linux-kernel - Re: [PATCH v2] ptrace: fix ptrace vs tasklist_lock race on PREEMPT

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Message-ID: <YlQSe6m2tidxwHt0@hirez.programming.kicks-ass.net>
Date:   Mon, 11 Apr 2022 13:35:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH v2] ptrace: fix ptrace vs tasklist_lock race on
 PREEMPT_RT.

On Fri, Apr 08, 2022 at 10:06:30PM +0200, Peter Zijlstra wrote:

> I'll ponder if wait_task_inactive() can simplify things..

This,.. so ptrace_check_attach(), which does ptrace_freeze_traced()
already does wait_task_inactive(), but on the 'wrong' side of things.

AFAICT, if we move that up, we're almost there, except that opens up a
detach+attach race. That could be fixed by doing another
wait_task_inactive(), but we can't due to locking :/

Let's see if I can make that work without making a mess of things.
Because ensuring the task is stuck in schedule() makes the whole
saved_state thing go away -- as you noted.