Re: CVS commit: src/lib/librumphijack

["Valeriy E. Ushakov" <uwe%stderr.spb.ru@localhost>]

On Fri, Feb 25, 2011 at 16:01:42 +0000, Antti Kantee wrote:

> Module Name:  src
> Committed By: pooka
> Date:         Fri Feb 25 16:01:42 UTC 2011
> 
> Modified Files:
>       src/lib/librumphijack: Makefile hijackdlsym.c
> 
> Log Message:
> Ok, for reasons I can't begin to understand, the binaries I tested
> yesterday on powerpc broke overnight.  Apparently adding one more
> function before the call to dlsym() fixes things again.  I hope
> I don't have to add another one tomorrow ....
> 
> 
> To generate a diff of this commit:
> cvs rdiff -u -r1.7 -r1.8 src/lib/librumphijack/Makefile
> cvs rdiff -u -r1.1 -r1.2 src/lib/librumphijack/hijackdlsym.c

I think this is caused by revision 1.121 of rtld.c (hi, mac!) that
added "hackish_return_address" for ppc.

#ifdef __powerpc__
static void *
hackish_return_address(void)
{
        return __builtin_return_address(1);
}
#endif

void *
dlsym(void *handle, const char *name)
{
        ...
#ifdef __powerpc__
                retaddr = hackish_return_address();
#else
                retaddr = __builtin_return_address(0);
#endif
        ...
}


hackish_return_address will be inlined (simple static function) and,
as far as I can tell, gcc does NOT adjust the "level" argument to
__builtin_return_address.

The net effect is that dlsym uses caller's caller address to detect
which module the call comes from, and if caller's caller is in a
different module wrong things happen.

That explains why you need an extra frame.

-uwe