linux-kernel - Re: [PATCH 08/15] x86/alternatives: Teach text_poke

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Date:   Tue, 11 Jun 2019 12:11:12 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Nadav Amit <namit@...are.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate
 instructions

On Tue, 11 Jun 2019 08:54:23 -0700
Andy Lutomirski <luto@...capital.net> wrote:


> How does that help?  If RIP == x+2 and you want to put a 5-byte jump
> at address x, no amount of 0xcc is going to change the fact that RIP
> is in the middle of the jump.
> 
> Live patching can handle this by detecting this condition on each
> CPU, but performance won’t be great.  Maybe some synchronize_sched
> trickery could help.

We have synchronize_rcu_tasks() which return after all tasks have
either entered user space or did a voluntary schedule (was not
preempted). Or have not run (still in a sleeping state).

That way we guarantee that all tasks are no longer on any trampoline
or code paths that do not call schedule. I use this to free dynamically
allocated trampolines used by ftrace. And kprobes uses this too for its
own trampolines.

-- Steve