1 files changed, 672 insertions, 87 deletions

diff --git a/fs/pidfs.c b/fs/pidfs.c
index 618abb1fa1b8..edc35522d75c 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/anon_inodes.h>
+#include <linux/exportfs.h>
 #include <linux/file.h>
 #include <linux/fs.h>
 #include <linux/cgroup.h>
@@ -19,10 +20,160 @@
 #include <linux/time_namespace.h>
 #include <linux/utsname.h>
 #include <net/net_namespace.h>
+#include <linux/coredump.h>
+#include <linux/xattr.h>
 
 #include "internal.h"
 #include "mount.h"
 
+#define PIDFS_PID_DEAD ERR_PTR(-ESRCH)
+
+static struct kmem_cache *pidfs_attr_cachep __ro_after_init;
+static struct kmem_cache *pidfs_xattr_cachep __ro_after_init;
+
+static struct path pidfs_root_path = {};
+
+void pidfs_get_root(struct path *path)
+{
+	*path = pidfs_root_path;
+	path_get(path);
+}
+
+/*
+ * Stashes information that userspace needs to access even after the
+ * process has been reaped.
+ */
+struct pidfs_exit_info {
+	__u64 cgroupid;
+	__s32 exit_code;
+	__u32 coredump_mask;
+};
+
+struct pidfs_attr {
+	struct simple_xattrs *xattrs;
+	struct pidfs_exit_info __pei;
+	struct pidfs_exit_info *exit_info;
+};
+
+static struct rb_root pidfs_ino_tree = RB_ROOT;
+
+#if BITS_PER_LONG == 32
+static inline unsigned long pidfs_ino(u64 ino)
+{
+	return lower_32_bits(ino);
+}
+
+/* On 32 bit the generation number are the upper 32 bits. */
+static inline u32 pidfs_gen(u64 ino)
+{
+	return upper_32_bits(ino);
+}
+
+#else
+
+/* On 64 bit simply return ino. */
+static inline unsigned long pidfs_ino(u64 ino)
+{
+	return ino;
+}
+
+/* On 64 bit the generation number is 0. */
+static inline u32 pidfs_gen(u64 ino)
+{
+	return 0;
+}
+#endif
+
+static int pidfs_ino_cmp(struct rb_node *a, const struct rb_node *b)
+{
+	struct pid *pid_a = rb_entry(a, struct pid, pidfs_node);
+	struct pid *pid_b = rb_entry(b, struct pid, pidfs_node);
+	u64 pid_ino_a = pid_a->ino;
+	u64 pid_ino_b = pid_b->ino;
+
+	if (pid_ino_a < pid_ino_b)
+		return -1;
+	if (pid_ino_a > pid_ino_b)
+		return 1;
+	return 0;
+}
+
+void pidfs_add_pid(struct pid *pid)
+{
+	static u64 pidfs_ino_nr = 2;
+
+	/*
+	 * On 64 bit nothing special happens. The 64bit number assigned
+	 * to struct pid is the inode number.
+	 *
+	 * On 32 bit the 64 bit number assigned to struct pid is split
+	 * into two 32 bit numbers. The lower 32 bits are used as the
+	 * inode number and the upper 32 bits are used as the inode
+	 * generation number.
+	 *
+	 * On 32 bit pidfs_ino() will return the lower 32 bit. When
+	 * pidfs_ino() returns zero a wrap around happened. When a
+	 * wraparound happens the 64 bit number will be incremented by 2
+	 * so inode numbering starts at 2 again.
+	 *
+	 * On 64 bit comparing two pidfds is as simple as comparing
+	 * inode numbers.
+	 *
+	 * When a wraparound happens on 32 bit multiple pidfds with the
+	 * same inode number are likely to exist (This isn't a problem
+	 * since before pidfs pidfds used the anonymous inode meaning
+	 * all pidfds had the same inode number.). Userspace can
+	 * reconstruct the 64 bit identifier by retrieving both the
+	 * inode number and the inode generation number to compare or
+	 * use file handles.
+	 */
+	if (pidfs_ino(pidfs_ino_nr) == 0)
+		pidfs_ino_nr += 2;
+
+	pid->ino = pidfs_ino_nr;
+	pid->stashed = NULL;
+	pid->attr = NULL;
+	pidfs_ino_nr++;
+
+	write_seqcount_begin(&pidmap_lock_seq);
+	rb_find_add_rcu(&pid->pidfs_node, &pidfs_ino_tree, pidfs_ino_cmp);
+	write_seqcount_end(&pidmap_lock_seq);
+}
+
+void pidfs_remove_pid(struct pid *pid)
+{
+	write_seqcount_begin(&pidmap_lock_seq);
+	rb_erase(&pid->pidfs_node, &pidfs_ino_tree);
+	write_seqcount_end(&pidmap_lock_seq);
+}
+
+void pidfs_free_pid(struct pid *pid)
+{
+	struct pidfs_attr *attr __free(kfree) = no_free_ptr(pid->attr);
+	struct simple_xattrs *xattrs __free(kfree) = NULL;
+
+	/*
+	 * Any dentry must've been wiped from the pid by now.
+	 * Otherwise there's a reference count bug.
+	 */
+	VFS_WARN_ON_ONCE(pid->stashed);
+
+	/*
+	 * This if an error occurred during e.g., task creation that
+	 * causes us to never go through the exit path.
+	 */
+	if (unlikely(!attr))
+		return;
+
+	/* This never had a pidfd created. */
+	if (IS_ERR(attr))
+		return;
+
+	xattrs = no_free_ptr(attr->xattrs);
+	if (xattrs)
+		simple_xattrs_free(xattrs, NULL);
+}
+
 #ifdef CONFIG_PROC_FS
 /**
  * pidfd_show_fdinfo - print information about a pidfd
@@ -96,36 +247,64 @@ static void pidfd_show_fdinfo(struct seq_file *m, struct file *f)
 static __poll_t pidfd_poll(struct file *file, struct poll_table_struct *pts)
 {
 	struct pid *pid = pidfd_pid(file);
-	bool thread = file->f_flags & PIDFD_THREAD;
 	struct task_struct *task;
 	__poll_t poll_flags = 0;
 
