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

H. Peter Anvin
2025-05-05T11:50:05Z

On May 5, 2025 3:42:46 AM PDT, Ard Biesheuvel <ardb@kernel.org> wrote:
>On Mon, 5 May 2025 at 08:09, H. Peter Anvin <hpa@zytor.com> wrote:
>>
>> On May 4, 2025 10:48:19 PM PDT, Ard Biesheuvel <ardb@kernel.org> wrote:
>> >On Mon, 5 May 2025 at 04:59, H. Peter Anvin <hpa@zytor.com> wrote:
>> >>
>> >> 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".
>> >> >
>> >
>> >Clang does not
>> >
>> >https://godbolt.org/z/5Y58T45f5
>> >
>> >> > No functional changes intended.
>> >> >
>> >
>> >NAK. There is a functional change with Clang, which does not emit
>> >%(rip), and this is the whole point of this thing.
>> >
>> >> > 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;
>> >> > }
>> >>
>> >> Also, this function really should be __attribute_const__ rather than __pure.
>> >>
>> >
>> >No it should really not.
>> >
>> >rip_rel_ptr() will yield different results depending on whether it is
>> >called [by the same code] from the 1:1 mapping or the ordinary kernel
>> >virtual mapping of memory, and this is basically the entire reason we
>> >need it in the first place.
>> >
>> >Lying to the compiler about this is not a good idea imo.
>>
>> Goddamnit, another clang bug. Someone fix the damned thing, please...
>
>How is this a bug? The %a modifier is documented as producing a memory
>reference - the fact that GCC on x86_64 always emits a RIP-relative
>reference regardless of whether we are generating PIC code or not is
>an implementation detail, and not fundamental to yield correct code.
>
>Clang produces something that matches the constraints that we gave it
>- we cannot blame the tools for the mess of our own making where we
>call C code from alternative mappings of memory, while we lie to the
>compiler that the code is position dependent and non-relocatable.

It is documented to produce an (%rip) reference in the gcc documentation, and gcc is the reference for inline assembly.

Furthermore, documentation is not reality. Code is.