge called Heisenbridge. This is what I use personally. It takes IRC to Matrix, which means that people on IRC will not know that you re using Matrix. This bridge is much simpler, and because it acts like a bouncer it s pretty much impossible for it to cause problems with the IRC network.

Due to the fact that I sometimes like to use other IRC clients, I still run a regular ZNC bouncer, and I use Heisenbridge to connect to my ZNC. This means that I can use, e.g., ERC inside Emacs and my Matrix bridge at the same time. But you don t necessarily need to run another bouncer; you can simply use Heisenbridge and connect directly to the IRC network(s) you want. A word of caution, though: unlike ZNC, Heisenbridge doesn t support per-user configuration when you use it in bouncer mode. This is the reason why you need to self-host it, and why it s not possible to offer the service to other users (they would have access to your IRC network configuration otherwise). It s also worth talking about logs. I find that keeping logs of everything that goes on IRC has saved me a bunch of times, and so I find it really important to continue doing that. Unfortunately, neither ement.el nor Element support logging things out of the box (at least not that I know). This is also one of the reasons why I still keep my ZNC around: I configure it to log everything.

Problem #3: Telegram I don t use Telegram myself, but unfortunately several people from the Debian community do, especially in Brazil. There is a whole Debian community on Telegram, and I wanted to be able to bridge our Debian Matrix channels to their Telegram counterparts. I am currently using mautrix-telegram for that, and it s working great. You need someone with a Telegram account to configure their credentials so that the bridge can connect to it, but afterwards it s really easy to bridge channels together.

Problem #4: GitLab webhooks Something else I wanted to be able to do was to receive notifications regarding new issues, merge requests and other activities from Salsa. For this, I m using maubot, which is awesome and has a huge list of plugins. I m using the gitlab one.

Final thoughts Overall, I m satisfied with the setup I have now. It has certainly taken some time and effort to find the right tool for each problem I needed to solve, and I still feel like there are some rough edges to soften (like the fact that my Emacs client doesn t support E2E encryption out of the box, or the whole logging situation), but otherwise things are working fine and I haven t had any big problems with the deployment. You do have to be much more careful about stuff (for example, when I installed an unrelated service that hijacked my Apache configuration and made Matrix s federation silently stop working), though. If you have more specific questions about any part of my setup, shoot me an email and I ll do my best to help. Happy chatting!

Focus This month I didn't have any particular focus. I just worked on issues in my info bubble.

Changes

• duck: add indicators (1 2)
• Debian wiki pages: PortsDocs/New, Rust

Issues

• Regressions in adequate (1 2 3 4)

Review

• Debian publicity: reviewed Debian birthday post

Administration

• Debian servers: contributed to a review of current Debian Partners

Communication

• Respond to queries from Debian users and contributors on IRC

Sponsors All work was done on a volunteer basis.

1. LWN: The history, status, and plans for reproducible builds
2. Intermediate Autotools build artifacts removed from PostgreSQL distribution tarballs
3. Distribution news
4. Mailing list news
5. diffoscope
6. Website updates
7. Upstream patches
8. Reproducibility testing framework

LWN: The history, status, and plans for reproducible builds The free software newspaper of record, Linux Weekly News, published an in-depth article based on Holger Levsen s talk, Reproducible Builds: The First Eleven Years which was presented at the recent DebConf24 conference in Busan, South Korea. Titled The history, status, and plans for reproducible builds and written by Jake Edge, LWN s article not only summarises Holger s talk and clarifies its message but it links to external information as well. Holger s original talk can also be watched on the DebConf24 webpage (direct .webm link and his HTML slides are available also). There are also a significant number of comments on LWN s page as well. Holger Levsen also headed a scheduled discussion session at DebConf24 on Preserving *other* build artifacts addressing a topic where a number of Debian packages are (or would like to) produce results that are neither the .deb files, the build logs nor the logs of CI tests. This is an issue for reproducible builds as this 4th type of build artifact are typically shipped within the binary .deb packages, and are invariably non-deterministic; thus making the .deb files unreproducible. (A direct .webm link and HTML slides are available).

Intermediate Autotools build artifacts removed from PostgreSQL distribution tarballs Peter Eisentraut wrote a detailed blog post on the subject of The new PostgreSQL 17 make dist . Like many projects, the PostgreSQL database has previously pre-built parts of its GNU Autotools build system: the reason for this is a mix of convenience and traditional practice . Peter astutely notes that this arrangement in the build system is quite tricky as:

You need to carefully maintain the different states of clean source code , partially built source code , and fully built source code , and the commands to transition between them.

However, Peter goes on to mention that:

a lot more attention is nowadays paid to the software supply chain. There are security and legal reasons for this. When users install software, they want to know where it came from, and they want to be sure that they got the right thing, not some fake version or some version of dubious legal provenance.

And cites the XZ Utils backdoor as a reason to care about transparent and reproducible ways of distributing and communicating a source tarball and provenance. Because of this, intermediate build artifacts are now henceforth essentially disallowed from PostgreSQL distribution tarballs.

Distribution news In Debian this month, 30 reviews of Debian packages were added, 17 were updated and 10 were removed this month adding to our knowledge about identified issues. One issue type was added by Chris Lamb, too. [ ] In addition, an issue was filed to update the Salsa CI pipeline (used by 1,000s of Debian packages) to no longer test for reproducibility with reprotest s build_path variation. Holger Levsen provided a rationale for this change in the issue, which has already been made to the tests being performed by tests.reproducible-builds.org.
In Arch Linux this month, Jelle van der Waa published a short blog post on the topic of Investigating creating reproducible images with mkosi, motivated by the desire to make it possible for anyone to re-recreate the official Arch cloud image bit-by-bit identical on their own machine as per [the] reproducible builds definition. In addition, Jelle filed a patch for pacman, the Arch Linux package manager, to respect the SOURCE_DATE_EPOCH environment variable when installing a package.
In openSUSE news, Bernhard M. Wiedemann published another report for that distribution.
In Android news, the IzzyOnDroid project added 49 new rebuilder recipes and now features 256 total reproducible applications representing 21% of the total offerings in the repository. IzzyOnDroid is an F-Droid style repository for Android apps[:] applications in this repository are official binaries built by the original application developers, taken from their resp. repositories (mostly GitHub).

Mailing list news From our mailing list this month:

• Bernhard M. Wiedemann posted a brief message to the list with some helpful information regarding nondeterminism within Rust binaries, positing the use of the codegen-units = 16 default and resulting in a bug being filed in the Rust issue tracker. [ ]
• Bernhard also wrote to the list, following up to a thread in November 2023, on attempts to make the LibreOffice suite of office applications build reproducibly. In the thread from this month, Bernhard could announce that the four patches previously mentioned have landed in LibreOffice upstream.
• Fay Stegerman linked the mailing list to a thread she made on the Signal issue tracker regarding whether device-specific binaries [can] ever be considered meaningfully reproducible . In particular: the whole part about allow[ing] multiple third parties to come to a consensus on a correct result breaks down completely when correct is device-specific and not something everyone can agree on. [ ]
• Developer kpcyrd posted an update for source code indexing project, whatsrc.org. Announcing that it now importing packages from live-bootstrap ( a usable Linux system [that is] created with only human-auditable, and wherever possible, human-written, source code ) into its database of provenance data.
• Lastly, Mechtilde Stehmann posted an update to an earlier thread about how Java builds are not reproducible on the armhf architecture, enquiring how they might gain temporary access to such a machine in order to perform some deeper testing. [ ]

diffoscope diffoscope is our in-depth and content-aware diff utility that can locate and diagnose reproducibility issues. This month, Chris Lamb released versions 274, 275, 276 and 277, uploaded these to Debian, and made the following changes as well:

• New features:
  • Strip ANSI escapes usually colour codes from the output of the Procyon Java decompiler. [ ]
  • Factor out a method for stripping ANSI escapes. [ ]
  • Append output from dumppdf(1) in more cases, avoiding situations where we fallback to a binary diff. [ ]
  • Add support for versions of Perl s IO::Compress::Zip version 2.212. [ ]
• Bug fixes:
  • Also catch RuntimeError exceptions when importing the PyPDF library so that it, or, crucially, its transitive dependencies, cannot not cause diffoscope to traceback at runtime and build time. [ ]
  • Do not call marshal.load( ) of precompiled Python bytecode as it, alas, inherently unsafe. Replace for now with a brief summary of the code section of .pyc. [ ][ ]
  • Don t include excessive debug output when calling dumppdf(1). [ ]
• Testsuite-related changes:
  • Don t bother to check version number in test_python.py: the fixture for this test is fixed. [ ][ ]
  • Update test_zip text fixtures and definitions to support new changes to the Perl IO::Compress library. [ ]

