Merge patch series "pidfs: persistent info & xattrs"

Christian Brauner <[e-mail]> says:

Persist exit and coredump information independent of whether anyone
currently holds a pidfd for the struct pid.

The current scheme allocated pidfs dentries on-demand repeatedly.
This scheme is reaching it's limits as it makes it impossible to pin
information that needs to be available after the task has exited or
coredumped and that should not be lost simply because the pidfd got
closed temporarily. The next opener should still see the stashed
information.

This is also a prerequisite for supporting extended attributes on
pidfds to allow attaching meta information to them.

If someone opens a pidfd for a struct pid a pidfs dentry is allocated
and stashed in pid->stashed. Once the last pidfd for the struct pid is
closed the pidfs dentry is released and removed from pid->stashed.

So if 10 callers create a pidfs dentry for the same struct pid
sequentially, i.e., each closing the pidfd before the other creates a
new one then a new pidfs dentry is allocated every time.

Because multiple tasks acquiring and releasing a pidfd for the same
struct pid can race with each another a task may still find a valid
pidfs entry from the previous task in pid->stashed and reuse it. Or it
might find a dead dentry in there and fail to reuse it and so stashes a
new pidfs dentry. Multiple tasks may race to stash a new pidfs dentry
but only one will succeed, the other ones will put their dentry.

The current scheme aims to ensure that a pidfs dentry for a struct pid
can only be created if the task is still alive or if a pidfs dentry
already existed before the task was reaped and so exit information has
been was stashed in the pidfs inode.

That's great except that it's buggy. If a pidfs dentry is stashed in
pid->stashed after pidfs_exit() but before __unhash_process() is called
we will return a pidfd for a reaped task without exit information being
available.

The pidfds_pid_valid() check does not guard against this race as it
doens't sync at all with pidfs_exit(). The pid_has_task() check might be
successful simply because we're before __unhash_process() but after
pidfs_exit().

Introduce a new scheme where the lifetime of information associated with
a pidfs entry (coredump and exit information) isn't bound to the
lifetime of the pidfs inode but the struct pid itself.

The first time a pidfs dentry is allocated for a struct pid a struct
pidfs_attr will be allocated which will be used to store exit and
coredump information.

If all pidfs for the pidfs dentry are closed the dentry and inode can be
cleaned up but the struct pidfs_attr will stick until the struct pid
itself is freed. This will ensure minimal memory usage while persisting
relevant information.

The new scheme has various advantages. First, it allows to close the
race where we end up handing out a pidfd for a reaped task for which no
exit information is available. Second, it minimizes memory usage.
Third, it allows to remove complex lifetime tracking via dentries when
registering a struct pid with pidfs. There's no need to get or put a
reference. Instead, the lifetime of exit and coredump information
associated with a struct pid is bound to the lifetime of struct pid
itself.

Now that we have a way to persist information for pidfs dentries we can
start supporting extended attributes on pidfds. This will allow
userspace to attach meta information to tasks.

One natural extension would be to introduce a custom pidfs.* extended
attribute space and allow for the inheritance of extended attributes
across fork() and exec().

The first simple scheme will allow privileged userspace to set trusted
extended attributes on pidfs inodes.

* patches from https://lore.kernel.org/[e-mail]:
pidfs: add some CONFIG_DEBUG_VFS asserts
selftests/pidfd: test setattr support
selftests/pidfd: test extended attribute support
selftests/pidfd: test extended attribute support
pidfs: support xattrs on pidfds
pidfs: make inodes mutable
libfs: prepare to allow for non-immutable pidfd inodes
pidfs: remove pidfs_pid_valid()
pidfs: remove pidfs_{get,put}_pid()
pidfs: remove custom inode allocation
pidfs: remove unused members from struct pidfs_inode
pidfs: persist information
pidfs: move to anonymous struct
libfs: massage path_from_stashed()
libfs: massage path_from_stashed() to allow custom stashing behavior
pidfs: raise SB_I_NODEV and SB_I_NOEXEC

Link: https://lore.kernel.org/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

fs/coredump.c

···
 		retval = kernel_connect(socket, (struct sockaddr *)(&addr),
 					addr_len, O_NONBLOCK | SOCK_COREDUMP);
 
