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Date:   Thu, 11 Mar 2021 16:21:23 +0100
From:   Oleg Nesterov <oleg@redhat.com>
To:     Jim Newsome <jnewsome@torproject.org>
Cc:     linux-kernel@vger.kernel.org
Subject: Re: [PATCH] ptrace: Allow other threads to access tracee
Message-ID: <20210311152123.GC15552@redhat.com>
References: <20210310205908.23447-1-jnewsome@torproject.org>
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On 03/10, Jim Newsome wrote:
>
> @@ -238,7 +238,7 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
>  	 * be changed by us so it's not changing right after this.
>  	 */
>  	read_lock(&tasklist_lock);
> -	if (child->ptrace && child->parent == current) {
> +	if (child->ptrace && same_thread_group(child->parent, current)) {

Cough... it is not that simple.

Just suppose that 2 threads call ptrace(tracee) at the same time. Say, the 1st
thread does PTRACE_CONT while the 2nd thread tries to change the registers.

Oleg.