In addition, Mattia Rizzolo updated the available architectures for a number of test dependencies [ ] and Sergei Trofimovich fixed an issue to avoid diffoscope crashing when hashing directory symlinks [ ] and Vagrant Cascadian proposed GNU Guix updates for diffoscope versions 275 and 276 and 277.

Website updates There were a rather substantial number of improvements made to our website this month, including:

• Alba Herrerias:
  • Substantially extend the guidance on the Contribute page. [ ]
• Chris Lamb:
  • Set the future: true configuration value so we render all files and documents in the website, regardless of whether they have a date property in the future. After all, we don t re-generate the website on a timer, and have other ways of making unpublished, draft posts. [ ][ ]
• Fay Stegerman:
  • Add IzzyOnDroid (IoD) to the Projects page. [ ]
• hulkoba:
  • Considerably overhaul the History page in the documentation, linking strip-nondeterminism and SOURCE_DATE_EPOCH [ ], fixing the test statistics link [ ], adjusting the Google Summer of Code application link [ ], a link to a Debian bug [ ], and removed a dead link to the debhelper utility [ ].
  • Use the jekyll-sitemap plugin to create a sitemap for the website. [ ]
  • Use raw HTML to avoid a literal .lead directive appearing in the page. [ ]
  • Fix a number of issues on the Virtual machine drivers page, such as keeping the Gitian info, linking (and then removing) an issue on the Bitcoin issue tracker [ ] and fixing a link to the Bazel website [ ].
  • Address a broken footnote link on the Timestamps page. [ ]
  • Unify the style on the Commandments of Reproducible Builds page in order to match other documentation entries. [ ]
  • Add a table of contents to the main Documentation page. [ ]
  • Avoid a number of so-called here links on the Variations in the build environment page. [ ]
  • Fix a link to the man2html patch on the SOURCE_DATE_EPOCH documentation page. [ ]
  • Fix a link to sources.debian.org on the Randomness page. [ ]
• kpcyrd:
  • Fix a typo on the Variations in the build environment page. [ ]
• Mattia Rizzolo:
  • Add all the sponsors to the Hamburg 2024 summit page. [ ][ ][ ][ ][ ]
  • Fix a link in the summit sponsor prospectus PDF. [ ]
• Pol Dellaiera:
  • Fix the DoI for their thesis on the Publications page. [ ]

Upstream patches The Reproducible Builds project detects, dissects and attempts to fix as many currently-unreproducible packages as possible. We endeavour to send all of our patches upstream where appropriate. This month, we wrote a large number of such patches, including:

• Bernhard M. Wiedemann:
  • agama-integration-tests (uses a random TCP-port number in .lock file)
  • ca-certificates-mozilla:ca-certificates-mozilla-prebuilt
  • cosmic (hash order issue)
  • openSUSE (meta-issue to test reproducibility in the openSUSE Build Service)
  • pop-launcher (parallelism-related issue)
  • post (toolchain-issue, avoiding Rust parallelism)
  • rpm-config-SUSE (date-related issue)
  • rust (Rust toolchain issue)
  • weblate (build gets stuck)
• Chris Lamb:
  • #1078048 filed against scikit-optimize.
  • #1078153 filed against pan.
  • #1078687 filed against tetzle.
  • #1079041 filed against apg.
  • #1079312 filed against receptor.
• James Addison:
  • #1064782 forwarded and merged in bind9-doc
  • #1066083 forwarded and merged in gnome-maps

Reproducibility testing framework The Reproducible Builds project operates a comprehensive testing framework running primarily at tests.reproducible-builds.org in order to check packages and other artifacts for reproducibility. In August, a number of changes were made by Holger Levsen, including:

• Temporarily install the openssl-provider-legacy package for the Debian unstable environments for running diffoscope due to Debian bug #1078944. [ ][ ][ ][ ]
• Mark Debian armhf architecture nodes as being down due to proxy down. [ ][ ]
• Detect proxy failures. [ ][ ][ ]
• Run the index-buildinfo for the builtin-pho script with the -q switch. [ ]
• Disable all Arch Linux reproducible jobs. [ ]

In addition, Mattia Rizzolo updated the website configuration to install the ruby-jekyll-sitemap package as it is now used in the website [ ], Roland Clobus updated the script to build Debian live images to treat openQA issues as warnings [ ], and Vagrant Cascadian marked the cbxi4b node as down [ ].