-		/*
-		 * ... Make sure to only put our reference after connect() took
-		 * its own reference keeping the pidfs entry alive ...
-		 */
-		pidfs_put_pid(cprm.pid);
-
 		if (retval) {
 			if (retval == -EAGAIN)
 				coredump_report_failure("Coredump socket %s receive queue full", addr.sun_path);

fs/internal.h

···
 struct mnt_idmap *mnt_idmap_get(struct mnt_idmap *idmap);
 void mnt_idmap_put(struct mnt_idmap *idmap);
 struct stashed_operations {
+	struct dentry *(*stash_dentry)(struct dentry **stashed,
+				       struct dentry *dentry);
 	void (*put_data)(void *data);
 	int (*init_inode)(struct inode *inode, void *data);
 };
 int path_from_stashed(struct dentry **stashed, struct vfsmount *mnt, void *data,
 		      struct path *path);
 void stashed_dentry_prune(struct dentry *dentry);
+struct dentry *stash_dentry(struct dentry **stashed, struct dentry *dentry);
 struct dentry *stashed_dentry_get(struct dentry **stashed);
 /**
  * path_mounted - check whether path is mounted

fs/libfs.c

···
 	dentry = rcu_dereference(*stashed);
 	if (!dentry)
 		return NULL;
+	if (IS_ERR(dentry))
+		return dentry;
 	if (!lockref_get_not_dead(&dentry->d_lockref))
 		return NULL;
 	return dentry;
···
 
 	/* Notice when this is changed. */
 	WARN_ON_ONCE(!S_ISREG(inode->i_mode));
-	WARN_ON_ONCE(!IS_IMMUTABLE(inode));
 
 	dentry = d_alloc_anon(sb);
 	if (!dentry) {
···
 	return dentry;
 }
 
-static struct dentry *stash_dentry(struct dentry **stashed,
-				   struct dentry *dentry)
+struct dentry *stash_dentry(struct dentry **stashed, struct dentry *dentry)
 {
 	guard(rcu)();
 	for (;;) {
···
 int path_from_stashed(struct dentry **stashed, struct vfsmount *mnt, void *data,
 		      struct path *path)
 {
-	struct dentry *dentry;
+	struct dentry *dentry, *res;
 	const struct stashed_operations *sops = mnt->mnt_sb->s_fs_info;
 
 	/* See if dentry can be reused. */
-	path->dentry = stashed_dentry_get(stashed);
-	if (path->dentry) {
+	res = stashed_dentry_get(stashed);
+	if (IS_ERR(res))
+		return PTR_ERR(res);
+	if (res) {
 		sops->put_data(data);
-		goto out_path;
+		goto make_path;
 	}
 
 	/* Allocate a new dentry. */
···
 		return PTR_ERR(dentry);
 
 	/* Added a new dentry. @data is now owned by the filesystem. */
-	path->dentry = stash_dentry(stashed, dentry);
-	if (path->dentry != dentry)
+	if (sops->stash_dentry)
+		res = sops->stash_dentry(stashed, dentry);
+	else
+		res = stash_dentry(stashed, dentry);
+	if (IS_ERR(res)) {
+		dput(dentry);
+		return PTR_ERR(res);
+	}
+	if (res != dentry)
 		dput(dentry);
 
-out_path:
-	WARN_ON_ONCE(path->dentry->d_fsdata != stashed);
-	WARN_ON_ONCE(d_inode(path->dentry)->i_private != data);
+make_path:
+	path->dentry = res;
 	path->mnt = mntget(mnt);
+	VFS_WARN_ON_ONCE(path->dentry->d_fsdata != stashed);
+	VFS_WARN_ON_ONCE(d_inode(path->dentry)->i_private != data);
 	return 0;
 }
 

fs/pidfs.c

···
 #include <linux/utsname.h>
 #include <net/net_namespace.h>
 #include <linux/coredump.h>
+#include <linux/xattr.h>
 
 #include "internal.h"
 #include "mount.h"
 
-static struct kmem_cache *pidfs_cachep __ro_after_init;
+#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;
 
