linux-kernel - Re: [PATCH 4/5] pidfd: add CLONE_WAIT

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Date:   Wed, 24 Jul 2019 20:27:21 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Jann Horn <jannh@...gle.com>
CC:     kernel list <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tejun Heo <tj@...nel.org>, David Howells <dhowells@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        kernel-team <kernel-team@...roid.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 4/5] pidfd: add CLONE_WAIT_PID

On July 24, 2019 8:14:26 PM GMT+02:00, Jann Horn <jannh@...gle.com> wrote:
>On Wed, Jul 24, 2019 at 4:48 PM Christian Brauner
><christian@...uner.io> wrote:
>> If CLONE_WAIT_PID is set the newly created process will not be
>> considered by process wait requests that wait generically on children
>> such as:
>>
>>         syscall(__NR_wait4, -1, wstatus, options, rusage)
>>         syscall(__NR_waitpid, -1, wstatus, options)
>>         syscall(__NR_waitid, P_ALL, -1, siginfo, options, rusage)
>>         syscall(__NR_waitid, P_PGID, -1, siginfo, options, rusage)
>>         syscall(__NR_waitpid, -pid, wstatus, options)
>>         syscall(__NR_wait4, -pid, wstatus, options, rusage)
>>
>> A process created with CLONE_WAIT_PID can only be waited upon with a
>> focussed wait call. This ensures that processes can be reaped even if
>> all file descriptors referring to it are closed.
>[...]
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index baaff6570517..a067f3876e2e 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -1910,6 +1910,8 @@ static __latent_entropy struct task_struct
>*copy_process(
>>         delayacct_tsk_init(p);  /* Must remain after
>dup_task_struct() */
>>         p->flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER | PF_IDLE);
>>         p->flags |= PF_FORKNOEXEC;
>> +       if (clone_flags & CLONE_WAIT_PID)
>> +               p->flags |= PF_WAIT_PID;
>>         INIT_LIST_HEAD(&p->children);
>>         INIT_LIST_HEAD(&p->sibling);
>>         rcu_copy_process(p);
>
>This means that if a process with PF_WAIT_PID forks, the child
>inherits the flag, right? That seems unintended? You might have to add
>something like "if (clone_flags & CLONE_THREAD == 0) p->flags &=
>~PF_WAIT_PID;" before this. (I think threads do have to inherit the
>flag so that the case where a non-leader thread of the child goes
>through execve and steals the leader's identity is handled properly.)
>Or you could cram it somewhere into signal_struct instead of on the
>task - that might be a more logical place for it?

Hm, CLONE_WAIT_PID is only useable with CLONE_PIDFD which in turn is
not useable with CLONE_THREAD.
But we should probably make that explicit for CLONE_WAIT_PID too.