[PATCH v2 10/11] fhandle, pidfs: support open_by_handle_at() purely based on file handle

[Christian Brauner <brauner@kernel.org>]

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@kernel.org>
---
 fs/fhandle.c | 22 ++++++++++++++++++++--
 fs/pidfs.c   |  5 ++++-
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/fs/fhandle.c b/fs/fhandle.c
index ab4891925b52..54081e19f594 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 +193,24 @@ 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;
+
+	if (fd != FD_INVALID)
+		return -EINVAL;
+
+	switch (handle_type & ~FILEID_USER_FLAGS_MASK) {
+	case FILEID_PIDFS:
+		pidfs_get_root(root);
+		break;
+	default:
+		return -EINVAL;
+	}
+
 	return 0;
 }
 
@@ -347,7 +365,7 @@ 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 69b0541042b5..2ab9b47fbfae 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