Thanks for providing this example. A new nits below.
On Wed, Apr 10, 2019 at 4:43 PM Christian Brauner <christian@xxxxxxxxxx> wrote:
>
> This is an sample program to show userspace how to get race-free access to
> process metadata from a pidfd.
> It is really not that difficult and instead of burdening the kernel with
> this task by using fds to /proc/<pid> we can simply add a helper to libc
> that does it for the user.
>
> Signed-off-by: Christian Brauner <christian@xxxxxxxxxx>
> Signed-off-by: Jann Horn <jann@xxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: David Howells <dhowells@xxxxxxxxxx>
> Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx>
> Cc: Jonathan Kowalski <bl0pbl33p@xxxxxxxxx>
> Cc: "Dmitry V. Levin" <ldv@xxxxxxxxxxxx>
> Cc: Andy Lutomirsky <luto@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Aleksa Sarai <cyphar@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> ---
> samples/Makefile | 2 +-
> samples/pidfd/Makefile | 6 ++
> samples/pidfd/pidfd-metadata.c | 169 +++++++++++++++++++++++++++++++++
> 3 files changed, 176 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..c46c6c34a012
> --- /dev/null
> +++ b/samples/pidfd/pidfd-metadata.c
> @@ -0,0 +1,169 @@
> +// 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;
> +
> +#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 | SIGCHLD);
> +#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 in_child;
> + int child_pid;
> + 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 | SIGCHLD)
> + : "%o1", "%o0", "%g1");
> +
> + if (in_child)
> + return 0;
> + else
> + return child_pid;
> + }
> +#elif defined(__ia64__)
> + /* On ia64 the stack and stack size are passed as separate arguments. */
> + return (int)syscall(__NR_clone, flags | SIGCHLD, NULL, prctl_arg(0));
> +#else
> + return (int)syscall(__NR_clone, flags | SIGCHLD, 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.
> + */
> + if (sys_pidfd_send_signal(pidfd, 0, NULL, 0) < 0) {
> + /* process does not exist */
> + if (errno == ESRCH) {
> + warn("The pid was recycled\n");
ITYM that the process was reaped.
> + close(procfd);
> + return -1;
> + }
> +
> + /* just not allowed to signal it */
I'd look for EPERM specifically instead of just assuming that any
error indicates that a permission failure. I'd also explicitly state
that EPERM still implies process existence.
> + }
> +
> + return procfd;
> +}
> +
> +int main(int argc, char *argv[])
> +{
> + int procfd, ret = EXIT_FAILURE;
> + ssize_t bytes;
> + char buf[4096] = { 0 };
> +
> + int pidfd = raw_clone_pidfd();
> + if (pidfd < 0)
> + return -1;
> +
> + if (pidfd == 0) {
> + printf("%d\n", getpid());
> + exit(EXIT_SUCCESS);
> + }
> +
> + procfd = pidfd_metadata_fd(pidfd);
> + close(pidfd);
> + if (procfd < 0)
> + goto out;
> +
> + int 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);
> +
> +out:
> + (void)wait(NULL);
> + if (bytes < 0 || ret)
> + exit(EXIT_FAILURE);
> +
> + exit(EXIT_SUCCESS);
> +}
> --
> 2.21.0
>
