When building a user space binary, the
-lc that gcc
inserts into the final link seems pretty straight forward link
the C library. As with all things system-library related there is
more to investigate.
If you look at
/usr/lib/libc.so; the "library" that gets
linked when you specify
-lc, it is not a library as such, but
a link script which specifies the libraries to link, which also
includes the dynamic linker itself:
$ cat /usr/lib/libc.so
/* GNU ld script
Use the shared library, but some functions are only in
the static library, so try that secondarily. */
OUTPUT_FORMAT(elf64-x86-64)
GROUP ( /lib/libc.so.6 /usr/lib/libc_nonshared.a AS_NEEDED ( /lib/ld-linux-x86-64.so.2 ) )
On
very
old glibc's the
AS_NEEDED didn't appear; so every binary
really did have a
DT_NEEDED entry for the dynamic linker
itself. This can be somewhat confusing if you're ever doing forensics
on an old binary which seems to have these entries for not apparent
reason. However, we can see that that
/lib/libc.so itself
does actually require symbols from the dynamic linker:
$ readelf --dynamic /lib/libc.so.6 grep NEEDED
0x0000000000000001 (NEEDED) Shared library: [ld-linux-x86-64.so.2]
Although you're unlikely to encounter it, a broken toolchain can
cause havoc if it gets the link order wrong here, because the glibc
dynamic linker defines
minimal
versions of
malloc and friends -- you've got a
chicken-and-egg problem using libc's
malloc before you've
loaded it! You can simulate this havoc with something like:
$ cat foo.c
#include <stdio.h>
#include <syslog.h>
int main(void)
syslog(LOG_DEBUG, "hello, world!");
return 0;
$ cat libbroken.so
GROUP ( /lib/ld-linux-x86-64.so.2 )
$ gcc -o -Wall -Wl,-rpath=. -L. -lbroken -g -o foo foo.c
$ ./foo
Inconsistency detected by ld.so: dl-minimal.c: 138: realloc: Assertion ptr == alloc_last_block' failed!
Depending on various versions of things, you might see that assert
or possibly just strange, corrupt output in your logs as
syslog calls the wrong
malloc. You could debug
something like this by asking the dynamic linker to show you its
bindings as it resolves them:
$ LD_DEBUG_OUTPUT=foo.txt LD_DEBUG=bindings ./foo
Inconsistency detected by ld.so: dl-minimal.c: 138: realloc: Assertion ptr == alloc_last_block' failed!
$ cat foo.txt.11360 grep "\ malloc'"
11360: binding file /lib/libc.so.6 [0] to /lib64/ld-linux-x86-64.so.2 [0]: normal symbol malloc' [GLIBC_2.2.5]
11360: binding file /lib64/ld-linux-x86-64.so.2 [0] to /lib64/ld-linux-x86-64.so.2 [0]: normal symbol malloc' [GLIBC_2.2.5]
11360: binding file /lib/libc.so.6 [0] to /lib64/ld-linux-x86-64.so.2 [0]: normal symbol malloc' [GLIBC_2.2.5]
Above, because the dynamic loader comes first in the link order,
libc.so.6's
malloc has bound to the minimal
implementation it provides, rather the full-featured one it provides
internally.
As an aside, AFAICT, there is really only one reason why a normal
library will link against the dynamic loader -- for the thread-local
storage support function
__tls_get_addr. You can try this
yourself:
$ cat tls.c
char __thread *foo;
char* moo(void)
return foo;
$ gcc -fPIC -o libtls.so -shared tls.c
$ readelf -d ./libtls.so grep NEED
0x00000001 (NEEDED) Shared library: [libc.so.6]
0x00000001 (NEEDED) Shared library: [ld-linux.so.2]
0x6ffffffe (VERNEED) 0x314
0x6fffffff (VERNEEDNUM) 2
Thread-local storage is worthy of a
book of its own,
but the gist is that this support function says "hey, give me an
address of
foo in
libtls.so", the magic being that
if the current thread has never accessed
foo then it may not
actually have any storage for
foo yet, so the dynamic linker
can allocate some memory for it lazily and then return the right
thing. Otherwise, every thread that started would need to reserve
room for
foo "just in case", even if it never cares about
moo.
But looking a little closer at the symbols of
libc.so is
also interesting.
libc.so doesn't actually have many
functions you can override. You can see what it is possible to
override by checking the relocations against the procdure-lookup table
(PLT).
