Re: For review: pidfd_send_signal(2) manual page

[Jann Horn <jannh@google.com>]

On Mon, Sep 23, 2019 at 1:26 PM Florian Weimer <fw@deneb.enyo.de> wrote:
> * Michael Kerrisk:
> >        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?

My favorite example here is that you could implement "killall" without
PID reuse races. With /proc/$pid file descriptors, you could do it
like this (rough pseudocode with missing error handling and resource
leaks and such):

for each pid {
  procfs_pid_fd = open("/proc/"+pid);
  if (procfs_pid_fd == -1) continue;
  comm_fd = openat(procfs_pid_fd, "comm");
  if (comm_fd == -1) continue;
  char buf[1000];
  int n = read(comm_fd, buf, sizeof(buf)-1);
  buf[n] = 0;
  if (strcmp(buf, expected_comm) == 0) {
    pidfd_send_signal(procfs_pid_fd, SIGKILL, NULL, 0);
  }
}

If you want to avoid using a procfs fd for this, I think you can still
do it, the dance just gets more complicated:

for each pid {
  procfs_pid_fd = open("/proc/"+pid);
  if (procfs_pid_fd == -1) continue;
  pid_fd = pidfd_open(pid, 0);
  if (pid_fd == -1) continue;
  /* at this point procfs_pid_fd and pid_fd may refer to different processes */
  comm_fd = openat(procfs_pid_fd, "comm");
  if (comm_fd == -1) continue;
  /* at this point we know that procfs_pid_fd and pid_fd refer to the
same struct pid, because otherwise the procfs_pid_fd must point to a
directory that throws -ESRCH for everything */
  char buf[1000];
  int n = read(comm_fd, buf, sizeof(buf)-1);
  buf[n] = 0;
  if (strcmp(buf, expected_comm) == 0) {
    pidfd_send_signal(pid_fd, SIGKILL, NULL, 0);
  }
}

But I don't think anyone is actually interested in using pidfds for
this kind of usecase right now.