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SubjectRe: For review: pidfd_send_signal(2) manual page
On Tue, Sep 24, 2019 at 09:57:04PM +0200, Christian Brauner wrote:
> On Tue, Sep 24, 2019 at 09:44:49PM +0200, Michael Kerrisk (man-pages) wrote:
> > Hello Christian,
> >
> > On 9/23/19 4:23 PM, Christian Brauner wrote:
> > > On Mon, Sep 23, 2019 at 01:26:34PM +0200, Florian Weimer wrote:
> > >> * Michael Kerrisk:
> > >>
> > >>> SYNOPSIS
> > >>> int pidfd_send_signal(int pidfd, int sig, siginfo_t info,
> > >>> unsigned int flags);
> > >>
> > >> This probably should reference a header for siginfo_t.
> > >
> > > Agreed.
> > >
> > >>
> > >>> ESRCH The target process does not exist.
> > >>
> > >> If the descriptor is valid, does this mean the process has been waited
> > >> for? Maybe this can be made more explicit.
> > >
> > > If by valid you mean "refers to a process/thread-group leader" aka is a
> > > pidfd then yes: Getting ESRCH means that the process has exited and has
> > > already been waited upon.
> > > If it had only exited but not waited upon aka is a zombie, then sending
> > > a signal will just work because that's currently how sending signals to
> > > zombies works, i.e. if you only send a signal and don't do any
> > > additional checks you won't notice a difference between a process being
> > > alive and a process being a zombie. The userspace visible behavior in
> > > terms of signaling them is identical.
> >
> > (Thanks for the clarification. I added the text "(i.e., it has
> > terminated and been waited on)" to the ESRCH error.)
> >
> > >>> The pidfd_send_signal() system call allows the avoidance of race
> > >>> conditions that occur when using traditional interfaces (such as
> > >>> kill(2)) to signal a process. The problem is that the traditional
> > >>> interfaces specify the target process via a process ID (PID), with
> > >>> the result that the sender may accidentally send a signal to the
> > >>> wrong process if the originally intended target process has termi‐
> > >>> nated and its PID has been recycled for another process. By con‐
> > >>> trast, a PID file descriptor is a stable reference to a specific
> > >>> process; if that process terminates, then the file descriptor
> > >>> ceases to be valid and the caller of pidfd_send_signal() is
> > >>> informed of this fact via an ESRCH error.
> > >>
> > >> It would be nice to explain somewhere how you can avoid the race using
> > >> a PID descriptor. Is there anything else besides CLONE_PIDFD?
> > >
> > > If you're the parent of the process you can do this without CLONE_PIDFD:
> > > pid = fork();
> > > pidfd = pidfd_open();
> > > ret = pidfd_send_signal(pidfd, 0, NULL, 0);
> > > if (ret < 0 && errno == ESRCH)
> > > /* pidfd refers to another, recycled process */
> >
> > Although there is still the race between the fork() and the
> > pidfd_open(), right?
>
> Actually no and my code is even too complex.
> If you are the parent, and this is really a sequence that obeys the
> ordering pidfd_open() before waiting:
>
> pid = fork();
> if (pid == 0)
> exit(EXIT_SUCCESS);
> pidfd = pidfd_open(pid, 0);
> waitid(pid, ...);
>
> Then you are guaranteed that pidfd will refer to pid. No recycling can
> happen since the process has not been waited upon yet (That is,
> excluding special cases such as where you have a mainloop where a
> callback reacts to a SIGCHLD event and waits on the child behind your
> back and your next callback in the mainloop calls pidfd_open() while the
> pid has been recycled etc.).

If we wanted to be super nitpicky one could also get in that situation
where you do:

signal(SIGCHLD,SIG_IGN);

// or

struct sigaction sa;
sa.sa_handler = SIG_IGN;
sigemptyset(&sa.sa_mask);
sa.sa_flags = 0;
sigaction(SIGCHLD, &sa, 0)

pid = fork();
if (pid == 0)
exit(EXIT_SUCCESS);
pidfd = pidfd_open();

because then the process gets autoreaped and can be recycled. But again,
that's just bad form and in that scenario one should again use
clone(CLONE_PIDFD) instead of fork().

Christian

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