---

If you are interested in contributing to the Reproducible Builds project, please visit our Contribute page on our website. However, you can get in touch with us via:

• IRC: #reproducible-builds on irc.oftc.net.
• Mastodon: @reproducible_builds@fosstodon.org
• Mailing list: rb-general@lists.reproducible-builds.org
• Twitter: @ReproBuilds

Changes in version 0.2.13 (2024-09-03)

• A test script was updated to account for the fact that it now returns a few instances of NULL under current Rcpp.
• Small package maintenance updates have been made to the README and DESCRIPTION files as well as to the continuous integration setup.

This post by Dirk Eddelbuettel originated on his Thinking inside the box blog. Please report excessive re-aggregation in third-party for-profit settings.

tl;dr DebConf24 was fun! A playlist of all of my talks, with subtitles (en, pt-br) and chapters is available on YouTube.

Overview DebConf24 was held in Busan, South Korea, between Sunday July 28th to Sunday August 4th 2024. As usual for DebConfs, I had a great time meeting my friends, but also met new people and got to learn a bit about the interesting things they're working on. I ended up getting too excited during the talk submission stage of the conference and as a result I presented 5 different activities (3 talks, 1 BoF and 1 lightning talk). Since I was too busy with the presentations, I did not have a lot of time to actually hang out with folks, or even to go out in the city, I guess I've learned my lesson for next time. The main purpose of this post is to write about all of the things I presented at the conference. I did want to list some of the interesting talks I've watched, but that I would not be able to be fair as I'm sure I would miss some. You can get the schedule and the recordings of any talks from the conference's website: https://debconf24.debconf.org/schedule/

wcurl Lightning Talk The most fun of my presentations, during the second-to-last day of the conference, I've asked for help from Sergio Durigan Junior <sergiodj> to setup an URL containing a whitespace and redirecting that to wcurl's manpage. I then did a little demo to showcase why me (and a lot others) struggle with downloading things with curl, and how wcurl solves that. https://www.youtube.com/watch?v=eM8M5qa4pPM

Fixing CVEs on Debian: Everything you probably know already I've always felt like DebConf was missing security-related talks, so I decided to do something about it and presented a few of the things I've learned when fixing CVEs for Debian. This is an area where we don't get a lot of new contributors, I'm trying to change that, and this talk can be used to introduce newcomers to it. https://www.youtube.com/watch?v=XzNVVILVyUM

The secret sauce of Debian Debian is not very vocal about all of the nice things it has regarding quality-assurance, testing, or CI, even though it's at the state-of-the-art for a lot of things. This talk is an initial step towards making people aware of the cool things happening behind the scenes. Ideally we should have it well-documented somewhere. https://www.youtube.com/watch?v=x_X2IBnpjic

"I use Debian BTW": fzf, tmux, zoxide and friends One of my earliest good memories of Debian was when it started coming with a colored PS1 by default, I still remember the feeling of relief whenever I jumped into a Debian server and didn't have to deal with a black and white PS1. There's still a lot of room for Debian to ship better defaults, and I think some of them can actually happen. This talk is a bit of a silly one where I'm just making people aware of the existence of a few Golang/Rust CLI tools, and also some dotfiles configurations that should probably be the default. https://www.youtube.com/watch?v=tfto3Seokn4

curl The curl project does such a great job with their security advisories that it will likely never receive the amount of praise it deserves, but I did my best at mentioning it throughout my CVEs talk. Maybe I will write more extensively about this someday, but in case I don't:

---

There's no other project which always consistently mentions the exact range of commits that are affected by a given CVE. Forget about whether the versions are EOL, curl doesn't have LTS releases, yet they do such a great job at clearly documenting their CVEs that I would take that over having LTS releases anytime (that's for curl at least, I acknowledge some types of projects have a different need for LTS releases). Not only that, but they are also always careful about explaining alternative mitigations such as configuration changes, build flags that defuse the exploitation, or parameters that you should not use.

---

Just like we tend to do every time we meet, me and the other Debian curl maintainers spent the first 2 or 3 days of the conference talking about how we wanted to eventually meet up to discuss the package. It was going to be informal, maybe during the Cheese and Wine party, but then I've realized we should make it part of the official schedule, which would also give us the recordings for later. And so the "curl maintainers BoF" happened, where we spoke about HTTP3, GnutTLS, wcurl and other things. https://www.youtube.com/watch?v=fL7hSypUTdM

wcurl Right after that BoF, Daniel Stenberg asked if we were interested in having wcurl adopted into curl, which we definitely were, so wcurl is now part of the curl project. Daniel was also kind enough to design a logo for the project, which makes me especially happy because I can stop with my own approach at a logo (which I had to redo every few days): And here is the new logo: Much better, I would say :)

curl Swag DebConf24 was my chance at forwarding some curl swag items to the other curl maintainers, so both Sergio Durigan Junior <sergiodj> and Carlos Henrique Lima Melara <charles> got the curl-up t-shirt and the very cool curl PCB coaster, both gifted by Daniel Stenberg. Unfortunately I didn't have any of that for DebConf attendees, but I did drop loads of curl stickers at the stickers table, they were gone very quickly.

For the future I used to think the most humbling experience you could have as someone who presented a talk was to have to watch it yourself, you notice a lot of mistakes and you instantly think about things that should be done differently. It turns out the most humbling thing to do is actually to write subtitles for your talks, I noticed every single mistake, often multiple times. So after spending more than 30 hours writing the subtitles for both English and Brazilian Portuguese for my talks, I feel like it's going to be much easier to avoid committing the same mistakes again. After some time you stop feeling shame about those mistakes and you're just left with feelings of annoyance, and at that point it becomes easier to consciously avoid them. I am collecting a list of things I wish I had done differently on all of those talks, so if I end up presenting any one of them again, it will be an improved version.

This post is a review for Computing Reviews for Free and open source software and other market failures , a article published in Communications of the ACM

Beer, cake and ISO testing amidst rugby and jazz band chaos On Saturday, the Debian South Africa team got together in Cape Town to celebrate Debian s 31st birthday and to perform ISO testing for the Debian 11.11 and 12.7 point releases.

We ran out of time to organise a fancy printed cake like we had last year, but our improvisation worked out just fine!

We thought that we had allotted plenty of time for all of our activities for the day, and that there would be plenty of time for everything including training, but the day zipped by really fast. We hired a venue at a brewery, which is usually really nice because they have an isolated area with lots of space and a big TV nice for presentations, demos, etc. But on this day, there was a big rugby match between South Africa and New Zealand, and as it got closer to the game, the place just got louder and louder (especially as a band started practicing and doing sound tests for their performance for that evening) and it turned out our space was also double-booked later in the afternoon, so we had to relocate.

Even amidst all the chaos, we ended up having a very productive day and we even managed to have some fun!

Four people from our local team performed ISO testing for the very first time, and in total we covered 44 test cases locally. Most of the other testers were the usual crowd in the UK, we also did a brief video call with them, but it was dinner time for them so we had to keep it short. Next time we ll probably have some party line open that any tester can also join.

Logo

We went through some more iterations of our local team logo that Tammy has been working on. They re turning out very nice and have been in progress for more than a year, I guess like most things Debian, it will be ready when it s ready!

Debian 11.11 and Debian 12.7 released, and looking ahead towards Debian 13 Both point releases tested just fine and was released later in the evening. I m very glad that we managed to be useful and reduce total testing time and that we managed to cover all the test cases in the end. A bunch of things we really wanted to fix by the time Debian 12 launched are now finally fixed in 12.7. There s still a few minor annoyances, but over all, Debian 13 (trixie) is looking even better than Debian 12 was around this time in the release cycle. Freeze dates for trixie has not yet been announced, I hope that the release team announces those sooner rather than later, also KDE Plasma 6 hasn t yet made its way into unstable, I ve seen quite a number of people ask about this online, so hopefully that works out. And by the way, the desktop artwork submissions for trixie ends in two weeks! More information about that is available on the Debian wiki if you re interested in making a contribution. There are already 4 great proposals.