 	poll_wait(file, &pid->wait_pidfd, pts);
 	/*
-	 * Depending on PIDFD_THREAD, inform pollers when the thread
-	 * or the whole thread-group exits.
+	 * Don't wake waiters if the thread-group leader exited
+	 * prematurely. They either get notified when the last subthread
+	 * exits or not at all if one of the remaining subthreads execs
+	 * and assumes the struct pid of the old thread-group leader.
 	 */
 	guard(rcu)();
 	task = pid_task(pid, PIDTYPE_PID);
 	if (!task)
 		poll_flags = EPOLLIN | EPOLLRDNORM | EPOLLHUP;
-	else if (task->exit_state && (thread || thread_group_empty(task)))
+	else if (task->exit_state && !delay_group_leader(task))
 		poll_flags = EPOLLIN | EPOLLRDNORM;
 
 	return poll_flags;
 }
 
-static long pidfd_info(struct task_struct *task, unsigned int cmd, unsigned long arg)
+static inline bool pid_in_current_pidns(const struct pid *pid)
+{
+	const struct pid_namespace *ns = task_active_pid_ns(current);
+
+	if (ns->level <= pid->level)
+		return pid->numbers[ns->level].ns == ns;
+
+	return false;
+}
+
+static __u32 pidfs_coredump_mask(unsigned long mm_flags)
+{
+	switch (__get_dumpable(mm_flags)) {
+	case SUID_DUMP_USER:
+		return PIDFD_COREDUMP_USER;
+	case SUID_DUMP_ROOT:
+		return PIDFD_COREDUMP_ROOT;
+	case SUID_DUMP_DISABLE:
+		return PIDFD_COREDUMP_SKIP;
+	default:
+		WARN_ON_ONCE(true);
+	}
+
+	return 0;
+}
+
+static long pidfd_info(struct file *file, unsigned int cmd, unsigned long arg)
 {
 	struct pidfd_info __user *uinfo = (struct pidfd_info __user *)arg;
+	struct pid *pid = pidfd_pid(file);
 	size_t usize = _IOC_SIZE(cmd);
 	struct pidfd_info kinfo = {};
+	struct pidfs_exit_info *exit_info;
 	struct user_namespace *user_ns;
+	struct task_struct *task;
+	struct pidfs_attr *attr;
 	const struct cred *c;
 	__u64 mask;
-#ifdef CONFIG_CGROUPS
-	struct cgroup *cgrp;
-#endif
 
 	if (!uinfo)
 		return -EINVAL;
@@ -135,10 +314,54 @@ static long pidfd_info(struct task_struct *task, unsigned int cmd, unsigned long
 	if (copy_from_user(&mask, &uinfo->mask, sizeof(mask)))
 		return -EFAULT;
 