 /*
  * Stashes information that userspace needs to access even after the
···
 	__u32 coredump_mask;
 };
 
-struct pidfs_inode {
+struct pidfs_attr {
+	struct simple_xattrs *xattrs;
 	struct pidfs_exit_info __pei;
 	struct pidfs_exit_info *exit_info;
-	struct inode vfs_inode;
 };
-
-static inline struct pidfs_inode *pidfs_i(struct inode *inode)
-{
-	return container_of(inode, struct pidfs_inode, vfs_inode);
-}
 
 static struct rb_root pidfs_ino_tree = RB_ROOT;
 
···
 
 	pid->ino = pidfs_ino_nr;
 	pid->stashed = NULL;
+	pid->attr = NULL;
 	pidfs_ino_nr++;
 
 	write_seqcount_begin(&pidmap_lock_seq);
···
 	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);
+
+	if (IS_ERR(attr))
+		return;
+
+	/*
+	 * Any dentry must've been wiped from the pid by now. Otherwise
+	 * there's a reference count bug.
+	 */
+	VFS_WARN_ON_ONCE(pid->stashed);
+
+	xattrs = attr->xattrs;
+	if (xattrs)
+		simple_xattrs_free(attr->xattrs, NULL);
 }
 
 #ifdef CONFIG_PROC_FS
···
 static long pidfd_info(struct file *file, unsigned int cmd, unsigned long arg)
 {
 	struct pidfd_info __user *uinfo = (struct pidfd_info __user *)arg;
-	struct inode *inode = file_inode(file);
 	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;
 
···
 	if (!pid_in_current_pidns(pid))
 		return -ESRCH;
 
+	attr = READ_ONCE(pid->attr);
 	if (mask & PIDFD_INFO_EXIT) {
-		exit_info = READ_ONCE(pidfs_i(inode)->exit_info);
+		exit_info = READ_ONCE(attr->exit_info);
 		if (exit_info) {
 			kinfo.mask |= PIDFD_INFO_EXIT;
 #ifdef CONFIG_CGROUPS
···
 
 	if (mask & PIDFD_INFO_COREDUMP) {
 		kinfo.mask |= PIDFD_INFO_COREDUMP;
-		kinfo.coredump_mask = READ_ONCE(pidfs_i(inode)->__pei.coredump_mask);
+		kinfo.coredump_mask = READ_ONCE(attr->__pei.coredump_mask);
 	}
 
 	task = get_pid_task(pid, PIDTYPE_PID);
···
  * task has been reaped which cannot happen until we're out of
  * release_task().
  *
- * If this struct pid is referred to by a pidfd then
- * stashed_dentry_get() will return the dentry and inode for that struct
- * pid. Since we've taken a reference on it there's now an additional
- * reference from the exit path on it. Which is fine. We're going to put
- * it again in a second and we know that the pid is kept alive anyway.
+ * 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 immediately free the
- * dentry and inode afterwards since the pidfd has been closed. Since
+ * 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 ourselves.
+ * it cannot be us aka the exiting task holding a pidfd to itself.
  */
 void pidfs_exit(struct task_struct *tsk)
 {
-	struct dentry *dentry;
+	struct pid *pid = task_pid(tsk);
+	struct pidfs_attr *attr;
+	struct pidfs_exit_info *exit_info;
+#ifdef CONFIG_CGROUPS
+	struct cgroup *cgrp;
+#endif
 
 	might_sleep();
 
-	dentry = stashed_dentry_get(&task_pid(tsk)->stashed);
-	if (dentry) {
-		struct inode *inode = d_inode(dentry);
-		struct pidfs_exit_info *exit_info = &pidfs_i(inode)->__pei;
-#ifdef CONFIG_CGROUPS
-		struct cgroup *cgrp;
-
-		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(&pidfs_i(inode)->exit_info, &pidfs_i(inode)->__pei);
-		dput(dentry);
+	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;
 	}
+
+	/*
+	 * 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);
 }
 
 #ifdef CONFIG_COREDUMP
···
 {
 	struct pid *pid = cprm->pid;
 	struct pidfs_exit_info *exit_info;
-	struct dentry *dentry;
-	struct inode *inode;
+	struct pidfs_attr *attr;
 	__u32 coredump_mask = 0;
 