Debian Local Groups Organising local events for Debian is probably easier than you think, and Debian does make funding available for events. So, if you want to grow Debian in your area, feel free to join us at #debian-localgroups on the OFTC IRC network, also plumbed on Matrix at #debian-localgroups:matrix.debian.social where we ll try to answer any questions you might have and guide you through the process! Oh and btw South Africa won the Rugby!

$ cat /var/lib/schroot/chroots/sid-amd64/etc/hosts
127.0.0.1       localhost
127.0.1.1       [...]
127.0.0.1       localhost ip6-localhost ip6-loopback

• python-aiohttp 3.10.0 needed fixes in blinkpy (#1077981, upstream PR) and python-yalexs (#1077985, upstream PR).
• twisted 24.7.0 needed fixes in pytest-twisted (cherry-picked existing upstream commit), python-daphne (cherry-picked existing upstream PR), and python-tornado (#1078411, upstream PR).

• Debug crash when swiping away keyboard on lockscreen (MR).
• Fix outdated clock when swiping back from lockscreen plugins (MR)
• Avoid deprecation warning (MR)
• Better handle mobile network generation bit masks (MR)
• Improve docs that end up in the libphosh-rs docs (MR)
• Modernize ModemManager backend in preparation for Cellbroadcast support (MR)
• Remove hacks from Cell Broadcast support MR (MR). Still draft but not much todo left once the ModemManager side landed
• Remove deprecated UI props and add a check so they don't creep back in (MR)
• Allow to use ASAN when feedbackd is a subproject (MR)
• Fix crash when Wi-Fi hot spot quick setting gets disabled (MR)
• Don't allow to change hotspot state on the lock screen (MR)
• Prepare and release Phosh 0.41.0~rc1 and Phosh 0.41.0
• Prepare 0.41.1 (MR)

• Don't reject gesture when we cross another surface (MR)

• Drop redundant enums (MR)
• Remember last used panel (MR)
• Fix initial state of move up/down popovers (MR)
• Allow to select OSK layouts (MR). This ensures only actually available layouts can be selected. Currently used by phosh-osk-stub but can easily be extended to squeekboard once it provides the information.

• Allow to run doc build locally and fix CI (MR)
• Release 0.0.2

• Allow to open OSK Settings panel when screen is not locked (MR)
• Unswap Enter and Backspace (MR)
• Bug fix release 0.41.1
• Use varnam_learn() for better completions in the varnam completer (MR)
• Export layout information (MR)
• Reduce flicker when launching settings (MR)

• Avoid new event sounds not being picked up due to stale caches (MR)
• Improve phone-hangup sound (MR)

• Add release helpers (MR)

• VM images to test nightly packages https://salsa.debian.org/agx/phosh-recipes (based on Mobian's recipes)

• Upload Phosh 0.41.0~rc1 and 0.41.0 releases
• Robustify release script a bit (MR)
• Enable binding lib in phosh (MR)
• Move govarnam and varnam schemes packages into the input method team
• Upload varnam schemes to sid (MR)
• Make varnam-schemes reproducible, add autopkgtests and run upstream test during build (MR)
• Build wlroots with xcb-errors support (MR)

• Help mobian-recipes with newer debos: (MR)

• Rework most bits of Cell Broadcast to move it closer to undraft status (MR). (Remaining bits affect enabling of unsolicited messages and setting channels).

• Use official notification category (MR)
• Use AdwAboutDialog (MR)

• Fix some deprecations (MR)
• Make pairing dialog adaptive (MR)
• Allow to use with Phosh without imposing more API/ABI guarantees (MR

• Fix crash when hitting an error condition (which could then bring down the whole session): (MR)

• Install the udev rule via meson (MR to makes it easier for distros to pick up rule changes
• Sync packaging with Debian (MR)
• Document used gsettings (MR)

• Update information at matrix.org (MR)
• Implement more unified push bits: (MR
• Document things a bit (MR
• Chase libcmatrix API changes (MR)

• More cleanups (MR, MR, MR)
• Publish docs (MR)
• Ship introspection data (MR)
• Release 0.0.2 (MR) (Release announcement)

• Catch up with libcmatrix API changes (MR)