Relocation section '.rela.plt' at offset 0x1e770 contains 8 entries:
Offset Info Type Sym. Value Sym. Name + Addend
00000035b000 084600000007 R_X86_64_JUMP_SLO 00000000000a2100 sysconf + 0
00000035b008 02e000000007 R_X86_64_JUMP_SLO 0000000000075e50 calloc + 0
00000035b010 01dd00000007 R_X86_64_JUMP_SLO 0000000000077910 realloc + 0
00000035b018 029300000007 R_X86_64_JUMP_SLO 00000000000661c0 feof + 0
00000035b020 046f00000007 R_X86_64_JUMP_SLO 00000000000768c0 malloc + 0
00000035b028 000400000007 R_X86_64_JUMP_SLO 0000000000000000 __tls_get_addr + 0
00000035b030 01b400000007 R_X86_64_JUMP_SLO 0000000000076dd0 memalign + 0
00000035b038 086000000007 R_X86_64_JUMP_SLO 00000000000767e0 free + 0
i.e. instead of jumping directly to the
malloc defined in
the
libc code section, any internal calls will jump to this
stub which, the first time, asks the dynamic linker to go out and find
the address of
malloc (it then saves it, so the second time
the stub just jumps to the saved location).
This is an interesting list, seemingly without much order.
feof, for example, stands out as worth checking out a bit
closer why would that be there when
fopen isn't, say?
We can track down where it comes from with a bit of detective work;
knowing that the value of the symbol
feof will be placed into
0x35b018 we can disassemble
libc.so to see that this
address is used by the
feof PLT stub at
0x1e870
(luckily,
objdump has done the math to offest from the
rip for us; i.e.
0x1e876 + 0x33c7a2 = 0x35b018)
000000000001e870 <feof@plt>:
1e870: ff 25 a2 c7 33 00 jmpq *0x33c7a2(%rip) # 35b018 <_IO_file_jumps+0xb18>
1e876: 68 03 00 00 00 pushq $0x3
1e87b: e9 b0 ff ff ff jmpq 1e830 <h_errno+0x1e7dc>
From there we can search for anyone jumping to that address, and
find out the caller:
$ objdump --disassemble-all /lib/libc.so.6 grep 1e870
000000000001e870 <feof@plt>:
1e870: ff 25 a2 c7 33 00 jmpq *0x33c7a2(%rip) # 35b018 <_IO_file_jumps+0xb18>
f19f7: e8 74 ce f2 ff callq 1e870 <feof@plt>
$ addr2line -e /lib/libc.so.6 f19f7
/home/aurel32/eglibc/eglibc-2.11.2/sunrpc/bindrsvprt.c:70
Which turns out to be part of a
local
patch which probably gets things a little wrong, as described
below. The
sysconf relocation is from a similar add-on patch
(being used to find the page size, it seems).
libc, like all sensible libraries, uses the
hidden
attribute on symbols to restrict what is exported by the library.
The benefit of this is that when the linker knows you are referencing
a hidden symbol it knows that the value can never be overridden, and
thus does not need to emit extra code to do indirection just in case
you ever wish to redirect the symbols. In the above, it appears that
feof has never been marked as hidden, probably because no
internal glibc functions used it until that add-on patch, and since it
is not considered an internal function the linker must allow for the
possibility that it will be overridden at run time and provide a PLT
slot for it. There are consequences; if this was on a fast-path then
the extra jumps required to go via the PLT may matter for performance
and it may also cause strange behaviour if somebody had preloaded
something that took over
feof.
Note, this is different from saying that your library can
override symbols provided by
libc.so; such as when you
LD_PRELOAD a library to wrap an
open call. What you
can not override is the
open call that say, the internal
libc.so function
getusershell does to read
/etc/shells.
Having the
malloc related calls as preemptable seems
intentional and sane; although I can not find a comment to the effect
of "we deliberately leave these like this so that users may use
alternative malloc implementations", it makes sense so that
libc.so is internally using the same malloc as everything
else if you choose to use something such as
tc-malloc.
tl;dr? Digging into your system libraries is always interesting.
Be careful with your link order when creating toolchains, and be
careful about symbol visibility when you're working with
libraries.
