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    SubjectRe: For review: pidfd_send_signal(2) manual page
    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.

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