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

[Uros Bizjak]

On 4. 03. 25 01:35, H. Peter Anvin wrote:
> On March 3, 2025 2:47:58 PM PST, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > On Mon, Mar 03, 2025 at 02:45:50PM -0800, Josh Poimboeuf wrote:
> > > On Mon, Mar 03, 2025 at 02:31:50PM -0800, H. Peter Anvin wrote:
> > > > > +#ifdef CONFIG_UNWINDER_FRAME_POINTER
> > > > > #define ASM_CALL_CONSTRAINT "r" (__builtin_frame_address(0))
> > > > > +#else
> > > > > +#define ASM_CALL_CONSTRAINT
> > > > > +#endif
> > > > >
> > > > > #endif /* __ASSEMBLY__ */
> > > >
> > > > Wait, why was this changed? I actually tested this form at least once
> > > > and found that it didn't work under all circumstances...
> > >
> > > Do you have any more details about where this didn't work?  I tested
> > > with several configs and it seems to work fine.  Objtool will complain
> > > if it doesn't work.
> > >
> > > See here for the justification (the previous version was producing crap
> > > code in Clang):
> >
> > Gah, that link doesn't work because I forgot to cc lkml.
> >
> > Here's the tip bot link:
> >
> >   https://lore.kernel.org/all/174099976253.10177.12542657892256193630.tip-bot2@tip-bot2/
>
> One more thing: if we remove ASM_CALL_CONSTRAINTS, we will not be able to use the redzone in future FRED only kernel builds.

Actually, GCC 15+ will introduce "redzone" clobber, so you will be able 
to write e.g.:

void foo (void) { asm ("" : : : "cc", "memory", "redzone"); }

Please see [1] and:

+@item "redzone"
+The @code{"redzone"} clobber tells the compiler that the assembly code
+may write to the stack red zone, area below the stack pointer which on
+some architectures in some calling conventions is guaranteed not to be
+changed by signal handlers, interrupts or exceptions and so the compiler
+can store there temporaries in leaf functions.  On targets which have
+no concept of the stack red zone, the clobber is ignored.
+It should be used e.g.@: in case the assembly code uses call instructions
+or pushes something to the stack without taking the red zone into account
+by subtracting red zone size from the stack pointer first and restoring
+it afterwards.
+

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117312

Uros.