On Mon 23-06-25 11:01:30, Christian Brauner wrote:
> Various filesystems such as pidfs (and likely drm in the future) have a
> use-case to support opening files purely based on the handle without
> having to require a file descriptor to another object. That's especially
> the case for filesystems that don't do any lookup whatsoever and there's
> zero relationship between the objects. Such filesystems are also
> singletons that stay around for the lifetime of the system meaning that
> they can be uniquely identified and accessed purely based on the file
> handle type. Enable that so that userspace doesn't have to allocate an
> object needlessly especially if they can't do that for whatever reason.
>
> Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
Hmm, maybe we should predefine some invalid fd value userspace should pass
when it wants to "autopick" fs root? Otherwise defining more special fd
values like AT_FDCWD would become difficult in the future. Or we could just
define that FILEID_PIDFS file handles *always* ignore the fd value and
auto-pick the root.
Honza
> ---
> fs/fhandle.c | 19 +++++++++++++++++--
> fs/pidfs.c | 5 ++++-
> 2 files changed, 21 insertions(+), 3 deletions(-)
>
> diff --git a/fs/fhandle.c b/fs/fhandle.c
> index ab4891925b52..20d6477b5a9e 100644
> --- a/fs/fhandle.c
> +++ b/fs/fhandle.c
> @@ -173,7 +173,7 @@ SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
> return err;
> }
>
> -static int get_path_anchor(int fd, struct path *root)
> +static int get_path_anchor(int fd, struct path *root, int handle_type)
> {
> if (fd >= 0) {
> CLASS(fd, f)(fd);
> @@ -193,6 +193,21 @@ static int get_path_anchor(int fd, struct path *root)
> return 0;
> }
>
> + /*
> + * Only autonomous handles can be decoded without a file
> + * descriptor.
> + */
> + if (!(handle_type & FILEID_IS_AUTONOMOUS))
> + return -EOPNOTSUPP;
> +
> + switch (handle_type & ~FILEID_USER_FLAGS_MASK) {
> + case FILEID_PIDFS:
> + pidfs_get_root(root);
> + break;
> + default:
> + return -EINVAL;
> + }
> +
> return 0;
> }
>
> @@ -347,7 +362,7 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
> FILEID_USER_FLAGS(f_handle.handle_type) & ~FILEID_VALID_USER_FLAGS)
> return -EINVAL;
>
> - retval = get_path_anchor(mountdirfd, &ctx.root);
> + retval = get_path_anchor(mountdirfd, &ctx.root, f_handle.handle_type);
> if (retval)
> return retval;
>
> diff --git a/fs/pidfs.c b/fs/pidfs.c
> index 1b7bd14366dc..ea50a6afc325 100644
> --- a/fs/pidfs.c
> +++ b/fs/pidfs.c
> @@ -747,7 +747,7 @@ static int pidfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
>
> *max_len = 2;
> *(u64 *)fh = pid->ino;
> - return FILEID_KERNFS;
> + return FILEID_PIDFS;
> }
>
> static int pidfs_ino_find(const void *key, const struct rb_node *node)
> @@ -802,6 +802,8 @@ static struct dentry *pidfs_fh_to_dentry(struct super_block *sb,
> return NULL;
>
> switch (fh_type) {
> + case FILEID_PIDFS:
> + fallthrough;
> case FILEID_KERNFS:
> pid_ino = *(u64 *)fid;
> break;
> @@ -860,6 +862,7 @@ static const struct export_operations pidfs_export_operations = {
> .fh_to_dentry = pidfs_fh_to_dentry,
> .open = pidfs_export_open,
> .permission = pidfs_export_permission,
> + .flags = EXPORT_OP_AUTONOMOUS_HANDLES,
> };
>
> static int pidfs_init_inode(struct inode *inode, void *data)
>
> --
> 2.47.2
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