+	/*
+	 * Restrict information retrieval to tasks within the caller's pid
+	 * namespace hierarchy.
+	 */
+	if (!pid_in_current_pidns(pid))
+		return -ESRCH;
+
+	attr = READ_ONCE(pid->attr);
+	if (mask & PIDFD_INFO_EXIT) {
+		exit_info = READ_ONCE(attr->exit_info);
+		if (exit_info) {
+			kinfo.mask |= PIDFD_INFO_EXIT;
+#ifdef CONFIG_CGROUPS
+			kinfo.cgroupid = exit_info->cgroupid;
+			kinfo.mask |= PIDFD_INFO_CGROUPID;
+#endif
+			kinfo.exit_code = exit_info->exit_code;
+		}
+	}
+
+	if (mask & PIDFD_INFO_COREDUMP) {
+		kinfo.mask |= PIDFD_INFO_COREDUMP;
+		kinfo.coredump_mask = READ_ONCE(attr->__pei.coredump_mask);
+	}
+
+	task = get_pid_task(pid, PIDTYPE_PID);
+	if (!task) {
+		/*
+		 * If the task has already been reaped, only exit
+		 * information is available
+		 */
+		if (!(mask & PIDFD_INFO_EXIT))
+			return -ESRCH;
+
+		goto copy_out;
+	}
+
 	c = get_task_cred(task);
 	if (!c)
 		return -ESRCH;
 
+	if ((kinfo.mask & PIDFD_INFO_COREDUMP) && !(kinfo.coredump_mask)) {
+		task_lock(task);
+		if (task->mm)
+			kinfo.coredump_mask = pidfs_coredump_mask(task->mm->flags);
+		task_unlock(task);
+	}
+
 	/* Unconditionally return identifiers and credentials, the rest only on request */
 
 	user_ns = current_user_ns();
@@ -154,11 +377,15 @@ static long pidfd_info(struct task_struct *task, unsigned int cmd, unsigned long
 	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();
+	if (!kinfo.cgroupid) {
+		struct cgroup *cgrp;
+
+		rcu_read_lock();
+		cgrp = task_dfl_cgroup(task);
+		kinfo.cgroupid = cgroup_id(cgrp);
+		kinfo.mask |= PIDFD_INFO_CGROUPID;
+		rcu_read_unlock();
+	}
 #endif
 
 	/*
@@ -175,36 +402,75 @@ static long pidfd_info(struct task_struct *task, unsigned int cmd, unsigned long
 	kinfo.pid = task_pid_vnr(task);
 	kinfo.mask |= PIDFD_INFO_PID;
 
-	if (kinfo.pid == 0 || kinfo.tgid == 0 || (kinfo.ppid == 0 && kinfo.pid != 1))
+	if (kinfo.pid == 0 || kinfo.tgid == 0)
 		return -ESRCH;
 
+copy_out:
 	/*
 	 * 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 copy_struct_to_user(uinfo, usize, &kinfo, sizeof(kinfo), NULL);
+}
 
-	return 0;
+static bool pidfs_ioctl_valid(unsigned int cmd)
+{
+	switch (cmd) {
+	case FS_IOC_GETVERSION:
+	case PIDFD_GET_CGROUP_NAMESPACE:
+	case PIDFD_GET_IPC_NAMESPACE:
+	case PIDFD_GET_MNT_NAMESPACE:
+	case PIDFD_GET_NET_NAMESPACE:
+	case PIDFD_GET_PID_FOR_CHILDREN_NAMESPACE:
+	case PIDFD_GET_TIME_NAMESPACE:
+	case PIDFD_GET_TIME_FOR_CHILDREN_NAMESPACE:
+	case PIDFD_GET_UTS_NAMESPACE:
+	case PIDFD_GET_USER_NAMESPACE:
+	case PIDFD_GET_PID_NAMESPACE:
+		return true;
+	}
+
+	/* Extensible ioctls require some more careful checks. */
+	switch (_IOC_NR(cmd)) {
+	case _IOC_NR(PIDFD_GET_INFO):
+		/*
+		 * Try to prevent performing a pidfd ioctl when someone
+		 * erronously mistook the file descriptor for a pidfd.
+		 * This is not perfect but will catch most cases.
+		 */
+		return (_IOC_TYPE(cmd) == _IOC_TYPE(PIDFD_GET_INFO));
+	}
+
+	return false;
 }
 
