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Date:   Wed, 27 Apr 2022 17:53:37 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     rjw@rjwysocki.net, mingo@kernel.org, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, mgorman@suse.de,
        ebiederm@xmission.com, bigeasy@linutronix.de,
        Will Deacon <will@kernel.org>, linux-kernel@vger.kernel.org,
        tj@kernel.org, linux-pm@vger.kernel.org
Subject: Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs
 PREEMPT_RT
Message-ID: <20220427155335.GH17421@redhat.com>
References: <20220421150248.667412396@infradead.org>
 <20220421150654.817117821@infradead.org>
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On 04/21, Peter Zijlstra wrote:
>
> @@ -1329,8 +1337,7 @@ SYSCALL_DEFINE4(ptrace, long, request, l
>  		goto out_put_task_struct;
>  
>  	ret = arch_ptrace(child, request, addr, data);
> -	if (ret || request != PTRACE_DETACH)
> -		ptrace_unfreeze_traced(child);
> +	ptrace_unfreeze_traced(child);

Forgot to mention... whatever we do this doesn't look right.

ptrace_unfreeze_traced() must not be called if the tracee was untraced,
anothet debugger can come after that. I agree, the current code looks
a bit confusing, perhaps it makes sense to re-write it:

	if (request == PTRACE_DETACH && ret == 0)
		; /* nothing to do, no longer traced by us */
	else
		ptrace_unfreeze_traced(child);

Oleg.