• Avoid broken URLs when using ntfy (MR)

• Improve error message when not running in CI (MR)

• Drop outdated comments (MR)

• propose some hints for Mobile clients (MR)

• propose new sound name for cell broadcasts (MR)

• Make reproducible (MR)
• Don't ignore errors in build scripts (MR)
• Allow to run test against installed schemes (MR
• Fix build with recent ruby (MR)

• Helped a bit to setup the event
• Gave a short Phosh status update

• [1] https://tinyurl.com/2d49n8al
• [2] https://tinyurl.com/26vnjl6e
• [3] https://tinyurl.com/27njkm72
• [4] https://tinyurl.com/yvrnxq6w
• [5] https://tinyurl.com/298p9zxv
• [6] https://www.youtube.com/watch?v=ouAG4vXFORc
• [7] https://www.youtube.com/watch?v=iMwepyyaj8I
• [8] https://www.youtube.com/watch?v=jW2NSrzcrIQ
• [9] https://tinyurl.com/2dfgty8z

Interactive tuning tool Instead of computing each property from font average metrics, I put together a tool for interactively tuning fallback fonts.¹

Instructions

1. Load your custom font.
2. Select a fallback font to tune.
3. Adjust the size-adjust property to match the width of your custom font with the fallback font. With a proportional font, it is not possible to achieve a perfect match.
4. Fine-tune the ascent-override property. Aim to align the final dot of the last paragraph while monitoring the font s baseline. For more precise adjustment, disable the Set line height option.
5. Modify the descent-override property. The goal is to make the two boxes match. You may need to alternate between this and the previous property for optimal results.
6. If necessary, adjust the line-gap-override property. This step is typically not required.

The process needs to be repeated for each fallback font. Some platforms may not include certain fonts. Notably, Android lacks most fonts found in other operating systems. It replaces Georgia with Noto Serif, which is not metrically-compatible.

Tool

This tool is not available from the Atom feed.

Results For the body text of this blog, I get the following CSS definition:

@font-face  
  font-family: Merriweather;
  font-style: normal;
  font-weight: 400;
  src: url("../fonts/merriweather.woff2") format("woff2");
  font-display: swap;
 
@font-face  
  font-family: "Fallback for Merriweather";
  src: local("Noto Serif"), local("Droid Serif");
  size-adjust: 98.3%;
  ascent-override: 99%;
  descent-override: 27%;
 
@font-face  
  font-family: "Fallback for Merriweather";
  src: local("Georgia");
  size-adjust: 106%;
  ascent-override: 90.4%;
  descent-override: 27.3%;
 
font-family: Merriweather, "Fallback for Merriweather", serif;

After a month, the CLS metric improved to 0:

Recent Core Web Vitals scores for vincent.bernat.ch

About custom fonts Using safe web fonts or a modern font stack is often simpler. However, I prefer custom web fonts. Merriweather and Iosevka, which are used in this blog, enhance the reading experience. An alternative approach could be to use Georgia as a serif option. Unfortunately, most default monospace fonts are ugly. Furthermore, paragraphs that combine proportional and monospace fonts can create visual disruption. This occurs due to mismatched vertical metrics or weights. To address this issue, I adjust Iosevka s metrics and weight to align with Merriweather s characteristics.

---

1. Similar tools already exist, like the Fallback Font Generator, but they were missing a few features, such as the ability to load the fallback font or to have decimals for the CSS properties. And no source code.

"Well," said Tiffany, "there's too much to be done and not enough people to do it." The smile that the kelda gave her was a strange one. The little woman said, "Do ye let them try? Ye mustn't be afraid to ask for help. Pride is a good thing, my girl, but it will kill you in time."

"Granny never said as she was better than others. She just got on with it and showed 'em and people worked it out for themselves."

Changes in version 0.2.4 (2024-08-30)

• The .Rbuildignore stanza now includes .github
• The support of and usage illustrations of tinytest are much enhanced (Paul Hudor in #18 adressing #19 and #20)
• The .gitignore file now includes C++ related files
• Improvements and polish to badges and continuous integration
• The DESCRIPTION file now contains an Authors@R entry

This post by Dirk Eddelbuettel originated on his Thinking inside the box blog. Please report excessive re-aggregation in third-party for-profit settings.