 static long pidfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {
 	struct task_struct *task __free(put_task) = NULL;
 	struct nsproxy *nsp __free(put_nsproxy) = NULL;
-	struct pid *pid = pidfd_pid(file);
 	struct ns_common *ns_common = NULL;
 	struct pid_namespace *pid_ns;
 
-	task = get_pid_task(pid, PIDTYPE_PID);
-	if (!task)
-		return -ESRCH;
+	if (!pidfs_ioctl_valid(cmd))
+		return -ENOIOCTLCMD;
+
+	if (cmd == FS_IOC_GETVERSION) {
+		if (!arg)
+			return -EINVAL;
+
+		__u32 __user *argp = (__u32 __user *)arg;
+		return put_user(file_inode(file)->i_generation, argp);
+	}
 
 	/* 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);
+		return pidfd_info(file, cmd, arg);
+
+	task = get_pid_task(pidfd_pid(file), PIDTYPE_PID);
+	if (!task)
+		return -ESRCH;
 
 	if (arg)
 		return -EINVAL;
@@ -316,42 +582,95 @@ struct pid *pidfd_pid(const struct file *file)
 	return file_inode(file)->i_private;
 }
 
-static struct vfsmount *pidfs_mnt __ro_after_init;
-
-#if BITS_PER_LONG == 32
 /*
- * Provide a fallback mechanism for 32-bit systems so processes remain
- * reliably comparable by inode number even on those systems.
+ * We're called from release_task(). We know there's at least one
+ * reference to struct pid being held that won't be released until the
+ * task has been reaped which cannot happen until we're out of
+ * release_task().
+ *
+ * If this struct pid has at least once been referred to by a pidfd then
+ * pid->attr will be allocated. If not we mark the struct pid as dead so
+ * anyone who is trying to register it with pidfs will fail to do so.
+ * Otherwise we would hand out pidfs for reaped tasks without having
+ * exit information available.
+ *
+ * Worst case is that we've filled in the info and the pid gets freed
+ * right away in free_pid() when no one holds a pidfd anymore. Since
+ * pidfs_exit() currently is placed after exit_task_work() we know that
+ * it cannot be us aka the exiting task holding a pidfd to itself.
  */
-static DEFINE_IDA(pidfd_inum_ida);
-
-static int pidfs_inum(struct pid *pid, unsigned long *ino)
+void pidfs_exit(struct task_struct *tsk)
 {
-	int ret;
+	struct pid *pid = task_pid(tsk);
+	struct pidfs_attr *attr;
+	struct pidfs_exit_info *exit_info;
+#ifdef CONFIG_CGROUPS
+	struct cgroup *cgrp;
+#endif
 
-	ret = ida_alloc_range(&pidfd_inum_ida, RESERVED_PIDS + 1,
-			      UINT_MAX, GFP_ATOMIC);
-	if (ret < 0)
-		return -ENOSPC;
+	might_sleep();
 
-	*ino = ret;
-	return 0;
-}
+	guard(spinlock_irq)(&pid->wait_pidfd.lock);
+	attr = pid->attr;
+	if (!attr) {
+		/*
+		 * No one ever held a pidfd for this struct pid.
+		 * Mark it as dead so no one can add a pidfs
+		 * entry anymore. We're about to be reaped and
+		 * so no exit information would be available.
+		 */
+		pid->attr = PIDFS_PID_DEAD;
+		return;
+	}
 
