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    SubjectRe: [PATCH 4/5] pidfd: add CLONE_WAIT_PID
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    On July 24, 2019 8:14:26 PM GMT+02:00, Jann Horn <jannh@google.com> wrote:
    >On Wed, Jul 24, 2019 at 4:48 PM Christian Brauner
    ><christian@brauner.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.

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