Re: [tip: x86/asm] x86/asm: Make ASM_CALL_CONSTRAINT conditional on frame pointers

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

On Tue, 4 Mar 2025 at 08:48, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> Random ugly code, untested, special versions for different config options.
>
> __builtin_frame_address() is much more complex than just the old "use
> a register variable".

On the gcc bugzilla that hpa opened, I also note that Pinski said that
the __builtin_frame_address() is likely to just work by accident.

Exactly like the %rsp case.

I'd be much more inclined to look for whether marking the asm
'volatile' would be a more reliable model. Or adding a memory clobber
or similar.

Those kinds of solutions would also hopefully not need different
sequences for different config options. Because
__builtin_frame_address() really *is* fundamentally fragile, and the
fact that frame pointers change behavior is a pretty big symptom of
that fragility.

             Linus