linux-ext4 - [PATCH] libext2fs: fix off-by-one bug in ext2fs_extent

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Message-Id: <1389847317-19016-1-git-send-email-tytso@mit.edu>
Date:	Wed, 15 Jan 2014 23:41:57 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Cc:	Theodore Ts'o <tytso@....edu>
Subject: [PATCH] libext2fs: fix off-by-one bug in ext2fs_extent_insert()

When inserting the first extent into an empty inode, the
ext2fs_extent_insert() leaves path->left set to 1 instead of 0.  Since
path->curr is pointing at the last (only) extent in the file,
path->left should be 0.

This is mostly harmless, and gets corrected fairly quickly if the
calling applicaton jumps to a different part of the extent tree ---
for example, by calling ext2fs_extent_goto(), or calling
ext2fs_extent_get with the flags argument set to EXT2_EXTENT_ROOT.
Which is why we hadn't noticed this problem until now.

However, if you insert four extents using ext2fs_extent_insert, the
fourth insert will end up copying many bytes in the i_block[] array,
since path->left is one larger than it should be.  This results in the
inode fields i_generation, i_file_acl, and i_size_high getting zeroed
out.

This problem can be replicated as follows:

% mke2fs -F -t /tmp/ext4 /tmp/foo.img 100
% debugfs -w /tmp/foo.img
debugfs: copy /dev/null foo
debugfs: set_inode_field foo i_size_hi 1
debugfs: stat foo
debugfs: extent_open foo
debugfs (extent ino 12): insert --after 0 1 100
debugfs (extent ino 12): insert --after 1 1 101
debugfs (extent ino 12): insert --after 2 1 102
debugfs (extent ino 12): insert --after 3 1 103
debugfs (extent ino 12): extent_close
debugfs: stat foo
debugfs: quit

Signed-off-by: "Theodore Ts'o" <tytso@....edu>
---
 lib/ext2fs/extent.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/ext2fs/extent.c b/lib/ext2fs/extent.c
index 5cdc2e4..6f4f1d2 100644
--- a/lib/ext2fs/extent.c
+++ b/lib/ext2fs/extent.c
@@ -1092,8 +1092,10 @@ errcode_t ext2fs_extent_insert(ext2_extent_handle_t handle, int flags,
 			ix++;
 			path->left--;
 		}
-	} else
+	} else {
 		ix = EXT_FIRST_INDEX(eh);
+		path->left = -1;
+	}

 	path->curr = ix;

-- 
1.8.5.rc3.362.gdf10213

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