LSDA unwind information is off by one (in __gcc_personality_v0)

[Florian Weimer <fweimer at redhat dot com>]

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.

I wonder how GCC itself copes with the decrement in 
__gcc_personality_v0.  This probably matters to Ada the most because it 
supports asynchronous exceptions by default.

with Ada.Strings.Unbounded; use Ada.Strings.Unbounded;
with System.Machine_Code; use System.Machine_Code;

procedure Proc is
begin
   Asm ("# BEFORE", Volatile => True);
   declare
      B : Unbounded_String;
   begin
      Asm ("# WITHIN", Volatile => True);
   end;
   Asm ("# AFTER", Volatile => True);
end Proc;

Looking at the assembly, there does not seem to be any adjustment for 
the off-by-one, but the asynchronous exception handler can be safely 
executed multiple times (it has an internal guard flag), so maybe this 
never matters.  This off-by-one only matters for the exit from exception 
handling regions because at the entry, we always get a PC value which is 
at least one greater than the initial label because on i386 and x86_64, 
the PC coming in from the kernel points after the last executed instruction.

I guess my question is if this is all working as designed.

If the decrement is ever removed from the libgcc unwinder, we should add 
a compensating NOP in the glibc code.  Otherwise, we can just adjust the 
unwind tables.

Florian