Re: [PATCH -tip] x86/asm: Use %a instead of %c(%%rip) in rip_rel_ptr()

H. Peter Anvin
2025-05-05T02:56:05Z

On 5/4/25 11:43, Uros Bizjak wrote:
> The "a" asm operand modifier substitutes a memory reference, with the
> actual operand treated as address. For x86_64, when a symbol is
> provided, the "a" modifier emits "sym(%rip)" instead of "sym".
>
> No functional changes intended.
>
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> ---
> arch/x86/include/asm/asm.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
> index f963848024a5..d7610b99b8d8 100644
> --- a/arch/x86/include/asm/asm.h
> +++ b/arch/x86/include/asm/asm.h
> @@ -116,7 +116,7 @@
> #ifndef __ASSEMBLER__
> static __always_inline __pure void *rip_rel_ptr(void *p)
> {
> - asm("leaq %c1(%%rip), %0" : "=r"(p) : "i"(p));
> + asm("leaq %a1, %0" : "=r"(p) : "i"(p));
>
> return p;
> }

Addendum to this: we recently had a discussion about this and it appears
that %a should work across i386/x86-64 (with i386 emitting the address
without (%rip)).

-hpa