Re: Finding the source code for ___tls_get_addr_internal()

[Saul Tamari <stamari at gmail dot com>]

Hi,

I'm using a GNU/Linux on an Intel 32bit x86.

I think that I might be digging too deep. There shouldn't be any
problem with the gs segment or whatever it points to.

The application is a C program that dynamically loads (using dlopen())
C++ libraries.
Do I need to set -ftls-model=global-dynamic when compiling the various
C++ libraries so TLS works correctly?


(gdb) disas __cxa_get_globals
Dump of assembler code for function __cxa_get_globals:
0xf6f536d0 <__cxa_get_globals+0>:       push   %ebp
0xf6f536d1 <__cxa_get_globals+1>:       mov    %esp,%ebp
0xf6f536d3 <__cxa_get_globals+3>:       push   %ebx
0xf6f536d4 <__cxa_get_globals+4>:       call   0xf6ee8f07 <pthread_join+247>
0xf6f536d9 <__cxa_get_globals+9>:       add    $0x3591b,%ebx
0xf6f536df <__cxa_get_globals+15>:      lea    -0x56c(,%ebx,1),%eax
0xf6f536e6 <__cxa_get_globals+22>:      call   0xf6ee7730 <___tls_get_addr@plt>
0xf6f536eb <__cxa_get_globals+27>:      pop    %ebx
0xf6f536ec <__cxa_get_globals+28>:      pop    %ebp
0xf6f536ed <__cxa_get_globals+29>:      ret

Now in order to get eax's value before the call to ___tls_get_attr() I do:
0xf6f536d9 + 0x3591b = 0xf6f88ff4
0xf6f88ff4 - 0x56c = 0xf6f88a88

Can this value help me locate which tls value is accessed here? Can I
find it with readelf or objdump?


Thanks,
Saul



On Wed, Jan 16, 2013 at 5:59 PM, Ian Lance Taylor <iant@google.com> wrote:
> On Wed, Jan 16, 2013 at 12:08 PM, Saul Tamari <stamari@gmail.com> wrote:
>>
>> Is ___tls_get_addr_internal() compiled from C source code or is it
>> generated directly in assembly code?
>> Where can I find the C source code that was used to generate
>> ___tls_get_addr_internal ?
>
> __tls_get_addr is provided by your C library.  I don't think you
> mentioned what system you are using, but if it is a GNU/Linux system
> then __tls_get_addr defined by glibc.  I'm not sure quite where
> __tls_get_addr_internal comes in.  __tls_get_addr is defined in
> elf/dl-tls.c in the glibc sources.  You seem to be using 32-bit x86;
> the interesting 32-bit x86 code is in nptl/sysdeps/i386/tls.h.
>
> It does indeed use %gs.  You can't change %gs yourself if your program
> uses TLS variables.
>
> Ian