Re: [PATCH] arm software watchpoint: return to epilogue

[Yao Qi <yao at codesourcery dot com>]

Yao Qi <yao@codesourcery.com> writes:

> This patch is to handle a software watchpoint case that program returns
> to caller's epilogue, and it causes the fail in thumb mode,
>
> finish^M
> Run till exit from #0  func () at gdb/testsuite/gdb.base/watchpoint-cond-gone.c:26^M
> 0x000001f6 in jumper ()^M
> (gdb) FAIL: gdb.base/watchpoint-cond-gone.exp: Catch the no longer valid watchpoint
>
> In the test, jumper calls func, and programs returns from func to
> jumper's epilogue, IOW, the branch instruction is the last instruction
> of jumper's function body.
>
>     jumper:
>     .....
>     0x000001f2 <+10>:    bl      0x200   [1] <---- indirect call to func
>     0x000001f6 <+14>:    mov     sp, r7  [2] <---- start of the epilogue
>     0x000001f8 <+16>:    add     sp, #8
>     0x000001fa <+18>:    pop     {r7}
>     0x000001fc <+20>:    pop     {r0}
>     0x000001fe <+22>:    bx      r0
>
> When the inferior returns from func back to jumper, it is expected
> that an expression of a software watchpoint becomes out-of-scope.
> GDB validates the expression by checking the corresponding frame,
> but this check is guarded by gdbarch_in_function_epilogue_p.  See
> breakpoint.c:watchpoint_check.
>
> It doesn't work in this case, because program returns from func's
> epilogue back to jumper's epilogue [2], GDB thinks the program is
> still within the epilogue, but in fact it goes to a different one.
> When PC points at [2], the sp-restore instruction is to be
> executed, so the stack frame isn't destroyed yet and we can still
> use the frame mechanism reliably.
>
> Note that when PC points to the first instruction of restoring SP,
> it is part of epilogue, but we still return zero.  When goes to
> the next instruction, the backward scan will still match the
> epilogue sequence correctly.  The reason for doing this is to
> handle the "return-to-epilogue" case.
>
> What this patch does is to restrict the epilogue matching that let
> GDB think the first SP restore instruction isn't part of the epilogue,
> and fall back to use frame mechanism.  We set 'found_stack_adjust'
> zero before backward scan (although found_stack_adjust is initialized
> to zero, it is safe to set it again before using it), and we've done
> this for arm mode counterpart (arm_in_function_epilogue_p) too.
>
> The patch is tested in arm-none-eabi and arm-none-linux-gnueabi with
> various multilibs.  OK to apply?
>
> gdb:
>
> 2014-08-06  Yao Qi  <yao@codesourcery.com>
>
> 	* arm-tdep.c (thumb_in_function_epilogue_p): Don't set
> 	found_stack_adjust in forward scan.  Set it zero before
> 	backward scan.    Remove condition check on
> 	found_stack_adjust which is always true.  Indent the code.

Ping^2 https://sourceware.org/ml/gdb-patches/2014-08/msg00060.html

-- 
Yao (éå)