Problems with _Unwind_Backtrace() vs signal handlers

[Bryce McKinlay <mckinlay at redhat dot com>]

libjava wants to use _Unwind_Backtrace() and friends to print diagnostic 
stack traces for exceptions (as well as examine the stack for security 
checks, etc). There appears to be a problem when trying to 
_Unwind_Backtrace through signal handler frames.

For a normal call, the IP returned by _Unwind_GetIP() is the return 
address for the frame above it - that is, the instruction following the 
"call" instruction. When unwinding through a signal handler, however, 
_Unwind_GetIP() returns a pointer to the faulting instruction. This 
inconsistency makes it difficult for libjava to print the correct source 
line number for NullPointerExceptions, and I suspect it also has 
something to do with the next problem:

The test case below is a simplification of what libjava is doing when it 
tries to print a stack trace for a NullPointerException. When compiled 
with no optimization, it works ok, but when compiled with -O2, the 
_Unwind_Backtrace() either misses/skips over some frames, or terminates 
prematurely. I tried this test on i686, x86-64, and ppc-32, and it seems 
more or less consistently broken on each of those platforms.

Note that the unwinder seems to work fine through the signal frame to 
find an exception handler, it is just _Unwind_Backtrace() that fails.

Bryce

/* Compile with -fnon-call-exceptions */

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>

#include <unwind.h>

extern void segv_handler(int sig);
extern int foo() __attribute__ ((noinline));
extern int bar(char *z) __attribute__ ((noinline));

int main(int argc, char *argv)
{
  /* Set up SEGV handler.  */
  struct sigaction action;
  action.sa_handler = segv_handler;
  action.sa_flags = 0;
  sigaction (SIGSEGV, &action, NULL);
  
  /* Cause a segfault. */
  printf ("%i\n", foo());
}

int bar(char *ptr)
{
  return ptr[10];  /* segfault */
}

int foo()
{
  return bar(NULL);
}

_Unwind_Reason_Code
unwind_trace_fn (struct _Unwind_Context *context, void *p)
{
  _Unwind_Ptr ip = _Unwind_GetIP (context);
  void *fn = _Unwind_FindEnclosingFunction((void *) ip);

  if (fn == main)
    printf ("main()\n");
  if (fn == foo)
    printf ("foo()\n");  
  if (fn == bar)
    printf ("bar()\n");
  return _URC_NO_REASON;
}

void segv_handler(int sig)
{
  _Unwind_Backtrace (unwind_trace_fn, NULL);
  exit(1);
}