Re: [PATCH 4/5] pidfd: add CLONE_WAIT_PID

[Christian Brauner]

On July 24, 2019 8:14:26 PM GMT+02:00, Jann Horn <jannh@xxxxxxxxxx> wrote:
>On Wed, Jul 24, 2019 at 4:48 PM Christian Brauner
><christian@xxxxxxxxxx> 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.