I think the old-fashioned coredump is a little under-appreciated
these days. I'm not sure when it changed, but I even had to add
myself to
/etc/security/limits.conf to raise my
ulimit to even create one.
Anyway, debugging
__thread variables from coredumps is a
bit of a pain. For the uninitiated, the
__thread identifier
specifies a thread-local variable, i.e. every thread gets its
own copy of the variable automatically.
The implementation of this is highly architecture specific. The
reason is that TLS entries need to be accessed via a register kept as
part of the thread state, and thus every architecture chooses their
own register and builds their own ABI. On x86-32, which is very
register-limited, you certainly don't want to dedicate a register to a
pointer to TLS variables and take it out of operation. Luckily there
is the hang-over from the 70's (60's? 50's?) segmentation.
Without going into real detail, segment registers can be used to
offset into a region of memory based on a look-up of a region
descriptor stored in a table.
Above, you see a simplified example of the
%gs register
loaded with the index value 2, and thus when you access
%gs:20 what you are saying is "find entry 2 in the global
descriptor table (GDT), follow it and offset 20 into that
region.
The kernel gives each thread its own GDT (i.e. the GDT register is
part of the thread-state). Thus
__thread variables are
stored based on segment offsets and voila thread-local
storage. Now, there's a few tricks here. For various reasons, a
process can not setup entries in the GDT; this is a privileged
operation that must be done by the kernel. There is actually a
special system call for threads to setup their TLS areas in the GDT
set_thread_area. When a new thread starts, the
thread-library and dynamic linker conspire to allocate and load any
static TLS data (i.e. if you have a global
__thread variable
initialised to some value, then every thread must see that value when
it starts) and then calls this to make sure the variables are ready to
go. After that, the
gs register is filled with the index of
that GDT entry, and all TLS access goes via it. That, in a nut-shell,
is TLS for x86-32.
Now, to the problem. Take, for example, the following short
program:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
int __thread foo;
void* thread(void *in)
foo = (int)in;
printf("foo is %d\n", foo);
while (1)
sleep(10);
int main(void)
pthread_t threads[5];
int i;
for (i=0; i<5; i++)
pthread_create(&threads[i], NULL,
thread, (void*)i);
sleep(5);
abort();
We start a few threads and then abort to make it dump core. But,
if you try and examine
foo:
$ gdb ./thread core
GNU gdb (GDB) 7.2-debian
Reading symbols from /home/ianw/tmp/thread/thread...done.
[New Thread 4970]
[New Thread 4975]
[New Thread 4974]
[New Thread 4973]
[New Thread 4972]
[New Thread 4971]
Core was generated by ./thread'.
Program terminated with signal 6, Aborted.
#0 0xffffe424 in __kernel_vsyscall ()
(gdb) print foo
Cannot find thread-local variables on this target
It seems that gdb doesn't know how to find the value of
foo because its not a variable in the usual sense ("this
target", in this case, means a coredump). It relies on accessing via
the
gs register, which relies on the current processes' GDT
state, which has since been destroyed. If you care to consult the
canonical source of
TLS info, you can find out exactly why this is so hard to figure out
generically. However, with some work, we can start to figure out the
value by hand.
A coredump is really a ELF file full of just two things: a bunch of
LOAD segments that are just dumps of the process memory
regions, and a
NOTE section that includes a bunch of notes
that the kernel dumps out for us such as the current register state,
the process id, the signal that killed us, etc. Here's an example of
a core file under
readelf
$ readelf --headers ./core
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
...
OS/ABI: UNIX - System V
...
Type: CORE (Core file)
...
There are no sections in this file.
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
NOTE 0x0003d4 0x00000000 0x00000000 0x006b4 0x00000 0
LOAD 0x001000 0x08048000 0x00000000 0x00000 0x01000 R E 0x1000
LOAD 0x001000 0x08049000 0x00000000 0x01000 0x01000 RW 0x1000
LOAD 0x002000 0x0804a000 0x00000000 0x21000 0x21000 RW 0x1000
LOAD 0x023000 0x46015000 0x00000000 0x00000 0x1b000 R E 0x1000
LOAD 0x023000 0x46030000 0x00000000 0x01000 0x01000 R 0x1000
LOAD 0x024000 0x46031000 0x00000000 0x01000 0x01000 RW 0x1000
...
