linux-kernel - Re: [PATCH v3 08/11] ptrace: Admit ptrace

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Date:   Thu, 5 May 2022 17:01:59 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, rjw@...ysocki.net, mingo@...nel.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, mgorman@...e.de, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, tj@...nel.org,
        linux-pm@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-xtensa@...ux-xtensa.org, Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, linux-ia64@...r.kernel.org
Subject: Re: [PATCH v3 08/11] ptrace: Admit ptrace_stop can generate spuriuos
 SIGTRAPs

On 05/04, Eric W. Biederman wrote:
>
> With the removal of the incomplete detection of the tracer going away
> in ptrace_stop, ptrace_stop always sleeps in schedule after
> ptrace_freeze_traced succeeds.  Modify ptrace_check_attach to
> warn if wait_task_inactive fails.

Oh. Again, I don't understand the changelog. If we forget about RT,
ptrace_stop() will always sleep if ptrace_freeze_traced() succeeds.
may_ptrace_stop() has gone.

IOW. Lets forget about RT

> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -266,17 +266,9 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
>  	}
>  	read_unlock(&tasklist_lock);
>
> -	if (!ret && !ignore_state) {
> -		if (!wait_task_inactive(child, __TASK_TRACED)) {
> -			/*
> -			 * This can only happen if may_ptrace_stop() fails and
> -			 * ptrace_stop() changes ->state back to TASK_RUNNING,
> -			 * so we should not worry about leaking __TASK_TRACED.
> -			 */
> -			WARN_ON(READ_ONCE(child->__state) == __TASK_TRACED);
> -			ret = -ESRCH;
> -		}
> -	}
> +	if (!ret && !ignore_state &&
> +	    WARN_ON_ONCE(!wait_task_inactive(child, __TASK_TRACED)))
> +		ret = -ESRCH;
>
>  	return ret;
>  }

Why do you think this change would be wrong without any other changes?

Oleg.