samples: show race-free pidfd metadata access
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>
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2151ad1b06
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@ -3,4 +3,4 @@
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obj-$(CONFIG_SAMPLES) += kobject/ kprobes/ trace_events/ livepatch/ \
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hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/ seccomp/ \
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configfs/ connector/ v4l/ trace_printk/ \
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vfio-mdev/ statx/ qmi/ binderfs/
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vfio-mdev/ statx/ qmi/ binderfs/ pidfd/
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6
samples/pidfd/Makefile
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6
samples/pidfd/Makefile
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# SPDX-License-Identifier: GPL-2.0
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hostprogs-y := pidfd-metadata
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always := $(hostprogs-y)
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HOSTCFLAGS_pidfd-metadata.o += -I$(objtree)/usr/include
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all: pidfd-metadata
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112
samples/pidfd/pidfd-metadata.c
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112
samples/pidfd/pidfd-metadata.c
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// 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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