-static inline void pidfs_free_inum(unsigned long ino)
-{
-	if (ino > 0)
-		ida_free(&pidfd_inum_ida, ino);
+	/*
+	 * If @pid->attr is set someone might still legitimately hold a
+	 * pidfd to @pid or someone might concurrently still be getting
+	 * a reference to an already stashed dentry from @pid->stashed.
+	 * So defer cleaning @pid->attr until the last reference to @pid
+	 * is put
+	 */
+
+	exit_info = &attr->__pei;
+
+#ifdef CONFIG_CGROUPS
+	rcu_read_lock();
+	cgrp = task_dfl_cgroup(tsk);
+	exit_info->cgroupid = cgroup_id(cgrp);
+	rcu_read_unlock();
+#endif
+	exit_info->exit_code = tsk->exit_code;
+
+	/* Ensure that PIDFD_GET_INFO sees either all or nothing. */
+	smp_store_release(&attr->exit_info, &attr->__pei);
 }
-#else
-static inline int pidfs_inum(struct pid *pid, unsigned long *ino)
+
+#ifdef CONFIG_COREDUMP
+void pidfs_coredump(const struct coredump_params *cprm)
 {
-	*ino = pid->ino;
-	return 0;
+	struct pid *pid = cprm->pid;
+	struct pidfs_exit_info *exit_info;
+	struct pidfs_attr *attr;
+	__u32 coredump_mask = 0;
+
+	attr = READ_ONCE(pid->attr);
+
+	VFS_WARN_ON_ONCE(!attr);
+	VFS_WARN_ON_ONCE(attr == PIDFS_PID_DEAD);
+
+	exit_info = &attr->__pei;
+	/* Note how we were coredumped. */
+	coredump_mask = pidfs_coredump_mask(cprm->mm_flags);
+	/* Note that we actually did coredump. */
+	coredump_mask |= PIDFD_COREDUMPED;
+	/* If coredumping is set to skip we should never end up here. */
+	VFS_WARN_ON_ONCE(coredump_mask & PIDFD_COREDUMP_SKIP);
+	smp_store_release(&exit_info->coredump_mask, coredump_mask);
 }
-#define pidfs_free_inum(ino) ((void)(ino))
 #endif
 
+static struct vfsmount *pidfs_mnt __ro_after_init;
+
 /*
  * The vfs falls back to simple_setattr() if i_op->setattr() isn't
  * implemented. Let's reject it completely until we have a clean
@@ -360,41 +679,34 @@ static inline int pidfs_inum(struct pid *pid, unsigned long *ino)
 static int pidfs_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 			 struct iattr *attr)
 {
-	return -EOPNOTSUPP;
+	return anon_inode_setattr(idmap, dentry, attr);
 }
 
-
-/*
- * User space expects pidfs inodes to have no file type in st_mode.
- *
- * In particular, 'lsof' has this legacy logic:
- *
- *	type = s->st_mode & S_IFMT;
- *	switch (type) {
- *	  ...
- *	case 0:
- *		if (!strcmp(p, "anon_inode"))
- *			Lf->ntype = Ntype = N_ANON_INODE;
- *
- * to detect our old anon_inode logic.
- *
- * Rather than mess with our internal sane inode data, just fix it
- * up here in getattr() by masking off the format bits.
- */
 static int pidfs_getattr(struct mnt_idmap *idmap, const struct path *path,
 			 struct kstat *stat, u32 request_mask,
 			 unsigned int query_flags)
 {
-	struct inode *inode = d_inode(path->dentry);
+	return anon_inode_getattr(idmap, path, stat, request_mask, query_flags);
+}
 
-	generic_fillattr(&nop_mnt_idmap, request_mask, inode, stat);
-	stat->mode &= ~S_IFMT;
-	return 0;
+static ssize_t pidfs_listxattr(struct dentry *dentry, char *buf, size_t size)
+{
+	struct inode *inode = d_inode(dentry);
+	struct pid *pid = inode->i_private;
+	struct pidfs_attr *attr = pid->attr;
+	struct simple_xattrs *xattrs;
+
+	xattrs = READ_ONCE(attr->xattrs);
+	if (!xattrs)
+		return 0;
+
+	return simple_xattr_list(inode, xattrs, buf, size);
 }
 
 static const struct inode_operations pidfs_inode_operations = {
-	.getattr = pidfs_getattr,
-	.setattr = pidfs_setattr,
+	.getattr	= pidfs_getattr,
+	.setattr	= pidfs_setattr,
+	.listxattr	= pidfs_listxattr,
 };
 
 static void pidfs_evict_inode(struct inode *inode)
@@ -403,7 +715,6 @@ static void pidfs_evict_inode(struct inode *inode)
 
 	clear_inode(inode);
 	put_pid(pid);
-	pidfs_free_inum(inode->i_ino);
 }
 
 static const struct super_operations pidfs_sops = {
@@ -421,25 +732,152 @@ static char *pidfs_dname(struct dentry *dentry, char *buffer, int buflen)
 	return dynamic_dname(buffer, buflen, "anon_inode:[pidfd]");
 }
 
