file: reclaim 24 bytes from f_owner

We do embedd struct fown_struct into struct file letting it take up 32
bytes in total. We could tweak struct fown_struct to be more compact but
really it shouldn't even be embedded in struct file in the first place.

Instead, actual users of struct fown_struct should allocate the struct
on demand. This frees up 24 bytes in struct file.

That will have some potentially user-visible changes for the ownership
fcntl()s. Some of them can now fail due to allocation failures.
Practically, that probably will almost never happen as the allocations
are small and they only happen once per file.

The fown_struct is used during kill_fasync() which is used by e.g.,
pipes to generate a SIGIO signal. Sending of such signals is conditional
on userspace having set an owner for the file using one of the F_OWNER
fcntl()s. Such users will be unaffected if struct fown_struct is
allocated during the fcntl() call.

There are a few subsystems that call __f_setown() expecting
file->f_owner to be allocated:

(1) tun devices
file->f_op->fasync::tun_chr_fasync()
-> __f_setown()

There are no callers of tun_chr_fasync().

(2) tty devices

file->f_op->fasync::tty_fasync()
-> __tty_fasync()
-> __f_setown()

tty_fasync() has no additional callers but __tty_fasync() has. Note
that __tty_fasync() only calls __f_setown() if the @on argument is
true. It's called from:

file->f_op->release::tty_release()
-> tty_release()
-> __tty_fasync()
-> __f_setown()

tty_release() calls __tty_fasync() with @on false
=> __f_setown() is never called from tty_release().
=> All callers of tty_release() are safe as well.

file->f_op->release::tty_open()
-> tty_release()
-> __tty_fasync()
-> __f_setown()

__tty_hangup() calls __tty_fasync() with @on false
=> __f_setown() is never called from tty_release().
=> All callers of __tty_hangup() are safe as well.

From the callchains it's obvious that (1) and (2) end up getting called
via file->f_op->fasync(). That can happen either through the F_SETFL
fcntl() with the FASYNC flag raised or via the FIOASYNC ioctl(). If
FASYNC is requested and the file isn't already FASYNC then
file->f_op->fasync() is called with @on true which ends up causing both
(1) and (2) to call __f_setown().

(1) and (2) are the only subsystems that call __f_setown() from the
file->f_op->fasync() handler. So both (1) and (2) have been updated to
allocate a struct fown_struct prior to calling fasync_helper() to
register with the fasync infrastructure. That's safe as they both call
fasync_helper() which also does allocations if @on is true.

The other interesting case are file leases:

(3) file leases
lease_manager_ops->lm_setup::lease_setup()
-> __f_setown()

Which in turn is called from:

generic_add_lease()
-> lease_manager_ops->lm_setup::lease_setup()
-> __f_setown()

So here again we can simply make generic_add_lease() allocate struct
fown_struct prior to the lease_manager_ops->lm_setup::lease_setup()
which happens under a spinlock.

With that the two remaining subsystems that call __f_setown() are:

(4) dnotify
(5) sockets

Both have their own custom ioctls to set struct fown_struct and both
have been converted to allocate a struct fown_struct on demand from
their respective ioctls.

Interactions with O_PATH are fine as well e.g., when opening a /dev/tty
as O_PATH then no file->f_op->open() happens thus no file->f_owner is
allocated. That's fine as no file operation will be set for those and
the device has never been opened. fcntl()s called on such things will
just allocate a ->f_owner on demand. Although I have zero idea why'd you
care about f_owner on an O_PATH fd.

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

drivers/net/tun.c

···
 	struct tun_file *tfile = file->private_data;
 	int ret;
 
+	if (on) {
+		ret = file_f_owner_allocate(file);
+		if (ret)
+			goto out;
+	}
+
 	if ((ret = fasync_helper(fd, file, on, &tfile->fasync)) < 0)
 		goto out;
 

drivers/tty/tty_io.c

···
 	if (tty_paranoia_check(tty, file_inode(filp), "tty_fasync"))
 		goto out;
 
+	if (on) {
+		retval = file_f_owner_allocate(filp);
+		if (retval)
+			goto out;
+	}
+
 	retval = fasync_helper(fd, filp, on, &tty->fasync);
 	if (retval <= 0)
 		goto out;

fs/fcntl.c

···
 #include <asm/siginfo.h>
 #include <linux/uaccess.h>
 
+#include "internal.h"
+
 #define SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | O_DIRECT | O_NOATIME)
 
 static int setfl(int fd, struct file * filp, unsigned int arg)
···
 	return error;
 }
 