-	dentry = pid->stashed;
-	if (WARN_ON_ONCE(!dentry))
-		return;
+	attr = READ_ONCE(pid->attr);
 
-	inode = d_inode(dentry);
-	exit_info = &pidfs_i(inode)->__pei;
+	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. */
···
 	return anon_inode_getattr(idmap, path, stat, request_mask, query_flags);
 }
 
+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)
···
 	put_pid(pid);
 }
 
-static struct inode *pidfs_alloc_inode(struct super_block *sb)
-{
-	struct pidfs_inode *pi;
-
-	pi = alloc_inode_sb(sb, pidfs_cachep, GFP_KERNEL);
-	if (!pi)
-		return NULL;
-
-	memset(&pi->__pei, 0, sizeof(pi->__pei));
-	pi->exit_info = NULL;
-
-	return &pi->vfs_inode;
-}
-
-static void pidfs_free_inode(struct inode *inode)
-{
-	kmem_cache_free(pidfs_cachep, pidfs_i(inode));
-}
-
 static const struct super_operations pidfs_sops = {
-	.alloc_inode	= pidfs_alloc_inode,
 	.drop_inode	= generic_delete_inode,
 	.evict_inode	= pidfs_evict_inode,
-	.free_inode	= pidfs_free_inode,
 	.statfs		= simple_statfs,
 };
 
···
 	if (ret < 0)
 		return ERR_PTR(ret);
 
+	VFS_WARN_ON_ONCE(!pid->attr);
+
 	mntput(path.mnt);
 	return path.dentry;
 }
···
 	return 0;
 }
 
-static inline bool pidfs_pid_valid(struct pid *pid, const struct path *path,
-				   unsigned int flags)
-{
-	enum pid_type type;
-
-	if (flags & PIDFD_STALE)
-		return true;
-
-	/*
-	 * Make sure that if a pidfd is created PIDFD_INFO_EXIT
-	 * information will be available. So after an inode for the
-	 * pidfd has been allocated perform another check that the pid
-	 * is still alive. If it is exit information is available even
-	 * if the task gets reaped before the pidfd is returned to
-	 * userspace. The only exception are indicated by PIDFD_STALE:
-	 *
-	 * (1) The kernel is in the middle of task creation and thus no
-	 *     task linkage has been established yet.
-	 * (2) The caller knows @pid has been registered in pidfs at a
-	 *     time when the task was still alive.
-	 *
-	 * In both cases exit information will have been reported.
-	 */
-	if (flags & PIDFD_THREAD)
-		type = PIDTYPE_PID;
-	else
-		type = PIDTYPE_TGID;
-
-	/*
-	 * Since pidfs_exit() is called before struct pid's task linkage
-	 * is removed the case where the task got reaped but a dentry
-	 * was already attached to struct pid and exit information was
-	 * recorded and published can be handled correctly.
-	 */
-	if (unlikely(!pid_has_task(pid, type))) {
-		struct inode *inode = d_inode(path->dentry);
-		return !!READ_ONCE(pidfs_i(inode)->exit_info);
-	}
-
-	return true;
-}
-
 static struct file *pidfs_export_open(struct path *path, unsigned int oflags)
 {
-	if (!pidfs_pid_valid(d_inode(path->dentry)->i_private, path, oflags))
-		return ERR_PTR(-ESRCH);
-
 	/*
 	 * Clear O_LARGEFILE as open_by_handle_at() forces it and raise
 	 * O_RDWR as pidfds always are.
···
 
 	inode->i_private = data;
 	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;
···
 	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)
···
 	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;
 }
···
 	if (ret < 0)
 		return ERR_PTR(ret);
 