You can examine the notes; with
readelf we see:
$ readelf --notes ./core
Notes at offset 0x000003d4 with length 0x000006b4:
Owner Data size Description
CORE 0x00000090 NT_PRSTATUS (prstatus structure)
CORE 0x0000007c NT_PRPSINFO (prpsinfo structure)
CORE 0x000000a0 NT_AUXV (auxiliary vector)
LINUX 0x00000030 Unknown note type: (0x00000200)
CORE 0x00000090 NT_PRSTATUS (prstatus structure)
LINUX 0x00000030 Unknown note type: (0x00000200)
CORE 0x00000090 NT_PRSTATUS (prstatus structure)
LINUX 0x00000030 Unknown note type: (0x00000200)
CORE 0x00000090 NT_PRSTATUS (prstatus structure)
LINUX 0x00000030 Unknown note type: (0x00000200)
CORE 0x00000090 NT_PRSTATUS (prstatus structure)
LINUX 0x00000030 Unknown note type: (0x00000200)
CORE 0x00000090 NT_PRSTATUS (prstatus structure)
LINUX 0x00000030 Unknown note type: (0x00000200)
This shows 5 notes of type
NT_PRSTATUS; it should be
little surprise that each of these notes describes the process status
of one running thread. When gdb pops up
[New Thread 4973]
that's because it just hit another note that describes the new
thread.
To actually make sense of the note, however, we need some other
tools that look deeper. The elfutils based tools give us a better
description of the various notes in the coredump; more akin to what
GDB is interpreting them as. Below I've extracted one thread's
info:
$ eu-readelf --notes core
...
CORE 144 PRSTATUS
info.si_signo: 6, info.si_code: 0, info.si_errno: 0, cursig: 6
sigpend: <>
sighold: <>
pid: 4970, ppid: 4960, pgrp: 4970, sid: 4960
utime: 0.000000, stime: 0.000000, cutime: 0.000000, cstime: 0.000000
orig_eax: 270, fpvalid: 0
ebx: 4970 ecx: 4970 edx: 6
esi: 0 edi: 1176018932 ebp: 0xbfa5b710
eax: 0 eip: 0xffffe424 eflags: 0x00000206
esp: 0xbfa5b6f8
ds: 0x007b es: 0x007b fs: 0x0000 gs: 0x0033 cs: 0x0073 ss: 0x007b
LINUX 48 386_TLS
index: 6, base: 0xb6043b70, limit: 0x000fffff, flags: 0x00000051
index: 7, base: 0x00000000, limit: 0x00000000, flags: 0x00000028
index: 8, base: 0x00000000, limit: 0x00000000, flags: 0x00000028
What's particularly interesting here is that the note type that was
previously unknown (
Unknown note type: (0x00000200)) has been
resolved for us it is in fact of type
NT_386_TLS and
is a dump of the GDT entries for the thread.
So, if we examine how our function is accessing our TLS variable by
disassembling the
thread function:
(gdb) disassemble thread
Dump of assembler code for function thread:
0x08048514 <+0>: push %ebp
0x08048515 <+1>: mov %esp,%ebp
0x08048517 <+3>: sub $0x18,%esp
0x0804851a <+6>: mov 0x8(%ebp),%eax
0x0804851d <+9>: mov %eax,%gs:0xfffffffc
0x08048523 <+15>: mov %gs:0xfffffffc,%edx
0x0804852a <+22>: mov $0x8048670,%eax
0x0804852f <+27>: mov %edx,0x4(%esp)
0x08048533 <+31>: mov %eax,(%esp)
0x08048536 <+34>: call 0x80483f4 <printf@plt>
0x0804853b <+39>: movl $0xa,(%esp)
0x08048542 <+46>: call 0x8048404 <sleep@plt>
0x08048547 <+51>: jmp 0x804853b <thread+39>
Examining the disassembly we can see that
foo is accessed
via an offset of -4 from
%gs (this is OK, as our limit value
is maxed out. See the TLS ABI doc for more info). Now, we can
examine
gs and see which selector it is telling us to
use:
(gdb) print $gs >> 3
$31 = 6
Above we shift out the last 3 bits, as these refer to the privilege
level (bits 0 and 1) and if this is a GDT or LDT reference (bit 2).