+/*
+ * Allocate an file->f_owner struct if it doesn't exist, handling racing
+ * allocations correctly.
+ */
+int file_f_owner_allocate(struct file *file)
+{
+	struct fown_struct *f_owner;
+
+	f_owner = file_f_owner(file);
+	if (f_owner)
+		return 0;
+
+	f_owner = kzalloc(sizeof(struct fown_struct), GFP_KERNEL);
+	if (!f_owner)
+		return -ENOMEM;
+
+	rwlock_init(&f_owner->lock);
+	f_owner->file = file;
+	/* If someone else raced us, drop our allocation. */
+	if (unlikely(cmpxchg(&file->f_owner, NULL, f_owner)))
+		kfree(f_owner);
+	return 0;
+}
+EXPORT_SYMBOL(file_f_owner_allocate);
+
+void file_f_owner_release(struct file *file)
+{
+	struct fown_struct *f_owner;
+
+	f_owner = file_f_owner(file);
+	if (f_owner) {
+		put_pid(f_owner->pid);
+		kfree(f_owner);
+	}
+}
+
 static void f_modown(struct file *filp, struct pid *pid, enum pid_type type,
                      int force)
 {
-	write_lock_irq(&filp->f_owner.lock);
-	if (force || !filp->f_owner.pid) {
-		put_pid(filp->f_owner.pid);
-		filp->f_owner.pid = get_pid(pid);
-		filp->f_owner.pid_type = type;
+	struct fown_struct *f_owner;
+
+	f_owner = file_f_owner(filp);
+	if (WARN_ON_ONCE(!f_owner))
+		return;
+
+	write_lock_irq(&f_owner->lock);
+	if (force || !f_owner->pid) {
+		put_pid(f_owner->pid);
+		f_owner->pid = get_pid(pid);
+		f_owner->pid_type = type;
 
 		if (pid) {
 			const struct cred *cred = current_cred();
-			filp->f_owner.uid = cred->uid;
-			filp->f_owner.euid = cred->euid;
+			f_owner->uid = cred->uid;
+			f_owner->euid = cred->euid;
 		}
 	}
-	write_unlock_irq(&filp->f_owner.lock);
+	write_unlock_irq(&f_owner->lock);
 }
 
 void __f_setown(struct file *filp, struct pid *pid, enum pid_type type,
···
 	struct pid *pid = NULL;
 	int ret = 0;
 
+	might_sleep();
+
 	type = PIDTYPE_TGID;
 	if (who < 0) {
 		/* avoid overflow below */
···
 		type = PIDTYPE_PGID;
 		who = -who;
 	}
+
+	ret = file_f_owner_allocate(filp);
+	if (ret)
+		return ret;
 
 	rcu_read_lock();
 	if (who) {
···
 pid_t f_getown(struct file *filp)
 {
 	pid_t pid = 0;
+	struct fown_struct *f_owner;
 
-	read_lock_irq(&filp->f_owner.lock);
+	f_owner = file_f_owner(filp);
+	if (!f_owner)
+		return pid;
+
+	read_lock_irq(&f_owner->lock);
 	rcu_read_lock();
-	if (pid_task(filp->f_owner.pid, filp->f_owner.pid_type)) {
-		pid = pid_vnr(filp->f_owner.pid);
-		if (filp->f_owner.pid_type == PIDTYPE_PGID)
+	if (pid_task(f_owner->pid, f_owner->pid_type)) {
+		pid = pid_vnr(f_owner->pid);
+		if (f_owner->pid_type == PIDTYPE_PGID)
 			pid = -pid;
 	}
 	rcu_read_unlock();
-	read_unlock_irq(&filp->f_owner.lock);
+	read_unlock_irq(&f_owner->lock);
 	return pid;
 }
 
···
 		return -EINVAL;
 	}
 
+	ret = file_f_owner_allocate(filp);
+	if (ret)
+		return ret;
+
 	rcu_read_lock();
 	pid = find_vpid(owner.pid);
 	if (owner.pid && !pid)
···
 	struct f_owner_ex __user *owner_p = (void __user *)arg;
 	struct f_owner_ex owner = {};
 	int ret = 0;
+	struct fown_struct *f_owner;
+	enum pid_type pid_type = PIDTYPE_PID;
 