Since this has been a major request from users for a long time, I can only cool with the idea of seeing the Debian project support a rolling release. However I m not pleased with the proposed ideas, since they don t actually include any serious plan to make this happen. Sorry guys, but a big GR that says We want a 
Here is my attempt at a summary of the rolling discussion currently happening on debian-devel@. It might not be complete, it s probably a bit biased, but I hope that it s still better than nothing. It was also posted on 
Branches are marked with green rectangles, and tags with yellow arrows.
What we have here (expected given our configuration of the tool) are
branches (e.g. 
Branches that were not merged using svn merge look like they were not
merged at all.
Before being in SVN, the repository was stored in CVS. When it was imported
into SVN, no special attention was given to the commit author. Hence I
got credited for changes I did not write.
The tool leaves all branches, including removed ones (with tag on their end)
so that you can decide what to do with them.
The branch 
Romain and I just posted our slides from yesterday's
A few days ago I did an 
I use 
The Linux kernel attempts to protect portions of its memory from unexpected modification (through potential future exploits) by setting areas read-only where the compiler has allowed it (
I'd like to buy most of my books as e-books, particularly novels.
I'd like buy to all of my movies and TV shows as downloads.
I'd like to buy maps for my Garmin GPS device as a download.
I can do none of these things, unless I consent to restrict my
freedoms in essential ways. For example, I can buy books, but only
if I read them on a Kindle or Nook or similar restricted device.
I can buy TV shows on iTunes, but I can't watch them on my Linux
laptop. I can buy the maps, but only if I get a Windows (or maybe
Mac) computer so I can run Garmin's proprietary software to download
the maps and put them on the device.
There are ways to circumvent these restrictions. Doing that is
illegal in various countries I would like to live in. If I'm going
to be breaking the law, the easier route would be to copy thing
illegally.
I can choose to succumb to corporate enslavement, where
they get to decide what I watch, and how. Even when
I try do this, I fail: I installed Amazon's Kindle software for
the Android, but it would still not let me buy anthing, saying
that each title is not available in my country. It said this even
for stuff long past copyright, while giving pointers to the same thing
in Project Gutenberg.
Alternatively, I can choose to reject doing things digitally,
and relying on paper for books and maps, and rental DVDs for
video stuff.
I've heard rumors that there is a lot of good stuff legally
downloadable for free. Maybe I should go find some more.
I have 
Linux 2.6.38 and 2.6.39
As you've probably seen, the testing suite finally got a new
upstream kernel version (Linux 2.6.38) and is currently synchronised
with unstable. I'm just about to upload a new version of linux-2.6
based on upstream stable update 2.6.38.4.
Linux 2.6.39 is well on its way to release, so 2.6.39-rc4 should
soon be in experimental. The Debian source package will add support
for the new armhf (ARM with hardware floating-point) architecture.
Splitting linux-2.6
The linux-base package, introduced to handle transitions and other
support functions for 'image' packages, is now built from a separate
source package. It won't be changed very often, so you will no
longer need to upgrade linux-base every time you upgrade an 'image'
package. (This doesn't apply within 'squeeze', but I may have a
way to work around that.)
The firmware-linux-free package, containing a few firmware and
configuration blobs from Linux that are DFSG-compliant, is also now
built from a separate source package. These blobs have not changed
for a very long time, so again you will no longer be recommended to
upgrade this package every time you upgrade an 'image' package.
Secondly, this allows us to remove the entire 'firmware' directory
from the upstream source rather than carefully removing and patching
to leave just the DFSG-compliant bits.
Wireless regulations
The '2.4 GHz' and '5 GHz' ISM bands used for wifi are not defined
identically around the world. The exact frequency ranges and
maximum permitted power levels vary between countries. Radio
equipment manufacturers have a legal responsibility to ensure their
equipment operates within these regulations. Since it is expensive
to implement such regulations in hardware (including many different
variants for different regions), these restrictions are usually
implemented in the driver using a country code retrieved from the
hardware's EEPROM or flash. So this is now our responsibility.
We need to be able to look up regulations by country codes, and we
also need to support the case where a device sold in one country is
used in another country with tighter regulations. For some time,
the wifi configuration library (cfg80211) included regulatory
information for a small number of countries and regions. However,
the default since Linux 2.6.34 has been to build-in only the 'world'
domain (intersection of all the restrictions) and to request
information from user-space. In Debian, we didn't have anything to
provide this information, so for example channels 12-13 became
unavailable for users in Europe.
The regulatory information should be provided by the Central
Regulatory Domain Agent (CRDA) from a current database. These are
now packaged as