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SubjectFor review: pidfd_send_signal(2) manual page
Date
Hello Christian and all,

Below, I have the rendered version of the current draft of
the pidfd_send_signal(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?

Thanks,

Michael


NAME
pidfd_send_signal - send a signal to a process specified by a file
descriptor

SYNOPSIS
int pidfd_send_signal(int pidfd, int sig, siginfo_t info,
unsigned int flags);

DESCRIPTION
The pidfd_send_signal() system call sends the signal sig to the
target process referred to by pidfd, a PID file descriptor that
refers to a process.

If the info argument points to a siginfo_t buffer, that buffer
should be populated as described in rt_sigqueueinfo(2).

If the info argument is a NULL pointer, this is equivalent to
specifying a pointer to a siginfo_t buffer whose fields match the
values that are implicitly supplied when a signal is sent using
kill(2):

* si_signo is set to the signal number;
* si_errno is set to 0;
* si_code is set to SI_USER;
* si_pid is set to the caller's PID; and
* si_uid is set to the caller's real user ID.

The calling process must either be in the same PID namespace as
the process referred to by pidfd, or be in an ancestor of that
namespace.

The flags argument is reserved for future use; currently, this
argument must be specified as 0.

RETURN VALUE
On success, pidfd_send_signal() returns 0. On success, -1 is
returned and errno is set to indicate the cause of the error.

ERRORS
EBADF pidfd is not a valid PID file descriptor.

EINVAL sig is not a valid signal.

EINVAL The calling process is not in a PID namespace from which it
can send a signal to the target process.

EINVAL flags is not 0.

EPERM The calling process does not have permission to send the
signal to the target process.

EPERM pidfd doesn't refer to the calling process, and
info.si_code is invalid (see rt_sigqueueinfo(2)).

ESRCH The target process does not exist.

VERSIONS
pidfd_send_signal() first appeared in Linux 5.1.

CONFORMING TO
pidfd_send_signal() is Linux specific.

NOTES
Currently, there is no glibc wrapper for this system call; call it
using syscall(2).

PID file descriptors
The pidfd argument is a PID file descriptor, a file descriptor
that refers to process. Such a file descriptor can be obtained
in any of the following ways:

* by opening a /proc/[pid] directory;

* using pidfd_open(2); or

* via the PID file descriptor that is returned by a call to
clone(2) or clone3(2) that specifies the CLONE_PIDFD flag.

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.

EXAMPLE
#define _GNU_SOURCE
#include <limits.h>
#include <signal.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/syscall.h>

#ifndef __NR_pidfd_send_signal
#define __NR_pidfd_send_signal 424
#endif

static
int pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
unsigned int flags)
{
return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
}

int
main(int argc, char *argv[])
{
siginfo_t info;
char path[PATH_MAX];
int pidfd, sig;

if (argc != 3) {
fprintf(stderr, "Usage: %s <pid> <signal>\n", argv[0]);
exit(EXIT_FAILURE);
}

sig = atoi(argv[2]);

/* Obtain a PID file descriptor by opening the /proc/PID directory
of the target process */

snprintf(path, sizeof(path), "/proc/%s", argv[1]);

pidfd = open(path, O_RDONLY);
if (pidfd == -1) {
perror("open");
exit(EXIT_FAILURE);
}

/* Populate a 'siginfo_t' structure for use with
pidfd_send_signal() */

memset(&info, 0, sizeof(info));
info.si_code = SI_QUEUE;
info.si_signo = sig;
info.si_errno = 0;
info.si_uid = getuid();
info.si_pid = getpid();
info.si_value.sival_int = 1234;

/* Send the signal */

if (pidfd_send_signal(pidfd, sig, &info, 0) == -1) {
perror("pidfd_send_signal");
exit(EXIT_FAILURE);
}

exit(EXIT_SUCCESS);
}

SEE ALSO
clone(2), kill(2), pidfd_open(2), rt_sigqueueinfo(2), sigac‐
tion(2), pid_namespaces(7), signal(7)

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