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>

2 files changed, 11 insertions, 1 deletions

diff --git a/fs/super.c b/fs/super.c
index 0e7207b9815c..4906e2d8f400 100644
--- a/fs/super.c
+++ b/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);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 955e34615cb7..cbb7724c11d3 100644
--- a/include/linux/fs.h
+++ b/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;