linux-kernel - Re: [PATCHv5 perf/core 10/22] uprobes/x86: Add support to optimize uprobes

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Message-ID: <20250714094824.GQ905792@noisy.programming.kicks-ass.net>
Date: Mon, 14 Jul 2025 11:48:24 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Jiri Olsa <jolsa@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>, Andrii Nakryiko <andrii@...nel.org>,
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, x86@...nel.org,
	Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
	John Fastabend <john.fastabend@...il.com>,
	Hao Luo <haoluo@...gle.com>, Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Alan Maguire <alan.maguire@...cle.com>,
	David Laight <David.Laight@...lab.com>,
	Thomas Weißschuh <thomas@...ch.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCHv5 perf/core 10/22] uprobes/x86: Add support to optimize
 uprobes

On Fri, Jul 11, 2025 at 10:29:18AM +0200, Jiri Olsa wrote:
> +enum {
> +	OPT_PART,
> +	OPT_INSN,
> +	UNOPT_INT3,
> +	UNOPT_PART,
> +};
> +
> +struct write_opcode_ctx {
> +	unsigned long base;
> +	int update;
> +};
> +
> +static int is_call_insn(uprobe_opcode_t *insn)
> +{
> +	return *insn == CALL_INSN_OPCODE;
> +}
> +
> +static int verify_insn(struct page *page, unsigned long vaddr, uprobe_opcode_t *new_opcode,
> +		       int nbytes, void *data)
> +{
> +	struct write_opcode_ctx *ctx = data;
> +	uprobe_opcode_t old_opcode[5];
> +
> +	uprobe_copy_from_page(page, ctx->base, (uprobe_opcode_t *) &old_opcode, 5);
> +
> +	switch (ctx->update) {
> +	case OPT_PART:
> +	case OPT_INSN:
> +		if (is_swbp_insn(&old_opcode[0]))
> +			return 1;
> +		break;
> +	case UNOPT_INT3:
> +		if (is_call_insn(&old_opcode[0]))
> +			return 1;
> +		break;
> +	case UNOPT_PART:
> +		if (is_swbp_insn(&old_opcode[0]))
> +			return 1;
> +		break;
> +	}
> +
> +	return -1;
> +}
> +
> +static int write_insn(struct arch_uprobe *auprobe, struct vm_area_struct *vma, unsigned long vaddr,
> +		      uprobe_opcode_t *insn, int nbytes, void *ctx)
> +{
> +	return uprobe_write(auprobe, vma, vaddr, insn, nbytes, verify_insn,
> +			    true /* is_register */, false /* do_update_ref_ctr */, ctx);
> +}
> +
> +static void relative_call(void *dest, long from, long to)
> +{
> +	struct __packed __arch_relative_insn {
> +		u8 op;
> +		s32 raddr;
> +	} *insn;
> +
> +	insn = (struct __arch_relative_insn *)dest;
> +	insn->raddr = (s32)(to - (from + 5));
> +	insn->op = CALL_INSN_OPCODE;
> +}

We already have this in asm/text-patching.h, its called
__text_gen_insn().

> +
> +static int swbp_optimize(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> +			 unsigned long vaddr, unsigned long tramp)
> +{
> +	struct write_opcode_ctx ctx = {
> +		.base = vaddr,
> +	};
> +	char call[5];
> +	int err;
> +
> +	relative_call(call, vaddr, tramp);

	__text_gen_insn(call, CALL_INSN_OPCODE, vaddr, tramp, CALL_INSN_SIZE);

> +
> +	/*
> +	 * We are in state where breakpoint (int3) is installed on top of first
> +	 * byte of the nop5 instruction. We will do following steps to overwrite
> +	 * this to call instruction:
> +	 *
> +	 * - sync cores
> +	 * - write last 4 bytes of the call instruction
> +	 * - sync cores
> +	 * - update the call instruction opcode

The sanctioned text poke sequence has another sync-core at the end.
Please also do this.

> +	 */
> +
> +	smp_text_poke_sync_each_cpu();
> +
> +	ctx.update = OPT_PART;
> +	err = write_insn(auprobe, vma, vaddr + 1, call + 1, 4, &ctx);
> +	if (err)
> +		return err;
> +
> +	smp_text_poke_sync_each_cpu();
> +
> +	ctx.update = OPT_INSN;
> +	return write_insn(auprobe, vma, vaddr, call, 1, &ctx);
> +}
> +
> +static int swbp_unoptimize(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> +			   unsigned long vaddr)
> +{
> +	uprobe_opcode_t int3 = UPROBE_SWBP_INSN;
> +	struct write_opcode_ctx ctx = {
> +		.base = vaddr,
> +	};
> +	int err;
> +
> +	/*
> +	 * We need to overwrite call instruction into nop5 instruction with
> +	 * breakpoint (int3) installed on top of its first byte. We will:
> +	 *
> +	 * - overwrite call opcode with breakpoint (int3)
> +	 * - sync cores
> +	 * - write last 4 bytes of the nop5 instruction
> +	 * - sync cores
> +	 */
> +
> +	ctx.update = UNOPT_INT3;
> +	err = write_insn(auprobe, vma, vaddr, &int3, 1, &ctx);
> +	if (err)
> +		return err;
> +
> +	smp_text_poke_sync_each_cpu();
> +
> +	ctx.update = UNOPT_PART;
> +	err = write_insn(auprobe, vma, vaddr + 1, (uprobe_opcode_t *) auprobe->insn + 1, 4, &ctx);
> +
> +	smp_text_poke_sync_each_cpu();
> +	return err;
> +}

Please unify these two functions; it makes absolutely no sense to have
two copies of this logic around.