-static const struct dentry_operations pidfs_dentry_operations = {
-	.d_delete	= always_delete_dentry,
+const struct dentry_operations pidfs_dentry_operations = {
 	.d_dname	= pidfs_dname,
 	.d_prune	= stashed_dentry_prune,
 };
 
+static int pidfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
+			   struct inode *parent)
+{
+	const struct pid *pid = inode->i_private;
+
+	if (*max_len < 2) {
+		*max_len = 2;
+		return FILEID_INVALID;
+	}
+
+	*max_len = 2;
+	*(u64 *)fh = pid->ino;
+	return FILEID_KERNFS;
+}
+
+static int pidfs_ino_find(const void *key, const struct rb_node *node)
+{
+	const u64 pid_ino = *(u64 *)key;
+	const struct pid *pid = rb_entry(node, struct pid, pidfs_node);
+
+	if (pid_ino < pid->ino)
+		return -1;
+	if (pid_ino > pid->ino)
+		return 1;
+	return 0;
+}
+
+/* Find a struct pid based on the inode number. */
+static struct pid *pidfs_ino_get_pid(u64 ino)
+{
+	struct pid *pid;
+	struct rb_node *node;
+	unsigned int seq;
+
+	guard(rcu)();
+	do {
+		seq = read_seqcount_begin(&pidmap_lock_seq);
+		node = rb_find_rcu(&ino, &pidfs_ino_tree, pidfs_ino_find);
+		if (node)
+			break;
+	} while (read_seqcount_retry(&pidmap_lock_seq, seq));
+
+	if (!node)
+		return NULL;
+
+	pid = rb_entry(node, struct pid, pidfs_node);
+
+	/* Within our pid namespace hierarchy? */
+	if (pid_vnr(pid) == 0)
+		return NULL;
+
+	return get_pid(pid);
+}
+
+static struct dentry *pidfs_fh_to_dentry(struct super_block *sb,
+					 struct fid *fid, int fh_len,
+					 int fh_type)
+{
+	int ret;
+	u64 pid_ino;
+	struct path path;
+	struct pid *pid;
+
+	if (fh_len < 2)
+		return NULL;
+
+	switch (fh_type) {
+	case FILEID_KERNFS:
+		pid_ino = *(u64 *)fid;
+		break;
+	default:
+		return NULL;
+	}
+
+	pid = pidfs_ino_get_pid(pid_ino);
+	if (!pid)
+		return NULL;
+
+	ret = path_from_stashed(&pid->stashed, pidfs_mnt, pid, &path);
+	if (ret < 0)
+		return ERR_PTR(ret);
+
+	VFS_WARN_ON_ONCE(!pid->attr);
+
+	mntput(path.mnt);
+	return path.dentry;
+}
+
+/*
+ * Make sure that we reject any nonsensical flags that users pass via
+ * open_by_handle_at(). Note that PIDFD_THREAD is defined as O_EXCL, and
+ * PIDFD_NONBLOCK as O_NONBLOCK.
+ */
+#define VALID_FILE_HANDLE_OPEN_FLAGS \
+	(O_RDONLY | O_WRONLY | O_RDWR | O_NONBLOCK | O_CLOEXEC | O_EXCL)
+
+static int pidfs_export_permission(struct handle_to_path_ctx *ctx,
+				   unsigned int oflags)
+{
+	if (oflags & ~(VALID_FILE_HANDLE_OPEN_FLAGS | O_LARGEFILE))
+		return -EINVAL;
+
+	/*
+	 * pidfd_ino_get_pid() will verify that the struct pid is part
+	 * of the caller's pid namespace hierarchy. No further
+	 * permission checks are needed.
+	 */
+	return 0;
+}
+
+static struct file *pidfs_export_open(struct path *path, unsigned int oflags)
+{
+	/*
+	 * Clear O_LARGEFILE as open_by_handle_at() forces it and raise
+	 * O_RDWR as pidfds always are.
+	 */
+	oflags &= ~O_LARGEFILE;
+	return dentry_open(path, oflags | O_RDWR, current_cred());
+}
+
+static const struct export_operations pidfs_export_operations = {
+	.encode_fh	= pidfs_encode_fh,
+	.fh_to_dentry	= pidfs_fh_to_dentry,
+	.open		= pidfs_export_open,
+	.permission	= pidfs_export_permission,
+};
+
 static int pidfs_init_inode(struct inode *inode, void *data)
 {
+	const struct pid *pid = data;
+
 	inode->i_private = data;
-	inode->i_flags |= S_PRIVATE;
+	inode->i_flags |= S_PRIVATE | S_ANON_INODE;
+	/* We allow to set xattrs. */
+	inode->i_flags &= ~S_IMMUTABLE;
 	inode->i_mode |= S_IRWXU;
 	inode->i_op = &pidfs_inode_operations;
 	inode->i_fop = &pidfs_file_operations;
-	/*
-	 * Inode numbering for pidfs start at RESERVED_PIDS + 1. This
-	 * avoids collisions with the root inode which is 1 for pseudo
-	 * filesystems.
-	 */
-	return pidfs_inum(data, &inode->i_ino);
+	inode->i_ino = pidfs_ino(pid->ino);
+	inode->i_generation = pidfs_gen(pid->ino);
+	return 0;
 }
 
