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Re: LSDA unwind information is off by one (in __gcc_personality_v0)


On Okt 20 2016, Florian Weimer <fweimer@redhat.com> wrote:

> We have a fun little glibc bug which causes pthread_cond_wait to write out
> of bounds on i386:
>
>   https://sourceware.org/bugzilla/show_bug.cgi?id=20719
>
> Root cause is this in libgcc/unwind-c.c:
>
>     130   int ip_before_insn = 0;
>     …
>     158   /* Parse the LSDA header.  */
>     159   p = parse_lsda_header (context, language_specific_data, &info);
>     160 #ifdef HAVE_GETIPINFO
>     161   ip = _Unwind_GetIPInfo (context, &ip_before_insn);
>     162 #else
>     163   ip = _Unwind_GetIP (context);
>     164 #endif
>     165   if (! ip_before_insn)
>     166     --ip;
>     167   landing_pad = 0;
>
> The PC decrement cannot be possibly meaningful on CISC architectures with
> variable instruction lengths.  I suspect it's an unintentional leftover
> from the ia64 port.

It is valid for all architectures, because the return IP usually points
_after_ the call instruction, which may already be inside the next
unwind region.

The situation is different for signal frame.  See
libgcc/unwind-dw2.c:_Unwind_GetIPInfo for the correct way to handle it.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."


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