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

[Christian Brauner]

On July 24, 2019 9:07:54 PM GMT+02:00, Jann Horn <jannh@xxxxxxxxxx> wrote:
>On Wed, Jul 24, 2019 at 8:27 PM Christian Brauner
><christian@xxxxxxxxxx> wrote:
>> 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.
>
>To clarify:
>
>This code looks buggy to me because p->flags is inherited from the
>parent, with the exception of flags that are explicitly stripped out.
>Since PF_WAIT_PID is not stripped out, this means that if task A
>creates a child B with clone(CLONE_WAIT_PID), and then task B uses
>fork() to create a child C, then B will not be able to use
>wait(&status) to wait for C since C inherited PF_WAIT_PID from B.
>
>The obvious way to fix that would be to always strip out PF_WAIT_PID;
>but that would also be wrong, because if task B creates a thread C,
>and then C calls execve(), the task_struct of B goes away and B's TGID
>is taken over by C. When C eventually exits, it should still obey the
>CLONE_WAIT_PID (since to A, it's all the same process). Therefore, if
>p->flags is used to track whether the task was created with
>CLONE_WAIT_PID, PF_WAIT_PID must be inherited if CLONE_THREAD is set.
>So:
>
>diff --git a/kernel/fork.c b/kernel/fork.c
>index d8ae0f1b4148..b32e1e9a6c9c 100644
>--- a/kernel/fork.c
>+++ b/kernel/fork.c
>@@ -1902,6 +1902,10 @@ 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_THREAD))
>+               p->flags &= ~PF_PF_WAIT_PID;
>+       if (clone_flags & CLONE_WAIT_PID)
>+               p->flags |= PF_PF_WAIT_PID;
>        INIT_LIST_HEAD(&p->children);
>        INIT_LIST_HEAD(&p->sibling);
>        rcu_copy_process(p);
>
>An alternative would be to not use p->flags at all, but instead make
>this a property of the signal_struct - since the property is shared by
>all threads, that might make more sense?

Yeah, thanks for clarifying.
Now it's more obvious.
I need to take a look at the signal struct before I can say anything about this.