autofs: improve ioctl sbi checks

Al Viro made some suggestions to improve the implementation of commit
0633da48 ("fix autofs_sbi() does not check super block type").

The check is unnecessary in all cases except for ioctl usage so placing
the check in the super block accessor function adds a small overhead to
the common case where it isn't needed.

So it's sufficient to do this in the ioctl code only.

Also the check in the ioctl code is needlessly complex.

[akpm@linux-foundation.org: declare autofs_fs_type in .h, not .c]
Link: http://lkml.kernel.org/r/154296970987.9889.1597442413573683096.stgit@pluto-themaw-netSigned-off-by: Ian Kent <raven@themaw.net>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/autofs/autofs_i.h

@@ -42,6 +42,8 @@
 #endif
 #define pr_fmt(fmt) KBUILD_MODNAME ":pid:%d:%s: " fmt, current->pid, __func__
 
+extern struct file_system_type autofs_fs_type;
+
 /*
  * Unified info structure.  This is pointed to by both the dentry and
  * inode structures.  Each file in the filesystem has an instance of this
@@ -126,8 +128,7 @@ struct autofs_sb_info {
 
 static inline struct autofs_sb_info *autofs_sbi(struct super_block *sb)
 {
-	return sb->s_magic != AUTOFS_SUPER_MAGIC ?
-		NULL : (struct autofs_sb_info *)(sb->s_fs_info);
+	return (struct autofs_sb_info *)(sb->s_fs_info);
 }
 
 static inline struct autofs_info *autofs_dentry_ino(struct dentry *dentry)

fs/autofs/dev-ioctl.c

@@ -151,22 +151,6 @@ static int validate_dev_ioctl(int cmd, struct autofs_dev_ioctl *param)
 	return err;
 }
 
-/*
- * Get the autofs super block info struct from the file opened on
- * the autofs mount point.
- */
-static struct autofs_sb_info *autofs_dev_ioctl_sbi(struct file *f)
-{
-	struct autofs_sb_info *sbi = NULL;
-	struct inode *inode;
-
-	if (f) {
-		inode = file_inode(f);
-		sbi = autofs_sbi(inode->i_sb);
-	}
-	return sbi;
-}
-
 /* Return autofs dev ioctl version */
 static int autofs_dev_ioctl_version(struct file *fp,
 				    struct autofs_sb_info *sbi,
@@ -658,6 +642,8 @@ static int _autofs_dev_ioctl(unsigned int command,
 	if (cmd != AUTOFS_DEV_IOCTL_VERSION_CMD &&
 	    cmd != AUTOFS_DEV_IOCTL_OPENMOUNT_CMD &&
 	    cmd != AUTOFS_DEV_IOCTL_CLOSEMOUNT_CMD) {
+		struct super_block *sb;
+
 		fp = fget(param->ioctlfd);
 		if (!fp) {
 			if (cmd == AUTOFS_DEV_IOCTL_ISMOUNTPOINT_CMD)
@@ -666,12 +652,13 @@ static int _autofs_dev_ioctl(unsigned int command,
 			goto out;
 		}
 
-		sbi = autofs_dev_ioctl_sbi(fp);
-		if (!sbi || sbi->magic != AUTOFS_SBI_MAGIC) {
+		sb = file_inode(fp)->i_sb;
+		if (sb->s_type != &autofs_fs_type) {
 			err = -EINVAL;
 			fput(fp);
 			goto out;
 		}
+		sbi = autofs_sbi(sb);
 
 		/*
 		 * Admin needs to be able to set the mount catatonic in

fs/autofs/init.c

@@ -16,7 +16,7 @@ static struct dentry *autofs_mount(struct file_system_type *fs_type,
 	return mount_nodev(fs_type, flags, data, autofs_fill_super);
 }
 
-static struct file_system_type autofs_fs_type = {
+struct file_system_type autofs_fs_type = {
 	.owner		= THIS_MODULE,
 	.name		= "autofs",
 	.mount		= autofs_mount,