[PATCH v7] arm64: implement support for static call trampolines

[Ard Biesheuvel]

On Tue, 17 Mar 2026, at 12:24, Peter Zijlstra wrote:
> On Tue, Mar 17, 2026 at 11:59:49AM +0100, Ard Biesheuvel wrote:
>
>> > +#define __ARCH_DEFINE_STATIC_CALL_TRAMP(name, target)			    \
>> > +	asm("	.pushsection .static_call.text, \"ax\"			\n" \
>> > +	    "	.align	3						\n" \
>> > +	    "	.globl	" STATIC_CALL_TRAMP_STR(name) "			\n" \
>> > +	    STATIC_CALL_TRAMP_STR(name) ":				\n" \
>> > +	    "	hint	34	/* BTI C */				\n" \
>> > +	    "	adrp	x16, 1f						\n" \
>> > +	    "	ldr	x16, [x16, :lo12:1f]				\n" \
>> > +	    "	cbz	x16, 0f						\n" \
>> > +	    "	br	x16						\n" \
>> > +	    "0:	ret							\n" \
>> > +	    "	.type	" STATIC_CALL_TRAMP_STR(name) ", %function	\n" \
>> > +	    "	.size	" STATIC_CALL_TRAMP_STR(name) ", . - " 
>> > STATIC_CALL_TRAMP_STR(name) " \n" \
>> > +	    "	.popsection						\n" \
>> > +	    "	.pushsection .rodata, \"a\"				\n" \
>> > +	    "	.align	3						\n" \
>> > +	    "1:	.quad	" target "					\n" \
>> > +	    "	.popsection						\n")
>
>> > +void arch_static_call_transform(void *site, void *tramp, void *func, 
>> > bool tail)
>> > +{
>> > +	u64 literal;
>> > +	int ret;
>> > +
>> 
>> Here, set func to &__static_call_return0 if it is NULL.
>
> I'm confused. NULL is for the static_call_cond() case, where we NO-OP.
> And the trampoline above does that cbz 0f to ret. So far so good.
>
> But ret0 should return 0, and IIRC arm64 uses x0 for the return value.
> But I don't see the above clearing it. Hmm?
>

The RET0 case will tail call __static_call_return0() which will take care of this.

I am just saying that the NULL case could just do the same, rather than have a conditional branch in the trampoline, as even in that case, the surrounding code must assume that X0 is clobbered. Alternatively, we could tail call __static_call_nop(), which would do just the 'ret'.

IOW, if we guarantee that the target is always set to something appropriate, we can elide the NULL check, and __static_call_return0() and __static_call_nop() are equally appropriate for the NULL case.