Re: For review: pidfd_send_signal(2) manual page

Hello Florian,

On 9/23/19 1:26 PM, 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.

Thanks. I added: #include <signal.h>

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.

Yes. I added "(i.e., it has terminated and been waited on)".

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?

Please see my comment in reply to Christian (which will be sent just
after this).

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.

Please see my open question in the thread on pidfd_open().

Thanks for the review, Florian.

Cheers,

Michael

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/