Re: [Tinycc-devel] Overflows in the dynamic linker on x86_64 when using a tcc-built shared library along with others

[Michael Matz]

Hi,

On Thu, 28 Jun 2018, Petr Skocik wrote:

> Symbol `__dso_handle' causes overflow in R_X86_64_PC32 relocation
> Symbol `fstatat' causes overflow in R_X86_64_PC32 relocation
> Symbol `fstat' causes overflow in R_X86_64_PC32 relocation
> Symbol `mknod' causes overflow in R_X86_64_PC32 relocation
> Symbol `pthread_atfork' causes overflow in R_X86_64_PC32 relocation

Indeed.  TCC doesn't link with crtbegin.o (or crtbeginS.o) coming from 
GCC, and hence doesn't provide the hack symbol __dso_handle (which 
pthread_atfork in glibc needs).  This is another case of an 
interdependency between glibc and compiler specifics.

> I tried to make an MCVE (minimal, complete, verifiable example) and this one
> should do it:

Thanks.  There's a work around for the pthread_atfork problem: add this 
definition to one source file for each shared library/executable that 
contains a call to pthread_atfork:

  void * __dso_handle __attribute((visibility("hidden"))) = &__dso_handle;

e.g. in your testcase, add it to liblib.c .  Eventually we probably want 
to create this symbol automatically from TCC on glibc systems, until then 
you can use the above.

That fstat/fstatat/mknod have overflows as well might have multiple 
reasons: on glibc systems all three functions have something in common: 
they are defined in the libc_nonshared.a archive, which normally should 
have been included via libc.so (a linker script, not a normal softlink to 
the real shared lib).  You can check with -vvv which should list that 
library to be included.  No matter what I can't reproduce that problem by 
e.g. adding a call to fstatat into your reproducer for the dso_handle 
problem, so if you can provide one for e.g. fstatat as well...

> If this is a bad place to post such bug reports, please let me know. 
> (Should  I post bug reports on
> https://savannah.nongnu.org/bugs/?group=tinycc or elsewhere instead?) 

Many people here don't have a bugzilla account there and so 
don't monitor it; it seems better to write here.


Ciao,
Michael.