Message-ID: <20241010155401.2268522-1-luca.boccassi@gmail.com>
Date: Thu, 10 Oct 2024 16:52:32 +0100
From: luca.boccassi@...il.com
To: linux-fsdevel@...r.kernel.org
Cc: christian@...uner.io,
	linux-kernel@...r.kernel.org,
	oleg@...hat.com
Subject: [PATCH v11] pidfd: add ioctl to retrieve pid info

From: Luca Boccassi <luca.boccassi@...il.com>

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@...il.com>
---
v11: add back flags for PID and CREDS, but always set them unconditionally
     always return cgroupid if possible
     update comments
v10: do not fail if we cannot get cgroupid, simply skip it (and document it)
v9: drop result_mask and reuse request_mask instead
v8: use RAII guard for rcu, call put_cred()
v7: fix RCU issue and style issue introduced by v6 found by reviewer
v6: use rcu_read_lock() when fetching cgroupid, use task_ppid_nr_ns() to
    get the ppid, return ESCHR if any of pid/tgid/ppid are 0 at the end
    of the call to avoid providing incomplete data, document what the
    callers should expect
v5: check again that the task hasn't exited immediately before copying
    the result out to userspace, to ensure we are not returning stale data
    add an ifdef around the cgroup structs usage to fix build errors when
    the feature is disabled
v4: fix arg check in pidfd_ioctl() by moving it after the new call
v3: switch from pid_vnr() to task_pid_vnr()
v2: Apply comments from Christian, apart from the one about pid namespaces
    as I need additional hints on how to implement it.
    Drop the security_context string as it is not the appropriate
    metadata to give userspace these days.

 fs/pidfs.c                                    | 92 ++++++++++++++++++-
 include/uapi/linux/pidfd.h                    | 48 ++++++++++
 .../testing/selftests/pidfd/pidfd_open_test.c | 82 ++++++++++++++++-
 3 files changed, 218 insertions(+), 4 deletions(-)

diff --git a/fs/pidfs.c b/fs/pidfs.c
index 80675b6bf884..711de3fddc09 100644
--- a/fs/pidfs.c
+++ b/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 +115,87 @@ 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 request_mask;
+#ifdef CONFIG_CGROUPS
+	struct cgroup *cgrp;
+#endif
+
+	if (!uinfo)
+		return -EINVAL;
+	if (usize < sizeof(struct pidfd_info))
+		return -EINVAL; /* First version, no smaller struct possible */
+
+	if (copy_from_user(&request_mask, &uinfo->request_mask, sizeof(request_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.request_mask |= PIDFD_INFO_CREDS;
+	put_cred(c);
+
+#ifdef CONFIG_CGROUPS
+	/*
+	 * The cgroup id cannot be retrieved anymore after the task has exited
+	 * (even if it has not been reaped yet), contrary to other fields. Set
+	 * the flag only if we can still access it. */
+	rcu_read_lock();
+	cgrp = task_cgroup(task, pids_cgrp_id);
+	if (cgrp) {
+		kinfo.cgroupid = cgroup_id(cgrp);
+		kinfo.request_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.request_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 +204,17 @@ 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)
diff --git a/include/uapi/linux/pidfd.h b/include/uapi/linux/pidfd.h
index 565fc0629fff..5ee51f234a87 100644
--- a/include/uapi/linux/pidfd.h
+++ b/include/uapi/linux/pidfd.h
@@ -16,6 +16,53 @@
 #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 */
+
+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 request_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 request_mask will not
+	 * contain a bit for that field.
+	 *
+	 * As such, userspace MUST verify that request_mask contains the
+	 * corresponding flags after the ioctl(2) returns to ensure that it is
+	 * using valid data.
+	 */
+	__u64 request_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 +75,6 @@
 #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 */
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 @@ 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:

base-commit: 8cf0b93919e13d1e8d4466eb4080a4c4d9d66d7b
-- 
2.45.2

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