Re: [PATCH 2/4] arm64: implement support for static call trampolines

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Oct 25, 2021 at 02:21:00PM +0200, Frederic Weisbecker wrote:

> +#define __ARCH_DEFINE_STATIC_CALL_TRAMP(name, insn)			    \
> +	asm("	.pushsection	.static_call.text, \"ax\"		\n" \
> +	    "	.align		4					\n" \
> +	    "	.globl		" STATIC_CALL_TRAMP_STR(name) "		\n" \
> +	    "0:	.quad	0x0						\n" \
> +	    STATIC_CALL_TRAMP_STR(name) ":				\n" \
> +	    "	hint 	34	/* BTI C */				\n" \
> +		insn "							\n" \
> +	    "	ldr	x16, 0b						\n" \
> +	    "	cbz	x16, 1f						\n" \
> +	    "	br	x16						\n" \
> +	    "1:	ret							\n" \
> +	    "	.popsection						\n")

> +void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
> +{
> +	/*
> +	 * -0x8	<literal>
> +	 *  0x0	bti c		<--- trampoline entry point
> +	 *  0x4	<branch or nop>
> +	 *  0x8	ldr x16, <literal>
> +	 *  0xc	cbz x16, 20
> +	 * 0x10	br x16
> +	 * 0x14	ret
> +	 */
> +	struct {
> +		u64	literal;
> +		__le32	insn[2];
> +	} insns;
> +	u32 insn;
> +	int ret;
> +
> +	insn = aarch64_insn_gen_hint(AARCH64_INSN_HINT_BTIC);
> +	insns.literal = (u64)func;
> +	insns.insn[0] = cpu_to_le32(insn);
> +
> +	if (!func) {
> +		insn = aarch64_insn_gen_branch_reg(AARCH64_INSN_REG_LR,
> +						   AARCH64_INSN_BRANCH_RETURN);
> +	} else {
> +		insn = aarch64_insn_gen_branch_imm((u64)tramp + 4, (u64)func,
> +						   AARCH64_INSN_BRANCH_NOLINK);
> +
> +		/*
> +		 * Use a NOP if the branch target is out of range, and rely on
> +		 * the indirect call instead.
> +		 */
> +		if (insn == AARCH64_BREAK_FAULT)
> +			insn = aarch64_insn_gen_hint(AARCH64_INSN_HINT_NOP);
> +	}
> +	insns.insn[1] = cpu_to_le32(insn);
> +
> +	ret = __aarch64_insn_write(tramp - 8, &insns, sizeof(insns));

OK, that's pretty magical...

So you're writing the literal and the two instructions with 2 u64
stores. Relying on alignment to guarantee both are in a single page and
that copy_to_kernel_nofault() selects u64 writes.

By unconditionally writing the literal, you avoid there ever being an
stale value, which in turn avoids there being a race where you switch
from 'J @func' relative addressing to 'NOP; do-literal-thing' and cross
CPU execution gets the ordering inverted.

Ooohh, but what if you go from !func to NOP.

assuming:

	.literal = 0
	BTI C
	RET

Then

	CPU0			CPU1

	[S] literal = func	[I] NOP
	[S] insn[1] = NOP	[L] x16 = literal (NULL)
				b x16
				*BANG*

Is that possible? (total lack of memory ordering etc..)

On IRC you just alluded to the fact that this relies on it all being in
a single cacheline (i-fetch windows don't need to be cacheline sized,
but provided they're at least 16 bytes, this should still work given the
alignment).

But is I$ and D$ coherent? One load is through I-fetch, the other is a
regular D-fetch.

However, Will has previously expressed reluctance to rely on such
things.

> +	if (!WARN_ON(ret))
> +		caches_clean_inval_pou((u64)tramp - 8, sizeof(insns));
>  }