vfs: expose delegation support to userland

Now that support for recallable directory delegations is available,
expose this functionality to userland with new F_SETDELEG and F_GETDELEG
commands for fcntl().

Note that this also allows userland to request a FL_DELEG type lease on
files too. Userland applications that do will get signalled when there
are metadata changes in addition to just data changes (which is a
limitation of FL_LEASE leases).

These commands accept a new "struct delegation" argument that contains a
flags field for future expansion.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20251111-dir-deleg-ro-v6-17-52f3feebb2f2@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/fcntl.c

@@ -445,6 +445,7 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 		struct file *filp)
 {
 	void __user *argp = (void __user *)arg;
+	struct delegation deleg;
 	int argi = (int)arg;
 	struct flock flock;
 	long err = -EINVAL;
@@ -550,6 +551,18 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 	case F_SET_RW_HINT:
 		err = fcntl_set_rw_hint(filp, arg);
 		break;
+	case F_GETDELEG:
+		if (copy_from_user(&deleg, argp, sizeof(deleg)))
+			return -EFAULT;
+		err = fcntl_getdeleg(filp, &deleg);
+		if (!err && copy_to_user(argp, &deleg, sizeof(deleg)))
+			return -EFAULT;
+		break;
+	case F_SETDELEG:
+		if (copy_from_user(&deleg, argp, sizeof(deleg)))
+			return -EFAULT;
+		err = fcntl_setdeleg(fd, filp, &deleg);
+		break;
 	default:
 		break;
 	}

fs/locks.c

@@ -1703,7 +1703,7 @@ EXPORT_SYMBOL(lease_get_mtime);
  *	XXX: sfr & willy disagree over whether F_INPROGRESS
  *	should be returned to userspace.
  */
-int fcntl_getlease(struct file *filp)
+static int __fcntl_getlease(struct file *filp, unsigned int flavor)
 {
 	struct file_lease *fl;
 	struct inode *inode = file_inode(filp);
@@ -1719,7 +1719,8 @@ int fcntl_getlease(struct file *filp)
 		list_for_each_entry(fl, &ctx->flc_lease, c.flc_list) {
 			if (fl->c.flc_file != filp)
 				continue;
-			type = target_leasetype(fl);
+			if (fl->c.flc_flags & flavor)
+				type = target_leasetype(fl);
 			break;
 		}
 		spin_unlock(&ctx->flc_lock);
@@ -1730,6 +1731,19 @@ int fcntl_getlease(struct file *filp)
 	return type;
 }
 
+int fcntl_getlease(struct file *filp)
+{
+	return __fcntl_getlease(filp, FL_LEASE);
+}
+
+int fcntl_getdeleg(struct file *filp, struct delegation *deleg)
+{
+	if (deleg->d_flags != 0 || deleg->__pad != 0)
+		return -EINVAL;
+	deleg->d_type = __fcntl_getlease(filp, FL_DELEG);
+	return 0;
+}
+
 /**
  * check_conflicting_open - see if the given file points to an inode that has
  *			    an existing open that would conflict with the
@@ -2039,13 +2053,13 @@ vfs_setlease(struct file *filp, int arg, struct file_lease **lease, void **priv)
 }
 EXPORT_SYMBOL_GPL(vfs_setlease);
 
-static int do_fcntl_add_lease(unsigned int fd, struct file *filp, int arg)
+static int do_fcntl_add_lease(unsigned int fd, struct file *filp, unsigned int flavor, int arg)
 {
 	struct file_lease *fl;
 	struct fasync_struct *new;
 	int error;
 
-	fl = lease_alloc(filp, FL_LEASE, arg);
+	fl = lease_alloc(filp, flavor, arg);
 	if (IS_ERR(fl))
 		return PTR_ERR(fl);
 
@@ -2081,7 +2095,28 @@ int fcntl_setlease(unsigned int fd, struct file *filp, int arg)
 
 	if (arg == F_UNLCK)
 		return vfs_setlease(filp, F_UNLCK, NULL, (void **)&filp);
-	return do_fcntl_add_lease(fd, filp, arg);
+	return do_fcntl_add_lease(fd, filp, FL_LEASE, arg);
+}
+
+/**
+ *	fcntl_setdeleg	-	sets a delegation on an open file
+ *	@fd: open file descriptor
+ *	@filp: file pointer
+ *	@deleg: delegation request from userland
+ *
+ *	Call this fcntl to establish a delegation on the file.
+ *	Note that you also need to call %F_SETSIG to
+ *	receive a signal when the lease is broken.
+ */
+int fcntl_setdeleg(unsigned int fd, struct file *filp, struct delegation *deleg)
+{
+	/* For now, no flags are supported */
+	if (deleg->d_flags != 0 || deleg->__pad != 0)
+		return -EINVAL;
+
+	if (deleg->d_type == F_UNLCK)
+		return vfs_setlease(filp, F_UNLCK, NULL, (void **)&filp);
+	return do_fcntl_add_lease(fd, filp, FL_DELEG, deleg->d_type);
 }
 
 /**

include/linux/filelock.h

@@ -159,6 +159,8 @@ int fcntl_setlk64(unsigned int, struct file *, unsigned int,
 
 int fcntl_setlease(unsigned int fd, struct file *filp, int arg);
 int fcntl_getlease(struct file *filp);
+int fcntl_setdeleg(unsigned int fd, struct file *filp, struct delegation *deleg);
+int fcntl_getdeleg(struct file *filp, struct delegation *deleg);
 
 static inline bool lock_is_unlock(struct file_lock *fl)
 {
@@ -278,6 +280,16 @@ static inline int fcntl_getlease(struct file *filp)
 	return F_UNLCK;
 }
 
+static inline int fcntl_setdeleg(unsigned int fd, struct file *filp, struct delegation *deleg)
+{
+	return -EINVAL;
+}
+
+static inline int fcntl_getdeleg(struct file *filp, struct delegation *deleg)
+{
+	return -EINVAL;
+}
+
 static inline bool lock_is_unlock(struct file_lock *fl)
 {
 	return false;

include/uapi/linux/fcntl.h

@@ -79,6 +79,17 @@
  */
 #define RWF_WRITE_LIFE_NOT_SET	RWH_WRITE_LIFE_NOT_SET
 
+/* Set/Get delegations */
+#define F_GETDELEG		(F_LINUX_SPECIFIC_BASE + 15)
+#define F_SETDELEG		(F_LINUX_SPECIFIC_BASE + 16)
+
+/* Argument structure for F_GETDELEG and F_SETDELEG */
+struct delegation {
+	uint32_t	d_flags;	/* Must be 0 */
+	uint16_t	d_type;		/* F_RDLCK, F_WRLCK, F_UNLCK */
+	uint16_t	__pad;		/* Must be 0 */
+};
+
 /*
  * Types of directory notifications that may be requested.
  */