 static void pidfs_put_data(void *data)
@@ -448,9 +886,127 @@ static void pidfs_put_data(void *data)
 	put_pid(pid);
 }
 
+/**
+ * pidfs_register_pid - register a struct pid in pidfs
+ * @pid: pid to pin
+ *
+ * Register a struct pid in pidfs.
+ *
+ * Return: On success zero, on error a negative error code is returned.
+ */
+int pidfs_register_pid(struct pid *pid)
+{
+	struct pidfs_attr *new_attr __free(kfree) = NULL;
+	struct pidfs_attr *attr;
+
+	might_sleep();
+
+	if (!pid)
+		return 0;
+
+	attr = READ_ONCE(pid->attr);
+	if (unlikely(attr == PIDFS_PID_DEAD))
+		return PTR_ERR(PIDFS_PID_DEAD);
+	if (attr)
+		return 0;
+
+	new_attr = kmem_cache_zalloc(pidfs_attr_cachep, GFP_KERNEL);
+	if (!new_attr)
+		return -ENOMEM;
+
+	/* Synchronize with pidfs_exit(). */
+	guard(spinlock_irq)(&pid->wait_pidfd.lock);
+
+	attr = pid->attr;
+	if (unlikely(attr == PIDFS_PID_DEAD))
+		return PTR_ERR(PIDFS_PID_DEAD);
+	if (unlikely(attr))
+		return 0;
+
+	pid->attr = no_free_ptr(new_attr);
+	return 0;
+}
+
+static struct dentry *pidfs_stash_dentry(struct dentry **stashed,
+					 struct dentry *dentry)
+{
+	int ret;
+	struct pid *pid = d_inode(dentry)->i_private;
+
+	VFS_WARN_ON_ONCE(stashed != &pid->stashed);
+
+	ret = pidfs_register_pid(pid);
+	if (ret)
+		return ERR_PTR(ret);
+
+	return stash_dentry(stashed, dentry);
+}
+
 static const struct stashed_operations pidfs_stashed_ops = {
-	.init_inode = pidfs_init_inode,
-	.put_data = pidfs_put_data,
+	.stash_dentry	= pidfs_stash_dentry,
+	.init_inode	= pidfs_init_inode,
+	.put_data	= pidfs_put_data,
+};
+
+static int pidfs_xattr_get(const struct xattr_handler *handler,
+			   struct dentry *unused, struct inode *inode,
+			   const char *suffix, void *value, size_t size)
+{
+	struct pid *pid = inode->i_private;
+	struct pidfs_attr *attr = pid->attr;
+	const char *name;
+	struct simple_xattrs *xattrs;
+
+	xattrs = READ_ONCE(attr->xattrs);
+	if (!xattrs)
+		return 0;
+
+	name = xattr_full_name(handler, suffix);
+	return simple_xattr_get(xattrs, name, value, size);
+}
+
+static int pidfs_xattr_set(const struct xattr_handler *handler,
+			   struct mnt_idmap *idmap, struct dentry *unused,
+			   struct inode *inode, const char *suffix,
+			   const void *value, size_t size, int flags)
+{
+	struct pid *pid = inode->i_private;
+	struct pidfs_attr *attr = pid->attr;
+	const char *name;
+	struct simple_xattrs *xattrs;
+	struct simple_xattr *old_xattr;
+
+	/* Ensure we're the only one to set @attr->xattrs. */
+	WARN_ON_ONCE(!inode_is_locked(inode));
+
+	xattrs = READ_ONCE(attr->xattrs);
+	if (!xattrs) {
+		xattrs = kmem_cache_zalloc(pidfs_xattr_cachep, GFP_KERNEL);
+		if (!xattrs)
+			return -ENOMEM;
+
+		simple_xattrs_init(xattrs);
+		smp_store_release(&pid->attr->xattrs, xattrs);
+	}
+
+	name = xattr_full_name(handler, suffix);
+	old_xattr = simple_xattr_set(xattrs, name, value, size, flags);
+	if (IS_ERR(old_xattr))
+		return PTR_ERR(old_xattr);
+
+	simple_xattr_free(old_xattr);
+	return 0;
+}
+
+static const struct xattr_handler pidfs_trusted_xattr_handler = {
+	.prefix = XATTR_TRUSTED_PREFIX,
+	.get	= pidfs_xattr_get,
+	.set	= pidfs_xattr_set,
+};
+
+static const struct xattr_handler *const pidfs_xattr_handlers[] = {
+	&pidfs_trusted_xattr_handler,
+	NULL
 };
 