-	if (!pidfs_pid_valid(pid, &path, flags))
-		return ERR_PTR(-ESRCH);
+	VFS_WARN_ON_ONCE(!pid->attr);
 
 	flags &= ~PIDFD_STALE;
 	flags |= O_RDWR;
···
 	return pidfd_file;
 }
 
-/**
- * pidfs_register_pid - register a struct pid in pidfs
- * @pid: pid to pin
- *
- * Register a struct pid in pidfs. Needs to be paired with
- * pidfs_put_pid() to not risk leaking the pidfs dentry and inode.
- *
- * Return: On success zero, on error a negative error code is returned.
- */
-int pidfs_register_pid(struct pid *pid)
-{
-	struct path path __free(path_put) = {};
-	int ret;
-
-	might_sleep();
-
-	if (!pid)
-		return 0;
-
-	ret = path_from_stashed(&pid->stashed, pidfs_mnt, get_pid(pid), &path);
-	if (unlikely(ret))
-		return ret;
-	/* Keep the dentry and only put the reference to the mount. */
-	path.dentry = NULL;
-	return 0;
-}
-
-/**
- * pidfs_get_pid - pin a struct pid through pidfs
- * @pid: pid to pin
- *
- * Similar to pidfs_register_pid() but only valid if the caller knows
- * there's a reference to the @pid through a dentry already that can't
- * go away.
- */
-void pidfs_get_pid(struct pid *pid)
-{
-	if (!pid)
-		return;
-	WARN_ON_ONCE(!stashed_dentry_get(&pid->stashed));
-}
-
-/**
- * pidfs_put_pid - drop a pidfs reference
- * @pid: pid to drop
- *
- * Drop a reference to @pid via pidfs. This is only safe if the
- * reference has been taken via pidfs_get_pid().
- */
-void pidfs_put_pid(struct pid *pid)
-{
-	might_sleep();
-
-	if (!pid)
-		return;
-	VFS_WARN_ON_ONCE(!pid->stashed);
-	dput(pid->stashed);
-}
-
-static void pidfs_inode_init_once(void *data)
-{
-	struct pidfs_inode *pi = data;
-
-	inode_init_once(&pi->vfs_inode);
-}
-
 void __init pidfs_init(void)
 {
-	pidfs_cachep = kmem_cache_create("pidfs_cache", sizeof(struct pidfs_inode), 0,
+	pidfs_attr_cachep = kmem_cache_create("pidfs_attr_cache", sizeof(struct pidfs_attr), 0,
 					 (SLAB_HWCACHE_ALIGN | SLAB_RECLAIM_ACCOUNT |
-					  SLAB_ACCOUNT | SLAB_PANIC),
-					 pidfs_inode_init_once);
+					  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");

include/linux/pid.h

···
 
 #define RESERVED_PIDS 300
 
+struct pidfs_attr;
+
 struct upid {
 	int nr;
 	struct pid_namespace *ns;
 };
 
-struct pid
-{
+struct pid {
 	refcount_t count;
 	unsigned int level;
 	spinlock_t lock;
-	struct dentry *stashed;
-	u64 ino;
-	struct rb_node pidfs_node;
+	struct {
+		u64 ino;
+		struct rb_node pidfs_node;
+		struct dentry *stashed;
+		struct pidfs_attr *attr;
+	};
 	/* lists of tasks that use this pid */
 	struct hlist_head tasks[PIDTYPE_MAX];
 	struct hlist_head inodes;

include/linux/pidfs.h

···
 #endif
 extern const struct dentry_operations pidfs_dentry_operations;
 int pidfs_register_pid(struct pid *pid);
-void pidfs_get_pid(struct pid *pid);
-void pidfs_put_pid(struct pid *pid);
+void pidfs_free_pid(struct pid *pid);
 
 #endif /* _LINUX_PID_FS_H */

kernel/pid.c

···
 
 	ns = pid->numbers[pid->level].ns;
 	if (refcount_dec_and_test(&pid->count)) {
-		WARN_ON_ONCE(pid->stashed);
+		pidfs_free_pid(pid);
 		kmem_cache_free(ns->pid_cachep, pid);
 		put_pid_ns(ns);
 	}

net/unix/af_unix.c

···
 		return;
 	}
 
-	if (sk->sk_peer_pid)
-		pidfs_put_pid(sk->sk_peer_pid);
-
 	if (u->addr)
 		unix_release_addr(u->addr);
 
···
 	swap(peercred->peer_pid, pid);
 	swap(peercred->peer_cred, cred);
 
