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

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Message-ID: <CAHk-=whqPZjtH6VwLT3vL5-b3ONL2F83yEzxMMco+uFXe8CdKg@mail.gmail.com>
Date: Tue, 4 Mar 2025 08:57:13 -1000
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org, 
	linux-tip-commits@...r.kernel.org, 
	"Peter Zijlstra (Intel)" <peterz@...radead.org>, Brian Gerst <brgerst@...il.com>, 
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [tip: x86/asm] x86/asm: Make ASM_CALL_CONSTRAINT conditional on
 frame pointers

On Tue, 4 Mar 2025 at 08:48, Linus Torvalds
<torvalds@...ux-foundation.org> 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