[RFA/alpha-osf] another next frame confusion in signal unwinder?

Joel Brobecker
2004-12-01T23:51:48Z

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Hello,

Using the following C program:

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

void blow_up (int x)
{
*((int*) (x|1)) = 3;
}

void handler (int sig) {
blow_up (sig);
printf ("This is a test.\n");
}

main ()
{
signal (SIGTERM, handler);

kill (getpid (), SIGTERM);
}

Compiled on Tru64 5.1a with GCC:

% gcc -g -o cause_signal cause_signal.c

The following transcript reveals that GDB is unable to unwind
the stack past the signal handler frame:

(gdb) run
Starting program: /usr/prague.a/brobecke/53-63/ex/cause_signal

Program received signal SIGTERM, Terminated.
0x000003ff800e8a78 in kill () from /usr/shlib/libc.so
(gdb) cont
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x0000000120001248 in blow_up (x=15) at cause_signal.c:7
7 *((int*) (x|1)) = 3;
(gdb) bt
#0 0x0000000120001248 in blow_up (x=15) at cause_signal.c:7
#1 0x0000000120001290 in handler (sig=15) at cause_signal.c:12
#2 <signal handler called>
#3 0xa79db5f0b43c0000 in ?? ()
warning: Hit heuristic-fence-post without finding
warning: enclosing function for address 0xa79db5f0b43c0000
This warning occurs if you are debugging a function without any symbols
(for example, in a stripped executable). In that case, you may wish to
increase the size of the search with the `set heuristic-fence-post'
+command.

Otherwise, you told GDB there was a function where there isn't one, or
(more likely) you have encountered a bug in GDB.
#4 0x2c9000006bfa8001 in ?? ()
warning: Hit heuristic-fence-post without finding
warning: enclosing function for address 0x2c9000006bfa8001
warning: Previous frame identical to this frame (corrupt stack?)

The expected output for the "bt" command is:

(gdb) bt
#0 0x0000000120001248 in blow_up (x=15) at cause_signal.c:7
#1 0x0000000120001290 in handler (sig=15) at cause_signal.c:12
#2 <signal handler called>
#3 0x000003ff800e8a78 in kill () from /usr/shlib/libc.so
#4 0x0000000120001318 in main () at cause_signal.c:20

When trying to compute the frame previous to the signal handler one,
GDB computes the frame cache, and as a consequence ends up calling

static struct alpha_sigtramp_unwind_cache *
alpha_sigtramp_frame_unwind_cache (struct frame_info *next_frame,
void **this_prologue_cache)
{
[...]
info->sigcontext_addr = tdep->sigcontext_addr (next_frame);

return info;
}

This results in a call to:

static CORE_ADDR
alpha_osf1_sigcontext_addr (struct frame_info *frame)
{
struct frame_info *next_frame = get_next_frame (frame);

if (next_frame != NULL)
return (read_memory_integer (get_frame_base (next_frame), 8));
else
return (read_memory_integer (get_frame_base (frame), 8));
}

It looks to me that we already have the next frame, so computing
the next frame of that frame in alpha_osf1_sigcontext_addr() is
making us using the wrong frame when getting the base address for
the signal context area. So I changed this function to:

static CORE_ADDR
alpha_osf1_sigcontext_addr (struct frame_info *next_frame)
{
return (read_memory_integer (get_frame_base (next_frame), 8));
}

This fixes the problem above, and I obtain the expected callstack.

2004-12-01 Joel Brobecker <brobecker@gnat.com>

* alpha-osf1-tdep.c (alpha_osf1_sigcontext_addr): Change
parameter name to make it clear that we already have a next
frame. Return the sigcontext from that next frame instead
of the frame following it.
Fixes [DB30-017].

Tested on alpha-tru64 5.1a. No regression.

OK to apply?
--
Joel

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Index: alpha-osf1-tdep.c
===================================================================
RCS file: /nile.c/cvs/Dev/gdb/gdb-6.3/gdb/alpha-osf1-tdep.c,v
retrieving revision 1.3
diff -u -p -r1.3 alpha-osf1-tdep.c
--- alpha-osf1-tdep.c 21 Oct 2004 00:08:34 -0000 1.3
+++ alpha-osf1-tdep.c 1 Dec 2004 17:54:46 -0000
@@ -35,14 +35,9 @@ alpha_osf1_pc_in_sigtramp (CORE_ADDR pc,
}

static CORE_ADDR
-alpha_osf1_sigcontext_addr (struct frame_info *frame)
+alpha_osf1_sigcontext_addr (struct frame_info *next_frame)
{
- struct frame_info *next_frame = get_next_frame (frame);
-
- if (next_frame != NULL)
- return (read_memory_integer (get_frame_base (next_frame), 8));
- else
- return (read_memory_integer (get_frame_base (frame), 8));
+ return (read_memory_integer (get_frame_base (next_frame), 8));
}

static void