-	read_lock_irq(&filp->f_owner.lock);
-	rcu_read_lock();
-	if (pid_task(filp->f_owner.pid, filp->f_owner.pid_type))
-		owner.pid = pid_vnr(filp->f_owner.pid);
-	rcu_read_unlock();
-	switch (filp->f_owner.pid_type) {
+	f_owner = file_f_owner(filp);
+	if (f_owner) {
+		read_lock_irq(&f_owner->lock);
+		rcu_read_lock();
+		if (pid_task(f_owner->pid, f_owner->pid_type))
+			owner.pid = pid_vnr(f_owner->pid);
+		rcu_read_unlock();
+		pid_type = f_owner->pid_type;
+	}
+
+	switch (pid_type) {
 	case PIDTYPE_PID:
 		owner.type = F_OWNER_TID;
 		break;
···
 		ret = -EINVAL;
 		break;
 	}
-	read_unlock_irq(&filp->f_owner.lock);
+	if (f_owner)
+		read_unlock_irq(&f_owner->lock);
 
 	if (!ret) {
 		ret = copy_to_user(owner_p, &owner, sizeof(owner));
···
 static int f_getowner_uids(struct file *filp, unsigned long arg)
 {
 	struct user_namespace *user_ns = current_user_ns();
+	struct fown_struct *f_owner;
 	uid_t __user *dst = (void __user *)arg;
-	uid_t src[2];
+	uid_t src[2] = {0, 0};
 	int err;
 
-	read_lock_irq(&filp->f_owner.lock);
-	src[0] = from_kuid(user_ns, filp->f_owner.uid);
-	src[1] = from_kuid(user_ns, filp->f_owner.euid);
-	read_unlock_irq(&filp->f_owner.lock);
+	f_owner = file_f_owner(filp);
+	if (f_owner) {
+		read_lock_irq(&f_owner->lock);
+		src[0] = from_kuid(user_ns, f_owner->uid);
+		src[1] = from_kuid(user_ns, f_owner->euid);
+		read_unlock_irq(&f_owner->lock);
+	}
 
 	err  = put_user(src[0], &dst[0]);
 	err |= put_user(src[1], &dst[1]);
···
 	return f.file == filp;
 }
 
+static int f_owner_sig(struct file *filp, int signum, bool setsig)
+{
+	int ret = 0;
+	struct fown_struct *f_owner;
+
+	might_sleep();
+
+	if (setsig) {
+		if (!valid_signal(signum))
+			return -EINVAL;
+
+		ret = file_f_owner_allocate(filp);
+		if (ret)
+			return ret;
+	}
+
+	f_owner = file_f_owner(filp);
+	if (setsig)
+		f_owner->signum = signum;
+	else if (f_owner)
+		ret = f_owner->signum;
+	return ret;
+}
+
 static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 		struct file *filp)
 {
···
 		err = f_getowner_uids(filp, arg);
 		break;
 	case F_GETSIG:
-		err = filp->f_owner.signum;
+		err = f_owner_sig(filp, 0, false);
 		break;
 	case F_SETSIG:
-		/* arg == 0 restores default behaviour. */
-		if (!valid_signal(argi)) {
-			break;
-		}
-		err = 0;
-		filp->f_owner.signum = argi;
+		err = f_owner_sig(filp, argi, true);
 		break;
 	case F_GETLEASE:
 		err = fcntl_getlease(filp);
···
 		do_send_sig_info(SIGURG, SEND_SIG_PRIV, p, type);
 }
 
-int send_sigurg(struct fown_struct *fown)
+int send_sigurg(struct file *file)
 {
+	struct fown_struct *fown;
 	struct task_struct *p;
 	enum pid_type type;
 	struct pid *pid;
 	unsigned long flags;
 	int ret = 0;
 	
+	fown = file_f_owner(file);
+	if (!fown)
+		return 0;
+
 	read_lock_irqsave(&fown->lock, flags);
 
 	type = fown->pid_type;
···
 		}
 		read_lock_irqsave(&fa->fa_lock, flags);
 		if (fa->fa_file) {
-			fown = &fa->fa_file->f_owner;
+			fown = file_f_owner(fa->fa_file);
+			if (!fown)
+				goto next;
 			/* Don't send SIGURG to processes which have not set a
 			   queued signum: SIGURG has its own default signalling
 			   mechanism. */
 			if (!(sig == SIGURG && fown->signum == 0))
 				send_sigio(fown, fa->fa_fd, band);
 		}
+next:
 		read_unlock_irqrestore(&fa->fa_lock, flags);
 		fa = rcu_dereference(fa->fa_next);
 	}

