linux-kernel - Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs PREEMPT

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Message-ID: <87sfpy42b0.fsf@email.froward.int.ebiederm.org>
Date:   Wed, 27 Apr 2022 16:57:39 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.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>,
        linux-kernel@...r.kernel.org, tj@...nel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs
 PREEMPT_RT

Oleg Nesterov <oleg@...hat.com> writes:

> 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);

This was a bug in my original JOBCTL_DELAY_WAITKILL patch and it was
just cut and pasted here.  I thought it made sense when I was throwing
things together but when I looked more closely I realized that it is
not safe.

Eric