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

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Message-ID: <20250427171143.GA27775@redhat.com>
Date: Sun, 27 Apr 2025 19:11:43 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Jiri Olsa <jolsa@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
	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: [PATCH perf/core 10/22] uprobes/x86: Add support to optimize
 uprobes

I didn't actually read this patch yet, but let me ask anyway...

On 04/21, Jiri Olsa wrote:
>
> +static int swbp_optimize(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);
> +
> +	/*
> +	 * 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
> +	 */
> +
> +	text_poke_sync();

Hmm. I would like to understand why exactly we need at least this first
text_poke_sync() before "write last 4 bytes of the call instruction".


And... I don't suggest to do this right now, but I am wondering if we can
use mm_cpumask(vma->vm_mm) later, I guess we don't care if we race with
switch_mm_irqs_off() which can add another CPU to this mask...

> +void arch_uprobe_optimize(struct arch_uprobe *auprobe, unsigned long vaddr)
> +{
> +	struct mm_struct *mm = current->mm;
> +	uprobe_opcode_t insn[5];
> +
> +	/*
> +	 * Do not optimize if shadow stack is enabled, the return address hijack
> +	 * code in arch_uretprobe_hijack_return_addr updates wrong frame when
> +	 * the entry uprobe is optimized and the shadow stack crashes the app.
> +	 */
> +	if (shstk_is_enabled())
> +		return;

Not sure I fully understand the comment/problem, but what if
prctl(ARCH_SHSTK_ENABLE) is called after arch_uprobe_optimize() succeeds?

Oleg.