Thus, looking at the GDT descriptor for index 6:
LINUX 48 386_TLS
index: 6, base: 0xb6043b70, limit: 0x000fffff, flags: 0x00000051
we can finally do some maths from the base-address to figure out
the value:
(gdb) print *(int*)(0xb6043b70 - 4)
$34 = 3
Thus we have found our TLS value; in this case for thread 3 the
value is indeed 3. I caution this is the simplest possible case;
other "models" (see the TLS doc, again) may not be so simple to work
out by hand, but this would certainly be how you would start.
I've been playing with C++ lately, and it's an interesting language. Some random thoughts go here. First, in which situation does "
Last year, I had all the intention in the world to go to Bosnia and Herzegovina this summer to attend 
Umh, so Google created yet-another-beta-service with yet-another-invite-only stage. Whee! And this time they choose a name yet more stupid to their previous attempts (Wave, Buzz now it's simply Google+).
And yes, some of my friends have mailed me invites (or offered them to me by chat) so I can join the fun and love, and lose the few free time I still have.
Guys, I care about you. I even love you all! But no, I don't care the least for so-called-social-network invites. I am quite a mess managing my time as it stands.
And I cannot reply this to you in person, as Google+ sends mails with a very helpful sender (noreply-929ac25c@plus.google.com or variations of it), I have just added this to my .procmailrc:
As I mentioned in my previous blog post, OMAP4 netboot images were available, but non-functional. I'm pleased to announce that these bugs have now been resolved and it is possible to have a functional install on OMAP4. This also has the added advantage of allowing one special partitioning layouts such as RAID, LVM, or simply having a non-SD based root device. The images are available here: http://ports.ubuntu.com/ubuntu-ports/dists/oneiric/main/installer-armel/current/images/omap4/netboot/
My 
Year ago, when 
Following a series of pre-releases, 
The thing that finally made me install Chromium? The refusal of Google+ to work with Firefox 3.5 (from Debian/testing). Well played Google. Well played.
I ve taken the plunge, and converted my blog from a venerable but largely
unmaintain(ed able) blosxom setup that I ve had since 2005 (OMG!) into a new
and shiny Jekyll-backed system. The main benefits are that I m more
comfortable hacking away at Jekyll (being written in Ruby), and my blog post
submission no longer requires me to e-mail my blog posts to myself to put
them up (that was just an artifact of how I set it up all those years ago,
but it had worked so I wasn t willing to change it just for the sake of it).
Jekyll s superior (for me) hackability has reaped dividends already I ve
put together a home-grown comments system, which fulfils my primary
requirements of being as close to completely statically-generated as it s
possible to be (about 10 lines of PHP is the only dynamic content), and
doesn t involve JavaScript or putting my blog comments in the cloud .
Tagging words.
I tried calculating TF/IDF for tokyodebian monthly report.
I found that mecab isn't very suited for this task.
Yeah! It's
A while ago I registered with 
Last week, the 
Let data be textarea->value and browser be (firefox or chrome).I want the user to be able to sign the data he entered in the textarea as I do not trust the website to store the data without modification.So far I found a couple of GnuPG/PGP based solutions:
Unfortunately Debian cannot sponsor my trip to Banja Luka this year and it s too expensive to pay it from my own budget. Thus I just cancelled my attendance for the conference. I m very sad, since my first DebConf last year in NY was a lot of fun and I was really looking forward to see you guys again this year.
I wish you a lot of fun anyways. Have a productive and fun conference!