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Subject[PATCH 4/4] samples: show race-free pidfd metadata access
Date
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.

Signed-off-by: Christian Brauner <christian@brauner.io>
Signed-off-by: Jann Horn <jann@thejh.net>
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: Oleg Nesterov <oleg@redhat.com>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
---
samples/Makefile | 2 +-
samples/pidfd/Makefile | 6 ++
samples/pidfd/pidfd-metadata.c | 172 +++++++++++++++++++++++++++++++++
3 files changed, 179 insertions(+), 1 deletion(-)
create mode 100644 samples/pidfd/Makefile
create mode 100644 samples/pidfd/pidfd-metadata.c

diff --git a/samples/Makefile b/samples/Makefile
index b1142a958811..fadadb1c3b05 100644
--- a/samples/Makefile
+++ b/samples/Makefile
@@ -3,4 +3,4 @@
obj-$(CONFIG_SAMPLES) += kobject/ kprobes/ trace_events/ livepatch/ \
hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/ seccomp/ \
configfs/ connector/ v4l/ trace_printk/ \
- vfio-mdev/ statx/ qmi/ binderfs/
+ vfio-mdev/ statx/ qmi/ binderfs/ pidfd/
diff --git a/samples/pidfd/Makefile b/samples/pidfd/Makefile
new file mode 100644
index 000000000000..0ff97784177a
--- /dev/null
+++ b/samples/pidfd/Makefile
@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: GPL-2.0
+
+hostprogs-y := pidfd-metadata
+always := $(hostprogs-y)
+HOSTCFLAGS_pidfd-metadata.o += -I$(objtree)/usr/include
+all: pidfd-metadata
diff --git a/samples/pidfd/pidfd-metadata.c b/samples/pidfd/pidfd-metadata.c
new file mode 100644
index 000000000000..23a44e582ccb
--- /dev/null
+++ b/samples/pidfd/pidfd-metadata.c
@@ -0,0 +1,172 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#define _GNU_SOURCE
+#include <err.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <inttypes.h>
+#include <limits.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#ifndef CLONE_PIDFD
+#define CLONE_PIDFD 0x00001000
+#endif
+
+static int raw_clone_pidfd(void)
+{
+ unsigned long flags = CLONE_PIDFD | SIGCHLD;
+
+#if defined(__s390x__) || defined(__s390__) || defined(__CRIS__)
+ /*
+ * On s390/s390x and cris the order of the first and second arguments
+ * of the system call is reversed.
+ */
+ return (int)syscall(__NR_clone, NULL, flags);
+#elif defined(__sparc__) && defined(__arch64__)
+ {
+ /*
+ * sparc64 always returns the other process id in %o0, and a
+ * boolean flag whether this is the child or the parent in %o1.
+ * Inline assembly is needed to get the flag returned in %o1.
+ */
+ int child_pid, in_child;
+
+ asm volatile("mov %2, %%g1\n\t"
+ "mov %3, %%o0\n\t"
+ "mov 0 , %%o1\n\t"
+ "t 0x6d\n\t"
+ "mov %%o1, %0\n\t"
+ "mov %%o0, %1"
+ : "=r"(in_child), "=r"(child_pid)
+ : "i"(__NR_clone), "r"(flags)
+ : "%o1", "%o0", "%g1");
+
+ if (in_child)
+ return 0;
+ else
+ return child_pid;
+ }
+#elif defined(__ia64__)
+ /* On ia64 stack and stack size are passed as separate arguments. */
+ return (int)syscall(__NR_clone, flags, NULL, 0UL);
+#else
+ return (int)syscall(__NR_clone, flags, NULL);
+#endif
+}
+
+static inline int sys_pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
+ unsigned int flags)
+{
+ return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
+}
+
+static int pidfd_metadata_fd(int pidfd)
+{
+ int procfd, ret;
+ char path[100];
+ FILE *f;
+ size_t n = 0;
+ char *line = NULL;
+
+ snprintf(path, sizeof(path), "/proc/self/fdinfo/%d", pidfd);
+
+ f = fopen(path, "re");
+ if (!f)
+ return -1;
+
+ ret = 0;
+ while (getline(&line, &n, f) != -1) {
+ char *numstr;
+ size_t len;
+
+ if (strncmp(line, "Pid:\t", 5))
+ continue;
+
+ numstr = line + 5;
+ len = strlen(numstr);
+ if (len > 0 && numstr[len - 1] == '\n')
+ numstr[len - 1] = '\0';
+ ret = snprintf(path, sizeof(path), "/proc/%s", numstr);
+ break;
+ }
+ free(line);
+ fclose(f);
+
+ if (!ret) {
+ errno = ENOENT;
+ warn("Failed to parse pid from fdinfo\n");
+ return -1;
+ }
+
+ procfd = open(path, O_DIRECTORY | O_RDONLY | O_CLOEXEC);
+ if (procfd < 0) {
+ warn("Failed to open %s\n", path);
+ return -1;
+ }
+
+ /*
+ * Verify that the pid has not been recycled and our /proc/<pid> handle
+ * is still valid.
+ */
+ ret = sys_pidfd_send_signal(pidfd, 0, NULL, 0);
+ if (ret < 0) {
+ switch (errno) {
+ case EPERM:
+ /* Process exists, just not allowed to signal it. */
+ break;
+ default:
+ warn("Failed to signal process\n");
+ close(procfd);
+ procfd = -1;
+ }
+ }
+
+ return procfd;
+}
+
+int main(int argc, char *argv[])
+{
+ int ret = EXIT_FAILURE;
+ char buf[4096] = { 0 };
+ int pidfd, procfd, statusfd;
+ ssize_t bytes;
+
+ pidfd = raw_clone_pidfd();
+ if (pidfd < 0)
+ exit(ret);
+
+ if (pidfd == 0) {
+ printf("%d\n", getpid());
+ exit(EXIT_SUCCESS);
+ }
+
+ procfd = pidfd_metadata_fd(pidfd);
+ close(pidfd);
+ if (procfd < 0)
+ goto out;
+
+ statusfd = openat(procfd, "status", O_RDONLY | O_CLOEXEC);
+ close(procfd);
+ if (statusfd < 0)
+ goto out;
+
+ bytes = read(statusfd, buf, sizeof(buf));
+ if (bytes > 0)
+ bytes = write(STDOUT_FILENO, buf, bytes);
+ close(statusfd);
+ ret = EXIT_SUCCESS;
+
+out:
+ (void)wait(NULL);
+
+ exit(ret);
+}
--
2.21.0
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