 static int pidfs_init_fs_context(struct fs_context *fc)
@@ -461,8 +1017,12 @@ static int pidfs_init_fs_context(struct fs_context *fc)
 	if (!ctx)
 		return -ENOMEM;
 
+	fc->s_iflags |= SB_I_NOEXEC;
+	fc->s_iflags |= SB_I_NODEV;
 	ctx->ops = &pidfs_sops;
+	ctx->eops = &pidfs_export_operations;
 	ctx->dops = &pidfs_dentry_operations;
+	ctx->xattr = pidfs_xattr_handlers;
 	fc->s_fs_info = (void *)&pidfs_stashed_ops;
 	return 0;
 }
@@ -475,23 +1035,48 @@ static struct file_system_type pidfs_type = {
 
 struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
 {
-
 	struct file *pidfd_file;
-	struct path path;
+	struct path path __free(path_put) = {};
 	int ret;
 
+	/*
+	 * Ensure that PIDFD_STALE can be passed as a flag without
+	 * overloading other uapi pidfd flags.
+	 */
+	BUILD_BUG_ON(PIDFD_STALE == PIDFD_THREAD);
+	BUILD_BUG_ON(PIDFD_STALE == PIDFD_NONBLOCK);
+
 	ret = path_from_stashed(&pid->stashed, pidfs_mnt, get_pid(pid), &path);
 	if (ret < 0)
 		return ERR_PTR(ret);
 
+	VFS_WARN_ON_ONCE(!pid->attr);
+
+	flags &= ~PIDFD_STALE;
+	flags |= O_RDWR;
 	pidfd_file = dentry_open(&path, flags, current_cred());
-	path_put(&path);
+	/* Raise PIDFD_THREAD explicitly as do_dentry_open() strips it. */
+	if (!IS_ERR(pidfd_file))
+		pidfd_file->f_flags |= (flags & PIDFD_THREAD);
+
 	return pidfd_file;
 }
 
 void __init pidfs_init(void)
 {
+	pidfs_attr_cachep = kmem_cache_create("pidfs_attr_cache", sizeof(struct pidfs_attr), 0,
+					 (SLAB_HWCACHE_ALIGN | SLAB_RECLAIM_ACCOUNT |
+					  SLAB_ACCOUNT | SLAB_PANIC), NULL);
+
+	pidfs_xattr_cachep = kmem_cache_create("pidfs_xattr_cache",
+					       sizeof(struct simple_xattrs), 0,
+					       (SLAB_HWCACHE_ALIGN | SLAB_RECLAIM_ACCOUNT |
+						SLAB_ACCOUNT | SLAB_PANIC), NULL);
+
 	pidfs_mnt = kern_mount(&pidfs_type);
 	if (IS_ERR(pidfs_mnt))
 		panic("Failed to mount pidfs pseudo filesystem");
+
+	pidfs_root_path.mnt = pidfs_mnt;
+	pidfs_root_path.dentry = pidfs_mnt->mnt_root;
 }