fs: Return ENOTTY directly if FS_IOC_GETUUID or FS_IOC_GETFSSYSFSPATH fail

These IOCTL commands should be implemented by setting attributes on the
superblock, rather than in the IOCTL hooks in struct file_operations.

By returning -ENOTTY instead of -ENOIOCTLCMD, we instruct the fs/ioctl.c
logic to return -ENOTTY immediately, rather than attempting to call
f_op->unlocked_ioctl() or f_op->compat_ioctl() as a fallback.

Why this is safe:

Before this change, fs/ioctl.c would unsuccessfully attempt calling the
IOCTL hooks, and then return -ENOTTY.  By returning -ENOTTY directly, we
return the same error code immediately, but save ourselves the fallback
attempt.

Motivation:

This simplifies the logic for these IOCTL commands and lets us reason about
the side effects of these IOCTLs more easily.  It will be possible to
permit these IOCTLs under LSM IOCTL policies, without having to worry about
them getting dispatched to problematic device drivers (which sometimes do
work before looking at the IOCTL command number).

Link: https://lore.kernel.org/all/cnwpkeovzbumhprco7q2c2y6zxzmxfpwpwe3tyy6c3gg2szgqd@vfzjaw5v5imr/
Cc: Kent Overstreet <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Darrick J. Wong <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Cc: Josef Bacik <[email protected]>
Signed-off-by: Günther Noack <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Acked-by: Kent Overstreet <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: gnoack

fs/ioctl.c

@@ -769,7 +769,7 @@ static int ioctl_getfsuuid(struct file *file, void __user *argp)
 	struct fsuuid2 u = { .len = sb->s_uuid_len, };
 
 	if (!sb->s_uuid_len)
-		return -ENOIOCTLCMD;
+		return -ENOTTY;
 
 	memcpy(&u.uuid[0], &sb->s_uuid, sb->s_uuid_len);
 
@@ -781,7 +781,7 @@ static int ioctl_get_fs_sysfs_path(struct file *file, void __user *argp)
 	struct super_block *sb = file_inode(file)->i_sb;
 
 	if (!strlen(sb->s_sysfs_name))
-		return -ENOIOCTLCMD;
+		return -ENOTTY;
 
 	struct fs_sysfs_path u = {};