[lsb-discuss] Checking Firefox for LSB-compliance

Robert Schweikert robert.schweikert at abaqus.com
Fri Apr 20 17:51:23 PDT 2007


Touche, was testing the 32 bit firefox binary with the 64 bit appchk. We 
probably should safeguard against that crash with a check.

I'll get a consistent check going next week.


Jeff Licquia wrote:
> On Fri, 2007-04-20 at 09:41 -0400, Robert Schweikert wrote:
>> I have reproduced the problem on x86_64 with the firefox-bin recently 
>> downloaded from mozilla for version The good news is appcheck 
>> still dumps, the bad new is it dumps in a different place as compared to 
>> the stack trace posted by Jeff.
>> However, the dumps may be related.
> Fun.
>> The rot cause for the dump is a bad address calculation hdr.c on line 
>> 28.  Here we add the address stored in the ElfFile struct  in the addr  
>> member (file1->addr) and the address we find in the Elf64_Ehdr struct 
>> for the shared offset (e_shoff) (hdr1->e_shoff).  While the earlier 
>> calculation for paddr worked (3 lines previous) the shared off set 
>> calculation fails. THerefore we are setting the saddr member in the 
>> ElfFile struct to garbage. Once this is accessed things of course blow up.
>> I don't know enough about ELF to keep debugging this effectively on my own.
> Here's the offending code:
> if( hdr1->e_shoff ) {
> 	file1->saddr=(Elf_Shdr *)((caddr_t)file1->addr+hdr1->e_shoff);
> 	file1->numsh = hdr1->e_shnum;
> 	}
> "file1" is the ElfFile * in question.  It has an "addr" member, which is
> set to point to an mmap() of the ELF file header.  "hdr1" is actually
> set to file1->addr, typecase to Elf_Phdr *.
> So if e_shoff is coming through incorrectly, that could mean that appchk
> is misreading the ELF header from the Mozilla executable.
> One possible reason for that is if we're checking an ia32 executable
> with an x86_64 appchk, or vice versa.  What architecture did the Mozilla
> binary represent itself as?
>> The stack trace posted by Jeff points to a "garbage pointer" culprit. 
>> The call that actually causes the dump in (I presume IA32) the original 
>> post is the strdup() call. strdup of course allocates and frees memory. 
>> Looks to me we are passing garbage to strdup(). This garbage could be 
>> triggered by a similar miscalculation of the saddr, by luck we don't 
>> blow up on the other platform as early as we do on x86_64 (my best guess).
> If we're passing garbage to strdup() in my trace, then I can't figure
> out where the garbage is.  Here's the full function being called:
> void output_purpose_start(unsigned int activity, unsigned int tpnumber,
>                           const char *message)
> {
>     current_tpnum = tpnumber;
>     if ((message != NULL) && (strlen(message) > 0))
>         current_purpose = strdup(message);
>     else
>         current_purpose = NULL;
> }
> current_purpose and current_tpnum are globals.  The "message" parameter
> is reported to be OK by gdb (the backtrace output lists its contents as
> "XSetInputFocus").
> On the other hand, here's the code which calls that function:
>     symbol_name = ElfGetStringIndex(file, syms1[i].st_name,
>                                     file->dynsymhdr->sh_link);
>     PURPOSE_START(tetj_activity_count, tetj_tp_count, symbol_name);
> (PURPOSE_START is a macro which, in this case, evaluates to the above
> function.)
> So, yes, the ultimate source for that string is an ElfFile *.  Which,
> given Robert's evaluation, is very suspect.  I wonder if we might be
> using the memory after freeing it, or something.
> Incidentally, if the program is run under valgrind, it does not throw a
> SIGABRT.  Ditto for MALLOC_CHECK_ (even at 2, which should immediately
> abort at these kinds of crashes).  As far as I can tell, building with
> mtrace() and running with MALLOC_TRACE on does not seem to make a
> difference, although it doesn't seem to tell us anything interesting
> when it does crash.
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Robert Schweikert                          MAY THE SOURCE BE WITH YOU
(Robert.Schweikert at abaqus.com)                        LINUX
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