Re: [PATCH v6 2/2] arm64: implement support for static call trampolines

[Peter Zijlstra]

On Fri, Nov 05, 2021 at 03:59:17PM +0100, Ard Biesheuvel wrote:
> diff --git a/arch/arm64/include/asm/static_call.h b/arch/arm64/include/asm/static_call.h
> new file mode 100644
> index 000000000000..6ee918991510
> --- /dev/null
> +++ b/arch/arm64/include/asm/static_call.h
> @@ -0,0 +1,40 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_STATIC_CALL_H
> +#define _ASM_STATIC_CALL_H
> +
> +/*
> + * The sequence below is laid out in a way that guarantees that the literal and
> + * the instruction are always covered by the same cacheline, and can be updated
> + * using a single store-pair instruction (provided that we rewrite the BTI C
> + * instruction as well). This means the literal and the instruction are always
> + * in sync when observed via the D-side.
> + *
> + * However, this does not guarantee that the I-side will catch up immediately
> + * as well: until the I-cache maintenance completes, CPUs may branch to the old
> + * target, or execute a stale NOP or RET. We deal with this by writing the
> + * literal unconditionally, even if it is 0x0 or the branch is in range. That
> + * way, a stale NOP will fall through and call the new target via an indirect
> + * call. Stale RETs or Bs will be taken as before, and branch to the old
> + * target.
> + */

Thanks for the comment!


> diff --git a/arch/arm64/kernel/patching.c b/arch/arm64/kernel/patching.c
> index 771f543464e0..a265a87d4d9e 100644
> --- a/arch/arm64/kernel/patching.c
> +++ b/arch/arm64/kernel/patching.c


> +static void *strip_cfi_jt(void *addr)
> +{
> +	if (IS_ENABLED(CONFIG_CFI_CLANG)) {
> +		void *p = addr;
> +		u32 insn;
> +
> +		/*
> +		 * Taking the address of a function produces the address of the
> +		 * jump table entry when Clang CFI is enabled. Such entries are
> +		 * ordinary jump instructions, preceded by a BTI C instruction
> +		 * if BTI is enabled for the kernel.
> +		 */
> +		if (IS_ENABLED(CONFIG_ARM64_BTI_KERNEL))
> +			p += 4;

Perhaps:
		if (aarch64_insn_is_bti(le32_to_cpup(p)))
			p += 4;

Perhapser still, add:
		else
			WARN_ON(IS_ENABLED(CONFIG_ARM64_BTI_KERNEL));

> +
> +		insn = le32_to_cpup(p);
> +		if (aarch64_insn_is_b(insn))
> +			return p + aarch64_get_branch_offset(insn);
> +
> +		WARN_ON(1);
> +	}
> +	return addr;
> +}

Also, can this please have a comment decrying the lack of built-in for
this?