fs/file_table.c

···
 		return error;
 	}
 
-	rwlock_init(&f->f_owner.lock);
 	spin_lock_init(&f->f_lock);
 	mutex_init(&f->f_pos_lock);
 	f->f_flags = flags;
···
 		cdev_put(inode->i_cdev);
 	}
 	fops_put(file->f_op);
-	put_pid(file->f_owner.pid);
+	file_f_owner_release(file);
 	put_file_access(file);
 	dput(dentry);
 	if (unlikely(mode & FMODE_NEED_UNMOUNT))

fs/internal.h

···
 {
 	return path->mnt->mnt_root == path->dentry;
 }
+void file_f_owner_release(struct file *file);

fs/locks.c

···
 		struct file *filp = fl->c.flc_file;
 
 		f_delown(filp);
-		filp->f_owner.signum = 0;
+		file_f_owner(filp)->signum = 0;
 		fasync_helper(0, fl->c.flc_file, 0, &fl->fl_fasync);
 		if (fl->fl_fasync != NULL) {
 			printk(KERN_ERR "locks_delete_lock: fasync == %p\n", fl->fl_fasync);
···
 
 	lease = *flp;
 	trace_generic_add_lease(inode, lease);
+
+	error = file_f_owner_allocate(filp);
+	if (error)
+		return error;
 
 	/* Note that arg is never F_UNLCK here */
 	ctx = locks_get_lock_context(inode, arg);

fs/notify/dnotify/dnotify.c

···
 			prev = &dn->dn_next;
 			continue;
 		}
-		fown = &dn->dn_filp->f_owner;
+		fown = file_f_owner(dn->dn_filp);
 		send_sigio(fown, dn->dn_fd, POLL_MSG);
 		if (dn->dn_mask & FS_DN_MULTISHOT)
 			prev = &dn->dn_next;
···
 		error = -ENOMEM;
 		goto out_err;
 	}
+
+	error = file_f_owner_allocate(filp);
+	if (error)
+		goto out_err;
 
 	/* set up the new_fsn_mark and new_dn_mark */
 	new_fsn_mark = &new_dn_mark->fsn_mark;

include/linux/fs.h

···
 }
 
 struct fown_struct {
+	struct file *file;	/* backpointer for security modules */
 	rwlock_t lock;          /* protects pid, uid, euid fields */
 	struct pid *pid;	/* pid or -pgrp where SIGIO should be sent */
 	enum pid_type pid_type;	/* Kind of process group SIGIO should be sent to */
···
 	struct mutex		f_pos_lock;
 	loff_t			f_pos;
 	unsigned int		f_flags;
-	struct fown_struct	f_owner;
+	struct fown_struct	*f_owner;
 	const struct cred	*f_cred;
 	struct file_ra_state	f_ra;
 	struct path		f_path;
···
 #define OFFT_OFFSET_MAX	type_max(off_t)
 #endif
 
+int file_f_owner_allocate(struct file *file);
+static inline struct fown_struct *file_f_owner(const struct file *file)
+{
+	return READ_ONCE(file->f_owner);
+}
+
 extern void send_sigio(struct fown_struct *fown, int fd, int band);
 
 static inline struct inode *file_inode(const struct file *f)
···
 extern int f_setown(struct file *filp, int who, int force);
 extern void f_delown(struct file *filp);
 extern pid_t f_getown(struct file *filp);
-extern int send_sigurg(struct fown_struct *fown);
+extern int send_sigurg(struct file *file);
 
 /*
  * sb->s_flags.  Note that these mirror the equivalent MS_* flags where

net/core/sock.c

···
 void sk_send_sigurg(struct sock *sk)
 {
 	if (sk->sk_socket && sk->sk_socket->file)
-		if (send_sigurg(&sk->sk_socket->file->f_owner))
+		if (send_sigurg(sk->sk_socket->file))
 			sk_wake_async(sk, SOCK_WAKE_URG, POLL_PRI);
 }
 EXPORT_SYMBOL(sk_send_sigurg);

security/selinux/hooks.c

···
 	struct file_security_struct *fsec;
 
 	/* struct fown_struct is never outside the context of a struct file */
-	file = container_of(fown, struct file, f_owner);
+	file = fown->file;
 
 	fsec = selinux_file(file);
 

security/smack/smack_lsm.c

···
 	/*
 	 * struct fown_struct is never outside the context of a struct file
 	 */
-	file = container_of(fown, struct file, f_owner);
+	file = fown->file;
 
 	/* we don't log here as rc can be overriden */
 	blob = smack_file(file);