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

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Message-ID: <Z1_gFymfO3sAwhiY@krava>
Date: Mon, 16 Dec 2024 09:08:55 +0100
From: Jiri Olsa <olsajiri@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: David Laight <David.Laight@...lab.com>,
	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 Sun, Dec 15, 2024 at 03:14:13PM +0100, Oleg Nesterov wrote:
> 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().

tbh I wasn't completely sure about that as well, I added selftest
in patch #11 trying to hit the issue you described and it seems to
work ok

jirka