Re: For review: pidfd_send_signal(2) manual page

On Mon, Sep 23, 2019 at 01:26:34PM +0200, Florian Weimer wrote:

* Michael Kerrisk:

quoted

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.

quoted

       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.

quoted

       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 */

quoted

       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.