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>

fs/pidfs.c

@@ -2,6 +2,7 @@
 #include <linux/anon_inodes.h>
 #include <linux/file.h>
 #include <linux/fs.h>
+#include <linux/cgroup.h>
 #include <linux/magic.h>
 #include <linux/mount.h>
 #include <linux/pid.h>
@@ -114,6 +115,81 @@ static __poll_t pidfd_poll(struct file *file, struct poll_table_struct *pts)
 	return poll_flags;
 }
 
+static long pidfd_info(struct task_struct *task, unsigned int cmd, unsigned long arg)
+{
+	struct pidfd_info __user *uinfo = (struct pidfd_info __user *)arg;
+	size_t usize = _IOC_SIZE(cmd);
+	struct pidfd_info kinfo = {};
+	struct user_namespace *user_ns;
+	const struct cred *c;
+	__u64 mask;
+#ifdef CONFIG_CGROUPS
+	struct cgroup *cgrp;
+#endif
+
+	if (!uinfo)
+		return -EINVAL;
+	if (usize < PIDFD_INFO_SIZE_VER0)
+		return -EINVAL; /* First version, no smaller struct possible */
+
+	if (copy_from_user(&mask, &uinfo->mask, sizeof(mask)))
+		return -EFAULT;
+
+	c = get_task_cred(task);
+	if (!c)
+		return -ESRCH;
+
+	/* Unconditionally return identifiers and credentials, the rest only on request */
+
+	user_ns = current_user_ns();
+	kinfo.ruid = from_kuid_munged(user_ns, c->uid);
+	kinfo.rgid = from_kgid_munged(user_ns, c->gid);
+	kinfo.euid = from_kuid_munged(user_ns, c->euid);
+	kinfo.egid = from_kgid_munged(user_ns, c->egid);
+	kinfo.suid = from_kuid_munged(user_ns, c->suid);
+	kinfo.sgid = from_kgid_munged(user_ns, c->sgid);
+	kinfo.fsuid = from_kuid_munged(user_ns, c->fsuid);
+	kinfo.fsgid = from_kgid_munged(user_ns, c->fsgid);
+	kinfo.mask |= PIDFD_INFO_CREDS;
+	put_cred(c);
+
+#ifdef CONFIG_CGROUPS
+	rcu_read_lock();
+	cgrp = task_dfl_cgroup(task);
+	kinfo.cgroupid = cgroup_id(cgrp);
+	kinfo.mask |= PIDFD_INFO_CGROUPID;
+	rcu_read_unlock();
+#endif
+
+	/*
+	 * Copy pid/tgid last, to reduce the chances the information might be
+	 * stale. Note that it is not possible to ensure it will be valid as the
+	 * task might return as soon as the copy_to_user finishes, but that's ok
+	 * and userspace expects that might happen and can act accordingly, so
+	 * this is just best-effort. What we can do however is checking that all
+	 * the fields are set correctly, or return ESRCH to avoid providing
+	 * incomplete information. */
+
+	kinfo.ppid = task_ppid_nr_ns(task, NULL);
+	kinfo.tgid = task_tgid_vnr(task);
+	kinfo.pid = task_pid_vnr(task);
+	kinfo.mask |= PIDFD_INFO_PID;
+
+	if (kinfo.pid == 0 || kinfo.tgid == 0 || (kinfo.ppid == 0 && kinfo.pid != 1))
+		return -ESRCH;
+
+	/*
+	 * If userspace and the kernel have the same struct size it can just
+	 * be copied. If userspace provides an older struct, only the bits that
+	 * userspace knows about will be copied. If userspace provides a new
+	 * struct, only the bits that the kernel knows about will be copied.
+	 */
+	if (copy_to_user(uinfo, &kinfo, min(usize, sizeof(kinfo))))
+		return -EFAULT;
+
+	return 0;
+}
+
 static long pidfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {
 	struct task_struct *task __free(put_task) = NULL;
@@ -122,13 +198,17 @@ static long pidfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	struct ns_common *ns_common = NULL;
 	struct pid_namespace *pid_ns;
 
-	if (arg)
-		return -EINVAL;
-
 	task = get_pid_task(pid, PIDTYPE_PID);
 	if (!task)
 		return -ESRCH;
 
+	/* Extensible IOCTL that does not open namespace FDs, take a shortcut */
+	if (_IOC_NR(cmd) == _IOC_NR(PIDFD_GET_INFO))
+		return pidfd_info(task, cmd, arg);
+
+	if (arg)
+		return -EINVAL;
+
 	scoped_guard(task_lock, task) {
 		nsp = task->nsproxy;
 		if (nsp)

include/uapi/linux/pidfd.h

@@ -16,6 +16,55 @@
 #define PIDFD_SIGNAL_THREAD_GROUP	(1UL << 1)
 #define PIDFD_SIGNAL_PROCESS_GROUP	(1UL << 2)
 
+/* Flags for pidfd_info. */
+#define PIDFD_INFO_PID			(1UL << 0) /* Always returned, even if not requested */
+#define PIDFD_INFO_CREDS		(1UL << 1) /* Always returned, even if not requested */
+#define PIDFD_INFO_CGROUPID		(1UL << 2) /* Always returned if available, even if not requested */
+
+#define PIDFD_INFO_SIZE_VER0		64 /* sizeof first published struct */
+
+struct pidfd_info {
+	/*
+	 * This mask is similar to the request_mask in statx(2).
+	 *
+	 * Userspace indicates what extensions or expensive-to-calculate fields
+	 * they want by setting the corresponding bits in mask. The kernel
+	 * will ignore bits that it does not know about.
+	 *
+	 * When filling the structure, the kernel will only set bits
+	 * corresponding to the fields that were actually filled by the kernel.
+	 * This also includes any future extensions that might be automatically
+	 * filled. If the structure size is too small to contain a field
+	 * (requested or not), to avoid confusion the mask will not
+	 * contain a bit for that field.
+	 *
+	 * As such, userspace MUST verify that mask contains the
+	 * corresponding flags after the ioctl(2) returns to ensure that it is
+	 * using valid data.
+	 */
+	__u64 mask;
+	/*
+	 * The information contained in the following fields might be stale at the
+	 * time it is received, as the target process might have exited as soon as
+	 * the IOCTL was processed, and there is no way to avoid that. However, it
+	 * is guaranteed that if the call was successful, then the information was
+	 * correct and referred to the intended process at the time the work was
+	 * performed. */
+	__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];
+};
+
 #define PIDFS_IOCTL_MAGIC 0xFF
 
 #define PIDFD_GET_CGROUP_NAMESPACE            _IO(PIDFS_IOCTL_MAGIC, 1)
@@ -28,5 +77,6 @@
 #define PIDFD_GET_TIME_FOR_CHILDREN_NAMESPACE _IO(PIDFS_IOCTL_MAGIC, 8)
 #define PIDFD_GET_USER_NAMESPACE              _IO(PIDFS_IOCTL_MAGIC, 9)
 #define PIDFD_GET_UTS_NAMESPACE               _IO(PIDFS_IOCTL_MAGIC, 10)
+#define PIDFD_GET_INFO                        _IOWR(PIDFS_IOCTL_MAGIC, 11, struct pidfd_info)
 
 #endif /* _UAPI_LINUX_PIDFD_H */

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 @@ static pid_t get_pid_from_fdinfo_file(int pidfd, const char *key, size_t keylen)
 
 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: