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Date: Mon, 23 Sep 2019 16:38:48 +0200
From: Christian Brauner <christian.brauner@...ntu.com>
To: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc: Christian Brauner <christian@...uner.io>,
Jann Horn <jannh@...gle.com>,
Daniel Colascione <dancol@...gle.com>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Joel Fernandes <joel@...lfernandes.org>,
Linux API <linux-api@...r.kernel.org>,
lkml <linux-kernel@...r.kernel.org>,
linux-man <linux-man@...r.kernel.org>,
Oleg Nesterov <oleg@...hat.com>
Subject: Re: For review: pidfd_open(2) manual page
On Mon, Sep 23, 2019 at 11:11:53AM +0200, Michael Kerrisk (man-pages) wrote:
> Hello Christian and all,
>
> Below, I have the rendered version of the current draft of
> the pidfd_open(2) manual page that I have written.
> The page source can be found in a Git branch at:
> https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/log/?h=draft_pidfd
>
> I would be pleased to receive corrections and notes on any
> details that should be added. (For example, are there error
> cases that I have missed?)
>
> Would you be able to review please?
Again, thank you Michael for doing this!
>
> Thanks,
>
> Michael
>
>
> NAME
> pidfd_open - obtain a file descriptor that refers to a process
>
> SYNOPSIS
> int pidfd_open(pid_t pid, unsigned int flags);
>
> DESCRIPTION
> The pidfd_open() system creates a file descriptor that refers to
s/system/system call/
> the process whose PID is specified in pid. The file descriptor is
> returned as the function result; the close-on-exec flag is set on
> the file descriptor.
>
> The flags argument is reserved for future use; currently, this
> argument must be specified as 0.
>
> RETURN VALUE
> On success, pidfd_open() returns a nonnegative file descriptor.
> On success, -1 is returned and errno is set to indicate the cause
s/On success/On error/g
> of the error.
>
> ERRORS
> EINVAL flags is not 0.
>
> EINVAL pid is not valid.
>
> ESRCH The process specified by pid does not exist.
>
> VERSIONS
> pidfd_open() first appeared in Linux 5.3.
>
> CONFORMING TO
> pidfd_open() is Linux specific.
>
> NOTES
> Currently, there is no glibc wrapper for this system call; call it
> using syscall(2).
>
> The pidfd_send_signal(2) system call can be used to send a signal
> to the process referred to by a PID file descriptor.
>
> A PID file descriptor can be monitored using poll(2), select(2),
> and epoll(7). When the process that it refers to terminates, the
> file descriptor indicates as readable. Note, however, that in the
Not a native English speaker but should this be "indicates it is
readable"?
> current implementation, nothing can be read from the file descrip‐
> tor.
>
> The pidfd_open() system call is the preferred way of obtaining a
> PID file descriptor. The alternative is to obtain a file descrip‐
> tor by opening a /proc/[pid] directory. However, the latter tech‐
> nique is possible only if the proc(5) file system is mounted; fur‐
> thermore, the file descriptor obtained in this way is not pol‐
> lable.
I mentioned this already in the CLONE_PIDFD manpage, we should probably
not make a big deal out of this and not mention /proc/<pid> here at all.
(Crazy idea, but we could also have a config option that allows you to
turn of proc-pid-dirfds as pidfds if we start to feel really strongly
about this or a sysctl whatever...)
>
> See also the discussion of the CLONE_PIDFD flag in clone(2).
>
> EXAMPLE
> The program below opens a PID file descriptor for the process
> whose PID is specified as its command-line argument. It then mon‐
> itors the file descriptor for readability (POLLIN) using poll(2).
Yeah, maybe say "monitors the file descriptor for process exit indicated
by an EPOLLIN event" or something. Readability might be confusing.
> When the process with the specified by PID terminates, poll(2)
> returns, and indicates that the file descriptor is readable.
See comment above "readable". (I'm on my phone and I think someone
pointed this out already.)
>
> Program source
>
> #define _GNU_SOURCE
> #include <sys/syscall.h>
> #include <unistd.h>
> #include <poll.h>
> #include <stdlib.h>
> #include <stdio.h>
>
> #ifndef __NR_pidfd_open
> #define __NR_pidfd_open 434
> #endif
Alpha is special... (and not in a good way).
So you would need to special case Alpha since that's the only arch where
we haven't been able to unify syscall numbering. :D
But it's not super important.
I like the program example.
>
> static
> int pidfd_open(pid_t pid, unsigned int flags)
> {
> return syscall(__NR_pidfd_open, pid, flags);
> }
>
> int
> main(int argc, char *argv[])
> {
> struct pollfd pollfd;
> int pidfd, ready;
>
> if (argc != 2) {
> fprintf(stderr, "Usage: %s <pid>\n", argv[0]);
> exit(EXIT_SUCCESS);
> }
>
> pidfd = pidfd_open(atoi(argv[1]), 0);
> if (pidfd == -1) {
> perror("pidfd_open");
> exit(EXIT_FAILURE);
> }
>
> pollfd.fd = pidfd;
> pollfd.events = POLLIN;
>
> ready = poll(&pollfd, 1, -1);
> if (ready == -1) {
> perror("poll");
> exit(EXIT_FAILURE);
> }
>
> printf("Events (0x%x): POLLIN is %sset\n", pollfd.revents,
> (pollfd.revents & POLLIN) ? "" : "not ");
>
> exit(EXIT_SUCCESS);
> }
>
> SEE ALSO
> clone(2), kill(2), pidfd_send_signal(2), poll(2), select(2),
> epoll(7)
>
>
> --
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/
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