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.
>
> > 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 */
>
> > static
> > int pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
> > unsigned int flags)
> > {
> > return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
> > }
>
> Please use a different function name. Thanks.
