Re: [PATCH 3/4] Stop prologue analysis when past the epilogue

Will Newton
2014-07-03T08:39:48Z

On 3 July 2014 07:09, Yao Qi <yao@codesourcery.com> wrote:
> We see a fail in gdb.trace/entry-values.exp on armv4t thumb,
>
> bt^M
> #0 0x000086fc in foo (i=0, i@entry=<optimized out>, j=2, j@entry=<optimized out>)^M
> #1 0x00000002 in ?? ()^M
> Backtrace stopped: previous frame identical to this frame (corrupt stack?)^M
> (gdb) FAIL: gdb.trace/entry-values.exp: bt (1) (pattern 1)
>
> The fail is caused by incorrect prologue analysis, which can be illustrated by
> setting a breakpoint on function foo,
>
> (gdb) disassemble foo
> Dump of assembler code for function foo:
> 0x000086e8 <+0>: push {r7, lr}
> 0x000086ea <+2>: sub sp, #8
> 0x000086ec <+4>: add r7, sp, #0
> 0x000086ee <+6>: str r0, [r7, #4]
> 0x000086f0 <+8>: str r1, [r7, #0]
> 0x000086f2 <+10>: movs r3, #0
> 0x000086f4 <+12>: adds r0, r3, #0
> 0x000086f6 <+14>: mov sp, r7
> 0x000086f8 <+16>: add sp, #8
> 0x000086fa <+18>: pop {r7}
> 0x000086fc <+20>: pop {r1}
> 0x000086fe <+22>: bx r1
> End of assembler dump.
> (gdb) b foo
> Breakpoint 1 at 0x86fc
>
> As we can see, GDB analyzes the prologue and skip the prologue to the last
> instruction but one. The breakpoint is set within the epilogue, and GDB
> skips too many instruction for prologue. This patch teaches GDB to stop
> prologue analysis when goes into the epilogue. With this patch applied,
> GDB is able to unwind correctly,
>
> (gdb) bt
> #0 0x000086f6 in foo (i=0, i@entry=2, j=2, j@entry=3)
> #1 0x00008718 in bar (i=<optimized out>)
> #2 0x00008758 in main ()
>
> gdb:
>
> 2014-07-02 Yao Qi <yao@codesourcery.com>
>
> * arm-tdep.c (thumb_analyze_prologue): Break the loop if
> thumb_instruction_restores_sp return true.
> ---
> gdb/arm-tdep.c | 5 +++++
> 1 file changed, 5 insertions(+)

This patch looks good to me too.

> diff --git a/gdb/arm-tdep.c b/gdb/arm-tdep.c
> index 153ef42..72beeb1 100644
> --- a/gdb/arm-tdep.c
> +++ b/gdb/arm-tdep.c
> @@ -754,6 +754,11 @@ thumb_analyze_prologue (struct gdbarch *gdbarch,
> regs[ARM_SP_REGNUM] = pv_add_constant (regs[ARM_SP_REGNUM],
> -offset);
> }
> + else if (thumb_instruction_restores_sp (insn))
> + {
> + /* Don't scan past the epilogue. */
> + break;
> + }
> else if ((insn & 0xf800) == 0xa800) /* add Rd, sp, #imm */
> regs[bits (insn, 8, 10)] = pv_add_constant (regs[ARM_SP_REGNUM],
> (insn & 0xff) << 2);
> --
> 1.9.0
>

--
Will Newton
Toolchain Working Group, Linaro