Subject: Re: [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends

From: Segher Boessenkool <[email-masked]>
Date: Thu, 4 Mar 2021 15:54:48 -0600

Hi!

On Thu, Mar 04, 2021 at 02:57:30PM +0000, Mark Rutland wrote:
> It looks like GCC is happy to give us the function-entry-time FP if we use
> __builtin_frame_address(1),

>From the GCC manual:
     Calling this function with a nonzero argument can have
     unpredictable effects, including crashing the calling program.  As
     a result, calls that are considered unsafe are diagnosed when the
     '-Wframe-address' option is in effect.  Such calls should only be
     made in debugging situations.

It *does* warn (the warning is in -Wall btw), on both powerpc and
aarch64.  Furthermore, using this builtin causes lousy code (it forces
the use of a frame pointer, which we normally try very hard to optimise
away, for good reason).

And, that warning is not an idle warning.  Non-zero arguments to
__builtin_frame_address can crash the program.  It won't on simpler
functions, but there is no real definition of what a simpler function
*is*.  It is meant for debugging, not for production use (this is also
why no one has bothered to make it faster).

On Power it should work, but on pretty much any other arch it won't.

> Unless we can get some strong guarantees from compiler folk such that we
> can guarantee a specific function acts boundary for unwinding (and
> doesn't itself get split, etc), the only reliable way I can think to
> solve this requires an assembly trampoline. Whatever we do is liable to
> need some invasive rework.

You cannot get such a guarantee, other than not letting the compiler
see into the routine at all, like with assembler code (not inline asm,
real assembler code).

The real way forward is to bite the bullet and to no longer pretend you
can do a full backtrace from just the stack contents.  You cannot.

Segher