-	pidfs_put_pid(pid);
 	put_pid(pid);
 	put_cred(cred);
 }
···
 
 	spin_lock(&sk->sk_peer_lock);
 	sk->sk_peer_pid = get_pid(peersk->sk_peer_pid);
-	pidfs_get_pid(sk->sk_peer_pid);
 	sk->sk_peer_cred = get_cred(peersk->sk_peer_cred);
 	spin_unlock(&sk->sk_peer_lock);
 }

tools/testing/selftests/pidfd/.gitignore

···
 pidfd_bind_mount
 pidfd_info_test
 pidfd_exec_helper
+pidfd_xattr_test
+pidfd_setattr_test

tools/testing/selftests/pidfd/Makefile

···
 
 TEST_GEN_PROGS := pidfd_test pidfd_fdinfo_test pidfd_open_test \
 	pidfd_poll_test pidfd_wait pidfd_getfd_test pidfd_setns_test \
-	pidfd_file_handle_test pidfd_bind_mount pidfd_info_test
+	pidfd_file_handle_test pidfd_bind_mount pidfd_info_test \
+	pidfd_xattr_test pidfd_setattr_test
 
 TEST_GEN_PROGS_EXTENDED := pidfd_exec_helper
 

tools/testing/selftests/pidfd/pidfd_setattr_test.c

···
+// SPDX-License-Identifier: GPL-2.0
+
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <linux/types.h>
+#include <poll.h>
+#include <pthread.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <syscall.h>
+#include <sys/prctl.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <sys/socket.h>
+#include <linux/kcmp.h>
+#include <sys/stat.h>
+#include <sys/xattr.h>
+
+#include "pidfd.h"
+#include "../kselftest_harness.h"
+
+FIXTURE(pidfs_setattr)
+{
+	pid_t child_pid;
+	int child_pidfd;
+};
+
+FIXTURE_SETUP(pidfs_setattr)
+{
+	self->child_pid = create_child(&self->child_pidfd, CLONE_NEWUSER | CLONE_NEWPID);
+	EXPECT_GE(self->child_pid, 0);
+
+	if (self->child_pid == 0)
+		_exit(EXIT_SUCCESS);
+}
+
+FIXTURE_TEARDOWN(pidfs_setattr)
+{
+	sys_waitid(P_PID, self->child_pid, NULL, WEXITED);
+	EXPECT_EQ(close(self->child_pidfd), 0);
+}
+
+TEST_F(pidfs_setattr, no_chown)
+{
+	ASSERT_LT(fchown(self->child_pidfd, 1234, 5678), 0);
+	ASSERT_EQ(errno, EOPNOTSUPP);
+}
+
+TEST_F(pidfs_setattr, no_chmod)
+{
+	ASSERT_LT(fchmod(self->child_pidfd, 0777), 0);
+	ASSERT_EQ(errno, EOPNOTSUPP);
+}
+
+TEST_F(pidfs_setattr, no_exec)
+{
+	char *const argv[] = { NULL };
+	char *const envp[] = { NULL };
+
+	ASSERT_LT(execveat(self->child_pidfd, "", argv, envp, AT_EMPTY_PATH), 0);
+	ASSERT_EQ(errno, EACCES);
+}
+
+TEST_HARNESS_MAIN

