Re: [PATCH] Remove some obfuscation from ${arch}_skip_prologue functions

Yao Qi
2014-09-09T01:29:58Z

Doug Evans <dje@google.com> writes:

> I have found two cases where, I think, ${arch}_skip_prologue can
> be called with pc != func_addr: vax and ppc.
> I'd be happy with simplifying the target API so that
> we could have such an assert, though I'd rather not put it
> in ${arch}_skip_prologue. I currently have the problem that
> the treatment of gcc vs clang is not as consistent as it could be
> across all ${arch}_skip_prologue functions, so I'm on a path of keeping
> as much code that should be common out of target-specific routines:
> cleaning it up later is not always fun or easy.

Agreed.

>
> > Note that this assert is triggered on arm in
> > gdb.cp/re-set-overloaded.exp, that is PC is [1] but FUNC_ADDR is [2].
> >
> > (gdb) disassemble _ZN1CC1Ei
> > Dump of assembler code for function _ZN1CC1Ev:
> > 0x0000090c <+0>: ldr r12, [pc, #4] ; 0x918 <_ZN1CC1Ev+12> <- [2]
> > 0x00000910 <+4>: add r12, r12, pc
> > 0x00000914 <+8>: bx r12
> > 0x00000918 <+12>: ; <UNDEFINED> instruction: 0xffffffc5
> > 0x0000091c <+0>: ldr r12, [pc, #4] ; 0x928 <_ZN1CC1Ei+12> <- [1]
> > 0x00000920 <+4>: add r12, r12, pc
> >
> > AFAICS, PC is still the function address but find_pc_partial_function
> > computes the FUNC_ADDR incorrectly and it is nothing wrong about your
> > patch.
>
> Thanks, this is good data.
>
> I did a similar experiment on amd64-linux after writing
> https://sourceware.org/ml/gdb-patches/2014-08/msg00539.html
> and got no hits.
>
> I'd be curious to see how arm_skip_prologue handles this.

I hacked arm_skip_prologue as the patch below shows.

> What flavor of arm and what version of gcc?

The test case is compiled with "-mthumb -march=armv4t", and gcc is 4.9.1
based sourcery lite gcc for arm-none-linux-gnueabi.

> I can't recreate that example with arm-linux-gnueabi-g++-4.7.
> Also, can you send me gdb.log plus re-set-overloaded{,.so} ?
> [don't cc the list :-)]

Yes, they are sent to you.

--
Yao (齐尧)
diff --git a/gdb/arm-tdep.c b/gdb/arm-tdep.c
index f9feb52..d2f2d52 100644
--- a/gdb/arm-tdep.c
+++ b/gdb/arm-tdep.c
@@ -1396,6 +1396,8 @@ arm_skip_prologue (struct gdbarch *gdbarch, CORE_ADDR pc)
= skip_prologue_using_sal (gdbarch, func_addr);
struct symtab *s = find_pc_symtab (func_addr);

+ gdb_assert (pc == func_addr);
+
if (post_prologue_pc)
post_prologue_pc
= arm_skip_stack_protector (post_prologue_pc, gdbarch);