On Tue, Sep 24, 2019 at 09:57:04PM +0200, Christian Brauner wrote: > On Tue, Sep 24, 2019 at 09:44:49PM +0200, Michael Kerrisk (man-pages) wrote: > > Hello Christian, > > > > On 9/23/19 4:23 PM, Christian Brauner wrote: > > > On Mon, Sep 23, 2019 at 01:26:34PM +0200, Florian Weimer wrote: > > >> * Michael Kerrisk: > > >> > > >>> 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. > > > > > >> > > >>> 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. > > > > (Thanks for the clarification. I added the text "(i.e., it has > > terminated and been waited on)" to the ESRCH error.) > > > > >>> 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 */ > > > > Although there is still the race between the fork() and the > > pidfd_open(), right? > > Actually no and my code is even too complex. > If you are the parent, and this is really a sequence that obeys the > ordering pidfd_open() before waiting: > > pid = fork(); > if (pid == 0) > exit(EXIT_SUCCESS); > pidfd = pidfd_open(pid, 0); > waitid(pid, ...); > > Then you are guaranteed that pidfd will refer to pid. No recycling can > happen since the process has not been waited upon yet (That is, > excluding special cases such as where you have a mainloop where a > callback reacts to a SIGCHLD event and waits on the child behind your > back and your next callback in the mainloop calls pidfd_open() while the > pid has been recycled etc.). If we wanted to be super nitpicky one could also get in that situation where you do: signal(SIGCHLD,SIG_IGN); // or struct sigaction sa; sa.sa_handler = SIG_IGN; sigemptyset(&sa.sa_mask); sa.sa_flags = 0; sigaction(SIGCHLD, &sa, 0) pid = fork(); if (pid == 0) exit(EXIT_SUCCESS); pidfd = pidfd_open(); because then the process gets autoreaped and can be recycled. But again, that's just bad form and in that scenario one should again use clone(CLONE_PIDFD) instead of fork(). Christian
