Re: [Patch ARM] Fix PR target/56846

[Ramana Radhakrishnan <ramana dot gcc at googlemail dot com>]

On Mon, Aug 25, 2014 at 11:32 AM, Tony Wang <tony.wang@arm.com> wrote:
> Hi all,
>
> The bug is reported at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56846, and itâs about the problem that
> when exception handler is involved in the function, then _Unwind_Backtrace function will run into deadloop on
> arm target.

You mean an infinite loop.

>
> Cmd line: arm-none-eabi-g++ -mthumb -mcpu=cortex-m3 -O0 -g -std=c++11 -specs=rdimon.specs main.c -o main.exe
> #include <unwind.h>
> #include <stdio.h>
> _Unwind_Reason_Code trace_func(struct _Unwind_Context * context, void* arg)
> {
>   void *ip = (void *)_Unwind_GetIP(context);
>   printf("Address: %p\n", ip);
>   return _URC_NO_REASON;
> }
> void bar()
> {
>   puts("This is in bar");
>   _Unwind_Backtrace((_Unwind_Trace_Fn)&trace_func, 0);
> }
> void foo()
> {
>   try
>   {
>     bar();
>   }
>   catch (...)
>   {
>     puts("Exception");
>   }
> }
>
> The potential of such a bug is discussed long time ago in mail:
> https://gcc.gnu.org/ml/gcc/2007-08/msg00235.html. Basically, as the ARM EHABI does not define how to implement
> the Unwind_Backtrace, Andrew give control to the personality routine to unwind the stack, and use the unwind
> state combination of â_US_VIRTUAL_UNWIND_FRAME | _US_FORCE_UNWINDâ to represent that the caller is asking the
> personality routine to only unwind the stack for it.
>
> However, the pr in the libstdc++-v3 doesnât handle such a unwind state pattern correctly. When the backtrace
> function passes such a pattern to it, it will still return _URC_HANDLER_FOUND to the caller in some cases.
> Itâs because the pr will think that the _Unwind_Backtrace is raising a none type exception to it, so if the
> exception handler in current stack frame can catch anything(like catch(â)), the pr will return
> _URC_HANDLER_FOUND to the caller and ask for next step. But definitely, the unwind backtrace function donât
> know what to do when pr return an exception handler to it.
>
> So this patch just evaluate such a unwind state pattern at the beginning of the personality routine in
> libstdc++-v3, if we meet with â_US_VIRTUAL_UNWIND_FRAME | _US_FORCE_UNWINDâ, then we directly call macro
> CONTINUE_UNWINDING to unwind the stack and return.
>
> Is this a reasonable fix?

I'd like another review here however it looks sane to me. You need to
CC libstdc++@gcc.gnu.org for libstdc++ patches. Your email doesn't say
how you tested this patch. Can you make sure you've run this through a
bootstrap and regression test on GNU/Linux and a cross regression test
on arm-none-eabi with no regressions ?


regards
Ramana



>
> gcc/libstdc++-v3/ChangeLog:
> 2014-8-25   Tony Wang <tony.wang@arm.com>
>
>                  PR target/56846
>                  * libsupc++/eh_personality.cc: Return with CONTINUE_UNWINDING
>                  when meet with the unwind state pattern: _US_VIRTUAL_UNWIND_FRAME |
>                  _US_FORCE_UNWIND
>
> diff --git a/libstdc++-v3/libsupc++/eh_personality.cc b/libstdc++-v3/libsupc++/eh_personality.cc
> index f315a83..c2b30e9 100644
> --- a/libstdc++-v3/libsupc++/eh_personality.cc
> +++ b/libstdc++-v3/libsupc++/eh_personality.cc
> @@ -378,6 +378,11 @@ PERSONALITY_FUNCTION (int version,
>    switch (state & _US_ACTION_MASK)
>      {
>      case _US_VIRTUAL_UNWIND_FRAME:
> +      // If the unwind state pattern is _US_VIRTUAL_UNWIND_FRAME |
> +      // _US_FORCE_UNWIND, we don't need to search for any handler
> +      // as it is not a real exception. Just unwind the stack.
> +      if (state & _US_FORCE_UNWIND)
> +        CONTINUE_UNWINDING;
>        actions = _UA_SEARCH_PHASE;
>        break;
>
>