Fix incorrect (or infinite loop) unwind on RHEL7 x86 32 bits.

On RHEL7 x86 32 bits, Valgrind unwinder cannot properly unwind
the stack just after a thread creation : the unwinder always retrieves
the same pc/sp/bp.
See below for an example.
This has as consequences that some stack traces are bigger than
needed (i.e. they always fill up the ips array). If
--merge-recursive-frames is given, then the unwinder enters in an
infinite loop (as identical frames will be merged, and the ips array
will never be filled in).
Thi patch adds an additional exit condition : after unwinding
a frame, if the previous sp is >= new sp, then unwinding stops.
Patch has been tested on debian 8/x86, RHEL7/x86.



   0x0417db67 <+55>:    mov    0x18(%esp),%ebx
   0x0417db6b <+59>:    mov    0x28(%esp),%edi
   0x0417db6f <+63>:    mov    $0x78,%eax
   0x0417db74 <+68>:    mov    %ebx,(%ecx)
   0x0417db76 <+70>:    int    $0x80
=> 0x0417db78 <+72>:    pop    %edi
   0x0417db79 <+73>:    pop    %esi
   0x0417db7a <+74>:    pop    %ebx
   0x0417db7b <+75>:    test   %eax,%eax

Valgrind stacktrace gives:
==21261==    at 0x417DB78: clone (clone.S:110)
==21261==    by 0x424702F: ???
==21261==    by 0x424702F: ???
==21261==    by 0x424702F: ???
==21261==    by 0x424702F: ???
==21261==    by 0x424702F: ???
==21261==    by 0x424702F: ???
==21261==    by 0x424702F: ???
...
(till the array of ips is full)

while gdb stacktrace gives:
(gdb) bt
#0  clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:110
svn2github#1  0x00000000 in ?? ()
(gdb) p $pc
$2 = (void (*)()) 0x417db78 <clone+72>
(gdb)


With the fix, valgrind gives:
==21261==    at 0x417DB78: clone (clone.S:110)
==21261==    by 0x424702F: ???
which looks more reasonable.




git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15729 a5019735-40e9-0310-863c-91ae7b9d1cf9

coregrind/m_stacktrace.c

@@ -350,6 +350,8 @@ UInt VG_(get_StackTrace_wrk) ( ThreadId tid_if_known,
           uregs.xbp <= fp_max - 1 * sizeof(UWord)/*see comment below*/ &&
           VG_IS_4_ALIGNED(uregs.xbp))
       {
+         Addr old_xsp;
+
          /* fp looks sane, so use it. */
          uregs.xip = (((UWord*)uregs.xbp)[1]);
          // We stop if we hit a zero (the traditional end-of-stack
@@ -382,6 +384,7 @@ UInt VG_(get_StackTrace_wrk) ( ThreadId tid_if_known,
             }
          }
 
+         old_xsp = uregs.xsp;
          uregs.xsp = uregs.xbp + sizeof(Addr) /*saved %ebp*/ 
                                + sizeof(Addr) /*ra*/;
          uregs.xbp = (((UWord*)uregs.xbp)[0]);
@@ -393,6 +396,12 @@ UInt VG_(get_StackTrace_wrk) ( ThreadId tid_if_known,
                if (debug) VG_(printf)("     cache FPUNWIND >2\n");
                if (debug) unwind_case = "FO";
                if (do_stats) stats.FO++;
+               if (old_xsp >= uregs.xsp) {
+                  if (debug)
+                    VG_(printf) ("     FO end of stack old_xsp %p >= xsp %p\n",
+                                 (void*)old_xsp, (void*)uregs.xsp);
+                  break;
+               }
             } else {
                fp_CF_verif_cache [hash] = xip_verified ^ CFUNWIND;
                if (debug) VG_(printf)("     cache CFUNWIND >2\n");
@@ -406,6 +415,12 @@ UInt VG_(get_StackTrace_wrk) ( ThreadId tid_if_known,
          } else {
             if (debug) unwind_case = "FF";
             if (do_stats) stats.FF++;
+            if (old_xsp >= uregs.xsp) {
+               if (debug)
+                  VG_(printf) ("     FF end of stack old_xsp %p >= xsp %p\n",
+                               (void*)old_xsp, (void*)uregs.xsp);
+               break;
+            }
          }
          goto unwind_done;
       } else {