Re: [RFA] epilogue unwinder for i386 (reverse 1/2)

Michael Snyder
2009-07-05T20:58:31Z

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Mark Kettenis wrote:
>> Date: Fri, 03 Jul 2009 17:34:59 -0700
>> From: Michael Snyder <msnyder@vmware.com>
>>
>> Michael Snyder wrote:
>>> This comes out of a discussion with Daniel, about how gcc
>>> does not generate the right dwarf info to allow correct
>>> frame unwinding in function epilogues, causing frame_unwind
>>> to return bad results.
>>>
>>> It's necessary for reverse-step, which will frequently step
>>> backward to the return instruction of a function. But it also
>>> provides an improvement for forward debugging, in that now,
>>> without this change, if you STEPI until you are at the return
>>> instruction, you will get a bad backtrace.
>>>
>>> The infrun changes that take advantage of this patch will follow
>>> separately.
>>>
>>> Michael
>> Oops, the patch wasn't meant to have that "#if 0" in it...
>> corrected patch below.
>
> Still has the #if 0 in there.

Sorry. ;-(
>
> I also think you should add a comment about the specific ordering of
> this unwinder. It has to come before the dwarf2 unwinder because GCC
> doesn't provide proper CFI for the epilogue, right?

Right. Since the others are similarly order-dependent, I will
expand on their comments as well.

>> + if (target_read_memory (pc, &insn, 1) != 0)
>> + return 0; /* Can't read memory at pc. */
>
> For consistency's sake, can you drop the != 0 here?

OK.

>> + if (insn != 0xc3) /* RET */
>> + return 0;
>
> Please use lowercase for instruction mnemonics.

OK. Revised patch attached.

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2009-07-03 Michael Snyder <msnyder@vmware.com>

* i386-tdep.c: Add a frame unwinder for function epilogues.
(i386_in_function_epilogue_p): New function.
(i386_epilogue_frame_sniffer): New function.
(i386_epilogue_frame_cache): New function.
(i386_epilogue_frame_this_id): New function.
(i386_epilogue_frame_unwind): New struct frame_unwind.
(i386_gdbarch_init): Hook the new unwinder.

Index: i386-tdep.c
===================================================================
RCS file: /cvs/src/src/gdb/i386-tdep.c,v
retrieving revision 1.280
diff -u -p -r1.280 i386-tdep.c
--- i386-tdep.c 2 Jul 2009 17:25:54 -0000 1.280
+++ i386-tdep.c 5 Jul 2009 20:56:08 -0000
@@ -1487,6 +1487,89 @@ static const struct frame_unwind i386_fr
NULL,
default_frame_sniffer
};
+
+/* Normal frames, but in a function epilogue. */
+
+/* The epilogue is defined here as the RET instruction, which will
+ follow any instruction such as LEAVE or POP EBP that destroys the
+ function's stack frame. */
+
+static int
+i386_in_function_epilogue_p (struct gdbarch *gdbarch, CORE_ADDR pc)
+{
+ gdb_byte insn;
+
+ if (target_read_memory (pc, &insn, 1))
+ return 0; /* Can't read memory at pc. */
+
+ if (insn != 0xc3) /* 'ret' instruction. */
+ return 0;
+
+ return 1;
+}
+
+static int
+i386_epilogue_frame_sniffer (const struct frame_unwind *self,
+ struct frame_info *this_frame,
+ void **this_prologue_cache)
+{
+ if (frame_relative_level (this_frame) == 0)
+ return i386_in_function_epilogue_p (get_frame_arch (this_frame),
+ get_frame_pc (this_frame));
+ else
+ return 0;
+}
+
+static struct i386_frame_cache *
+i386_epilogue_frame_cache (struct frame_info *this_frame, void **this_cache)
+{
+ struct gdbarch *gdbarch = get_frame_arch (this_frame);
+ enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
+ struct i386_frame_cache *cache;
+ gdb_byte buf[4];
+
+ if (*this_cache)
+ return *this_cache;
+
+ cache = i386_alloc_frame_cache ();
+ *this_cache = cache;
+
+ /* Cache base will be ESP plus cache->sp_offset (-4). */
+ get_frame_register (this_frame, I386_ESP_REGNUM, buf);
+ cache->base = extract_unsigned_integer (buf, 4,
+ byte_order) + cache->sp_offset;
+
+ /* Cache pc will be the frame func. */
+ cache->pc = get_frame_pc (this_frame);
+
+ /* The saved ESP will be at cache->base plus 8. */
+ cache->saved_sp = cache->base + 8;
+
+ /* The saved EIP will be at cache->base plus 4. */
+ cache->saved_regs[I386_EIP_REGNUM] = cache->base + 4;
+
+ return cache;
+}
+
+static void
+i386_epilogue_frame_this_id (struct frame_info *this_frame,
+ void **this_cache,
+ struct frame_id *this_id)
+{
+ struct i386_frame_cache *cache = i386_epilogue_frame_cache (this_frame,
+ this_cache);
+
+ (*this_id) = frame_id_build (cache->base + 8, cache->pc);
+}
+
+static const struct frame_unwind i386_epilogue_frame_unwind =
+{
+ NORMAL_FRAME,
+ i386_epilogue_frame_this_id,
+ i386_frame_prev_register,
+ NULL,
+ i386_epilogue_frame_sniffer
+};
\f

/* Signal trampolines. */
@@ -5329,7 +5412,15 @@ i386_gdbarch_init (struct gdbarch_info i
/* Helper for function argument information. */
set_gdbarch_fetch_pointer_argument (gdbarch, i386_fetch_pointer_argument);

- /* Hook in the DWARF CFI frame unwinder. */
+ /* Hook the function epilogue frame unwinder. This unwinder is
+ appended to the list first, so that it supercedes the Dwarf
+ unwinder in function epilogues (where the Dwarf unwinder
+ currently fails). */
+ frame_unwind_append_unwinder (gdbarch, &i386_epilogue_frame_unwind);
+
+ /* Hook in the DWARF CFI frame unwinder. This unwinder is appended
+ to the list before the prologue-based unwinders, so that Dwarf
+ CFI info will be used if it is available. */
dwarf2_append_unwinders (gdbarch);

frame_base_set_default (gdbarch, &i386_frame_base);
@@ -5337,6 +5428,7 @@ i386_gdbarch_init (struct gdbarch_info i
/* Hook in ABI-specific overrides, if they have been registered. */
gdbarch_init_osabi (info, gdbarch);

+ /* Hook in the legacy prologue-based unwinders last (fallback). */
frame_unwind_append_unwinder (gdbarch, &i386_sigtramp_frame_unwind);
frame_unwind_append_unwinder (gdbarch, &i386_frame_unwind);