On Mon, Sep 23, 2019 at 1:26 PM Florian Weimer <fw@xxxxxxxxxxxxx> 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.
