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

[Mark Rutland]

On Thu, Mar 04, 2021 at 03:54:48PM -0600, Segher Boessenkool wrote:
> Hi!

Hi Segher,

> 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.

I understand this is true generally, and cannot be relied upon in
portable code. However as you hint here for Power, I believe that on
arm64 __builtin_frame_address(1) shouldn't crash the program due to the
way frame records work on arm64, but I'll go check with some local
compiler folk. I agree that __builtin_frame_address(2) and beyond
certainly can, e.g.  by NULL dereference and similar.

For context, why do you think this would work on power specifically? I
wonder if our rationale is similar.

Are you aware of anything in particular that breaks using
__builtin_frame_address(1) in non-portable code, or is this just a
general sentiment of this not being a supported use-case?

> > 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).

If we cannot reliably ensure this then I'm happy to go write an assembly
trampoline to snapshot the state at a function call boundary (where our
procedure call standard mandates the state of the LR, FP, and frame
records pointed to by the FP). This'll require reworking a reasonable
amount of code cross-architecture, so I'll need to get some more
concrete justification (e.g. examples of things that can go wrong in
practice).

> 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.

I think what you mean here is that there's no reliable way to handle the
current/leaf function, right? If so I do agree.

Beyond that I believe that arm64's frame records should be sufficient.

Thanks,
Mark.