Re: [PATCH v6 17/17] vfs: expose delegation support to userland

Jan Kara
2025-11-11T14:25:56Z

On Tue 11-11-25 09:12:58, Jeff Layton wrote:
> 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>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

> ---
> fs/fcntl.c | 13 +++++++++++++
> fs/locks.c | 45 ++++++++++++++++++++++++++++++++++++++++-----
> include/linux/filelock.h | 12 ++++++++++++
> include/uapi/linux/fcntl.h | 11 +++++++++++
> 4 files changed, 76 insertions(+), 5 deletions(-)
>
> diff --git a/fs/fcntl.c b/fs/fcntl.c
> index 72f8433d9109889eecef56b32d20a85b4e12ea44..f93dbca0843557d197bd1e023519cfa0f00ad78f 100644
> --- a/fs/fcntl.c
> +++ b/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;
> }
> diff --git a/fs/locks.c b/fs/locks.c
> index dd290a87f58eb5d522f03fa99d612fbad84dacf3..7f4ccc7974bc8d3e82500ee692c6520b53f2280f 100644
> --- a/fs/locks.c
> +++ b/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);
> }
>
> /**
> diff --git a/include/linux/filelock.h b/include/linux/filelock.h
> index 208d108df2d73a9df65e5dc9968d074af385f881..54b824c05299261e6bd6acc4175cb277ea35b35d 100644
> --- a/include/linux/filelock.h
> +++ b/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;
> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> index 3741ea1b73d8500061567b6590ccf5fb4c6770f0..008fac15e573084a9b48e4e991528b4363c54047 100644
> --- a/include/uapi/linux/fcntl.h
> +++ b/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.
> */
>
> --
> 2.51.1
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR