linux-kernel - Re: [PATCH bpf-next 08/13] uprobes/x86: Add support to optimize uprobes

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Message-ID: <20241215141412.GA13580@redhat.com>
Date: Sun, 15 Dec 2024 15:14:13 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: David Laight <David.Laight@...LAB.COM>
Cc: 'Jiri Olsa' <jolsa@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	"bpf@...r.kernel.org" <bpf@...r.kernel.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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-trace-kernel@...r.kernel.org" <linux-trace-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next 08/13] uprobes/x86: Add support to optimize
 uprobes

On 12/15, David Laight wrote:
>
> From: Jiri Olsa
> > The optimized uprobe path
> >
> >   - checks the original instruction is 5-byte nop (plus other checks)
> >   - adds (or uses existing) user space trampoline and overwrites original
> >     instruction (5-byte nop) with call to user space trampoline
> >   - the user space trampoline executes uprobe syscall that calls related uprobe
> >     consumers
> >   - trampoline returns back to next instruction
> ...
>
> How on earth can you safely overwrite a randomly aligned 5 byte instruction
> that might be being prefetched and executed by another thread of the
> same process.

uprobe_write_opcode() doesn't overwrite the instruction in place.

It creates the new page with the same content, overwrites the probed insn in
that page, then calls __replace_page().

Oleg.