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https://github.com/torvalds/linux
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43c6afee48
This is a sample program showing userspace how to get race-free access
to process metadata from a pidfd. It is rather easy to do and userspace
can actually simply reuse code that currently parses a process's status
file in procfs.
The program can easily be extended into a generic helper suitable for
inclusion in a libc to make it even easier for userspace to gain metadata
access.
Since this came up in a discussion because this API is going to be used
in various service managers: A lot of programs will have a whitelist
seccomp filter that returns <some-errno> for all new syscalls. This
means that programs might get confused if CLONE_PIDFD works but the
later pidfd_send_signal() syscall doesn't. Hence, here's a ahead of
time check that pidfd_send_signal() is supported:
bool pidfd_send_signal_supported()
{
int procfd = open("/proc/self", O_DIRECTORY | O_RDONLY | O_CLOEXEC);
if (procfd < 0)
return false;
/*
* A process is always allowed to signal itself so
* pidfd_send_signal() should never fail this test. If it does
* it must mean it is not available, blocked by an LSM, seccomp,
* or other.
*/
return pidfd_send_signal(procfd, 0, NULL, 0) == 0;
}
Signed-off-by: Christian Brauner <christian@brauner.io>
Co-developed-by: Jann Horn <jannh@google.com>
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: David Howells <dhowells@redhat.com>
Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Cc: Andy Lutomirsky <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
112 lines
2.2 KiB
C
112 lines
2.2 KiB
C
// SPDX-License-Identifier: GPL-2.0
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#define _GNU_SOURCE
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#include <err.h>
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#include <errno.h>
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#include <fcntl.h>
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#include <inttypes.h>
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#include <limits.h>
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#include <sched.h>
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#include <signal.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <sys/stat.h>
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#include <sys/syscall.h>
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#include <sys/types.h>
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#include <sys/wait.h>
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#include <unistd.h>
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#ifndef CLONE_PIDFD
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#define CLONE_PIDFD 0x00001000
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#endif
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static int do_child(void *args)
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{
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printf("%d\n", getpid());
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_exit(EXIT_SUCCESS);
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}
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static pid_t pidfd_clone(int flags, int *pidfd)
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{
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size_t stack_size = 1024;
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char *stack[1024] = { 0 };
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#ifdef __ia64__
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return __clone2(do_child, stack, stack_size, flags | SIGCHLD, NULL, pidfd);
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#else
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return clone(do_child, stack + stack_size, flags | SIGCHLD, NULL, pidfd);
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#endif
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}
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static inline int sys_pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
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unsigned int flags)
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{
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return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
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}
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static int pidfd_metadata_fd(pid_t pid, int pidfd)
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{
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int procfd, ret;
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char path[100];
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snprintf(path, sizeof(path), "/proc/%d", pid);
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procfd = open(path, O_DIRECTORY | O_RDONLY | O_CLOEXEC);
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if (procfd < 0) {
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warn("Failed to open %s\n", path);
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return -1;
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}
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/*
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* Verify that the pid has not been recycled and our /proc/<pid> handle
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* is still valid.
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*/
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ret = sys_pidfd_send_signal(pidfd, 0, NULL, 0);
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if (ret < 0) {
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switch (errno) {
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case EPERM:
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/* Process exists, just not allowed to signal it. */
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break;
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default:
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warn("Failed to signal process\n");
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close(procfd);
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procfd = -1;
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}
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}
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return procfd;
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}
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int main(int argc, char *argv[])
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{
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int pidfd = 0, ret = EXIT_FAILURE;
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char buf[4096] = { 0 };
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pid_t pid;
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int procfd, statusfd;
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ssize_t bytes;
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pid = pidfd_clone(CLONE_PIDFD, &pidfd);
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if (pid < 0)
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exit(ret);
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procfd = pidfd_metadata_fd(pid, pidfd);
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close(pidfd);
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if (procfd < 0)
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goto out;
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statusfd = openat(procfd, "status", O_RDONLY | O_CLOEXEC);
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close(procfd);
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if (statusfd < 0)
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goto out;
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bytes = read(statusfd, buf, sizeof(buf));
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if (bytes > 0)
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bytes = write(STDOUT_FILENO, buf, bytes);
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close(statusfd);
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ret = EXIT_SUCCESS;
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out:
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(void)wait(NULL);
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exit(ret);
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}
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