tools/testing/selftests/pidfd/pidfd_xattr_test.c

···
+// SPDX-License-Identifier: GPL-2.0
+
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <linux/types.h>
+#include <poll.h>
+#include <pthread.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <syscall.h>
+#include <sys/prctl.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <sys/socket.h>
+#include <linux/kcmp.h>
+#include <sys/stat.h>
+#include <sys/xattr.h>
+
+#include "pidfd.h"
+#include "../kselftest_harness.h"
+
+FIXTURE(pidfs_xattr)
+{
+	pid_t child_pid;
+	int child_pidfd;
+};
+
+FIXTURE_SETUP(pidfs_xattr)
+{
+	self->child_pid = create_child(&self->child_pidfd, CLONE_NEWUSER | CLONE_NEWPID);
+	EXPECT_GE(self->child_pid, 0);
+
+	if (self->child_pid == 0)
+		_exit(EXIT_SUCCESS);
+}
+
+FIXTURE_TEARDOWN(pidfs_xattr)
+{
+	sys_waitid(P_PID, self->child_pid, NULL, WEXITED);
+}
+
+TEST_F(pidfs_xattr, set_get_list_xattr_multiple)
+{
+	int ret, i;
+	char xattr_name[32];
+	char xattr_value[32];
+	char buf[32];
+	const int num_xattrs = 10;
+	char list[PATH_MAX] = {};
+
+	for (i = 0; i < num_xattrs; i++) {
+		snprintf(xattr_name, sizeof(xattr_name), "trusted.testattr%d", i);
+		snprintf(xattr_value, sizeof(xattr_value), "testvalue%d", i);
+		ret = fsetxattr(self->child_pidfd, xattr_name, xattr_value, strlen(xattr_value), 0);
+		ASSERT_EQ(ret, 0);
+	}
+
+	for (i = 0; i < num_xattrs; i++) {
+		snprintf(xattr_name, sizeof(xattr_name), "trusted.testattr%d", i);
+		snprintf(xattr_value, sizeof(xattr_value), "testvalue%d", i);
+		memset(buf, 0, sizeof(buf));
+		ret = fgetxattr(self->child_pidfd, xattr_name, buf, sizeof(buf));
+		ASSERT_EQ(ret, strlen(xattr_value));
+		ASSERT_EQ(strcmp(buf, xattr_value), 0);
+	}
+
+	ret = flistxattr(self->child_pidfd, list, sizeof(list));
+	ASSERT_GT(ret, 0);
+	for (i = 0; i < num_xattrs; i++) {
+		snprintf(xattr_name, sizeof(xattr_name), "trusted.testattr%d", i);
+		bool found = false;
+		for (char *it = list; it < list + ret; it += strlen(it) + 1) {
+			if (strcmp(it, xattr_name))
+				continue;
+			found = true;
+			break;
+		}
+		ASSERT_TRUE(found);
+	}
+
+	for (i = 0; i < num_xattrs; i++) {
+		snprintf(xattr_name, sizeof(xattr_name), "trusted.testattr%d", i);
+		ret = fremovexattr(self->child_pidfd, xattr_name);
+		ASSERT_EQ(ret, 0);
+
+		ret = fgetxattr(self->child_pidfd, xattr_name, buf, sizeof(buf));
+		ASSERT_EQ(ret, -1);
+		ASSERT_EQ(errno, ENODATA);
+	}
+}
+
+TEST_F(pidfs_xattr, set_get_list_xattr_persistent)
+{
+	int ret;
+	char buf[32];
+	char list[PATH_MAX] = {};
+
+	ret = fsetxattr(self->child_pidfd, "trusted.persistent", "persistent value", strlen("persistent value"), 0);
+	ASSERT_EQ(ret, 0);
+
+	memset(buf, 0, sizeof(buf));
+	ret = fgetxattr(self->child_pidfd, "trusted.persistent", buf, sizeof(buf));
+	ASSERT_EQ(ret, strlen("persistent value"));
+	ASSERT_EQ(strcmp(buf, "persistent value"), 0);
+
+	ret = flistxattr(self->child_pidfd, list, sizeof(list));
+	ASSERT_GT(ret, 0);
+	ASSERT_EQ(strcmp(list, "trusted.persistent"), 0)
+
+	ASSERT_EQ(close(self->child_pidfd), 0);
+	self->child_pidfd = -EBADF;
+	sleep(2);
+
+	self->child_pidfd = sys_pidfd_open(self->child_pid, 0);
+	ASSERT_GE(self->child_pidfd, 0);
+
+	memset(buf, 0, sizeof(buf));
+	ret = fgetxattr(self->child_pidfd, "trusted.persistent", buf, sizeof(buf));
+	ASSERT_EQ(ret, strlen("persistent value"));
+	ASSERT_EQ(strcmp(buf, "persistent value"), 0);
+
+	ret = flistxattr(self->child_pidfd, list, sizeof(list));
+	ASSERT_GT(ret, 0);
+	ASSERT_EQ(strcmp(list, "trusted.persistent"), 0);
+}
+
+TEST_HARNESS_MAIN