Segfault in re_string_reconstruct()

[Bernhard Reutner-Fischer]

On Thu, Oct 30, 2008 at 10:06:45AM +1300, Jeremy Bowen wrote:
>On Monday 20 October 2008 5:54:14 pm Rob Landley wrote:
>
>> You need to figure out what actual line is segfaulting.  I'd stick
>> dprintf(2,"here\n") calls into the busybox source to track down exactly
>> where the segfault happens.  (You can of course use gdb or some such, if
>> you're feeling posh. :)
>>
>> If you can figure out what line is causing the segfault, and what the
>> values of the variables used on that line are, it'd help a lot.  Right now
>> we don't know if it's a busybox problem or a uClibc problem.
>
>OK after a lot of effort re-validating my toolchain, I'm now certain 
>the problem occurs within uClibc and not busybox.
>
>Specifically ./libc/misc/regex/regex_internal.c on line 703/4 >> <<
>==============================================
>#endif /* RE_ENABLE_I18N */
>	    {
>	      int c = pstr->raw_mbs[pstr->raw_mbs_idx + offset - 1];
>	      if (pstr->trans)
>        	c = pstr->trans[c];
>==============================================
>
>However I'm now very nervous about my C compiler....
>
>I don't believe uClibc is actually broken because changing the above code to 
>
>	      if (pstr->trans)
>+	      {
>+		dprintf(2, "Hello");
>        	c = pstr->trans[c];
>+	      }
>
>eliminates the segfault behaviour even though "Hello" is never printed.
>
>The fact that the expression evaluates to false and the code continues on OK 
>makes me believe there is a problem with compiler optimisation or something 
>else, which I'm at a loss to explain at the moment.
>
>The code looks perfectly legal and simply bracketing the statement in the 'if' 
>doesn't help (but adding dprintf does). 
>
>It has to be a compiler bug right ??

Sounds a bit like it is.
sprinkle some __asm__("# 1"); round the spot to see what the compiler
generates there. Build this file with -O0 (either via
__attribute__((optimize(0))) or by setting e.g. CFLAGS-regex_internal.os)
if that works then reduce the spot that is miscompiled as described
somewhere in http://gcc.gnu.org/bugs.html and report it in bugzilla.

HTH,