vfs: change sb->s_maxbytes to a loff_t

sb->s_maxbytes is supposed to indicate the maximum size of a file that can
exist on the filesystem.  It's declared as an unsigned long long.

Even if a filesystem has no inherent limit that prevents it from using
every bit in that unsigned long long, it's still problematic to set it to
anything larger than MAX_LFS_FILESIZE.  There are places in the kernel
that cast s_maxbytes to a signed value.  If it's set too large then this
cast makes it a negative number and generally breaks the comparison.

Change s_maxbytes to be loff_t instead.  That should help eliminate the
temptation to set it too large by making it a signed value.

Also, add a warning for couple of releases to help catch filesystems that
set s_maxbytes too large.  Eventually we can either convert this to a
BUG() or just remove it and in the hope that no one will get it wrong now
that it's a signed value.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Robert Love <rlove@google.com>
Cc: Mandeep Singh Baines <msb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/super.c

@@ -892,6 +892,16 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
  	if (error)
  		goto out_sb;
 
+	/*
+	 * filesystems should never set s_maxbytes larger than MAX_LFS_FILESIZE
+	 * but s_maxbytes was an unsigned long long for many releases. Throw
+	 * this warning for a little while to try and catch filesystems that
+	 * violate this rule. This warning should be either removed or
+	 * converted to a BUG() in 2.6.34.
+	 */
+	WARN((mnt->mnt_sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
+		"negative value (%lld)\n", type->name, mnt->mnt_sb->s_maxbytes);
+
 	mnt->mnt_mountpoint = mnt->mnt_root;
 	mnt->mnt_parent = mnt;
 	up_write(&mnt->mnt_sb->s_umount);

include/linux/fs.h

@@ -1315,7 +1315,7 @@ struct super_block {
 	unsigned long		s_blocksize;
 	unsigned char		s_blocksize_bits;
 	unsigned char		s_dirt;
-	unsigned long long	s_maxbytes;	/* Max file size */
+	loff_t			s_maxbytes;	/* Max file size */
 	struct file_system_type	*s_type;
 	const struct super_operations	*s_op;
 	const struct dquot_operations	*dq_op;