pidfd: add ioctl to retrieve pid info

A common pattern when using pid fds is having to get information
about the process, which currently requires /proc being mounted,
resolving the fd to a pid, and then do manual string parsing of
/proc/N/status and friends. This needs to be reimplemented over
and over in all userspace projects (e.g.: I have reimplemented
resolving in systemd, dbus, dbus-daemon, polkit so far), and
requires additional care in checking that the fd is still valid
after having parsed the data, to avoid races.

Having a programmatic API that can be used directly removes all
these requirements, including having /proc mounted.

As discussed at LPC24, add an ioctl with an extensible struct
so that more parameters can be added later if needed. Start with
returning pid/tgid/ppid and creds unconditionally, and cgroupid
optionally.

Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>
Link: https://lore.kernel.org/r/20241010155401.2268522-1-luca.boccassi@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 81 insertions, 1 deletions

diff --git a/tools/testing/selftests/pidfd/pidfd_open_test.c b/tools/testing/selftests/pidfd/pidfd_open_test.c
index c62564c264b1..ce413a221bac 100644
--- a/tools/testing/selftests/pidfd/pidfd_open_test.c
+++ b/tools/testing/selftests/pidfd/pidfd_open_test.c
@@ -13,6 +13,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <syscall.h>
+#include <sys/ioctl.h>
 #include <sys/mount.h>
 #include <sys/prctl.h>
 #include <sys/wait.h>
@@ -21,6 +22,32 @@
 #include "pidfd.h"
 #include "../kselftest.h"
 
+#ifndef PIDFS_IOCTL_MAGIC
+#define PIDFS_IOCTL_MAGIC 0xFF
+#endif
+
+#ifndef PIDFD_GET_INFO
+#define PIDFD_GET_INFO _IOWR(PIDFS_IOCTL_MAGIC, 11, struct pidfd_info)
+#define PIDFD_INFO_CGROUPID		(1UL << 0)
+
+struct pidfd_info {
+	__u64 request_mask;
+	__u64 cgroupid;
+	__u32 pid;
+	__u32 tgid;
+	__u32 ppid;
+	__u32 ruid;
+	__u32 rgid;
+	__u32 euid;
+	__u32 egid;
+	__u32 suid;
+	__u32 sgid;
+	__u32 fsuid;
+	__u32 fsgid;
+	__u32 spare0[1];
+};
+#endif
+
 static int safe_int(const char *numstr, int *converted)
 {
 	char *err = NULL;
@@ -120,10 +147,13 @@ out:
 
 int main(int argc, char **argv)
 {
+	struct pidfd_info info = {
+		.request_mask = PIDFD_INFO_CGROUPID,
+	};
 	int pidfd = -1, ret = 1;
 	pid_t pid;
 
-	ksft_set_plan(3);
+	ksft_set_plan(4);
 
 	pidfd = sys_pidfd_open(-1, 0);
 	if (pidfd >= 0) {
@@ -153,6 +183,56 @@ int main(int argc, char **argv)
 	pid = get_pid_from_fdinfo_file(pidfd, "Pid:", sizeof("Pid:") - 1);
 	ksft_print_msg("pidfd %d refers to process with pid %d\n", pidfd, pid);
 
+	if (ioctl(pidfd, PIDFD_GET_INFO, &info) < 0) {
+		ksft_print_msg("%s - failed to get info from pidfd\n", strerror(errno));
+		goto on_error;
+	}
+	if (info.pid != pid) {
+		ksft_print_msg("pid from fdinfo file %d does not match pid from ioctl %d\n",
+			       pid, info.pid);
+		goto on_error;
+	}
+	if (info.ppid != getppid()) {
+		ksft_print_msg("ppid %d does not match ppid from ioctl %d\n",
+			       pid, info.pid);
+		goto on_error;
+	}
+	if (info.ruid != getuid()) {
+		ksft_print_msg("uid %d does not match uid from ioctl %d\n",
+			       getuid(), info.ruid);
+		goto on_error;
+	}
+	if (info.rgid != getgid()) {
+		ksft_print_msg("gid %d does not match gid from ioctl %d\n",
+			       getgid(), info.rgid);
+		goto on_error;
+	}
+	if (info.euid != geteuid()) {
+		ksft_print_msg("euid %d does not match euid from ioctl %d\n",
+			       geteuid(), info.euid);
+		goto on_error;
+	}
+	if (info.egid != getegid()) {
+		ksft_print_msg("egid %d does not match egid from ioctl %d\n",
+			       getegid(), info.egid);
+		goto on_error;
+	}
+	if (info.suid != geteuid()) {
+		ksft_print_msg("suid %d does not match suid from ioctl %d\n",
+			       geteuid(), info.suid);
+		goto on_error;
+	}
+	if (info.sgid != getegid()) {
+		ksft_print_msg("sgid %d does not match sgid from ioctl %d\n",
+			       getegid(), info.sgid);
+		goto on_error;
+	}
+	if ((info.request_mask & PIDFD_INFO_CGROUPID) && info.cgroupid == 0) {
+		ksft_print_msg("cgroupid should not be 0 when PIDFD_INFO_CGROUPID is set\n");
+		goto on_error;
+	}
+	ksft_test_result_pass("get info from pidfd test: passed\n");
+
 	ret = 0;
 
 on_error: