[PATCH] ext2 FS corruption in 2.5.3-pre[3-5]

[Mikael Pettersson <mikpe@csd.uu.se>]

Al & Linus,

There is a file system corruption problem in 2.5.3-pre[3-5], caused
by leaking uninitialised memory to in-core and later on-disk inodes.

I can easily observe the problem by creating lots of files in an
ext2 FS, reboot to single-user, and fsck the ext2 FS partition.
fsck will then give a lot of warnings about:

"i_fsize for inode NNN (...) is XX, should be zero"

where XX is a random non-zero 8-bit number.

I traced it to the pre2->pre3 transition, which contains:
> - Al Viro: VFS inode allocation moved down to filesystem, trim inodes

When FS-specific data was stored in inode->u, that data was cleared by
the memset() in fs/inode.c:alloc_inode(). The FS didn't have to bother.
Since -pre3, ext2 (and perhaps other FSs, I haven't checked) uses an
in-core inode layout consisting of an FS-specific head followed by the
generic inode. ext2 allocates these in fs/ext2/super.c:ext2_alloc_inode(),
but doesn't clear the ext2-specific fields. fs/inode.c:alloc_inode()
neither knows about these fields nor clears them, so the ext2-specific
data remains uninitialised when the new inode is returned for use. Ouch.

The patch below fixes this by adding the missing memset() to
fs/ext2/super.c:ext2_alloc_inode(). Works fine over here.

/Mikael

--- linux-2.5.3-pre5/fs/ext2/super.c.~1~	Fri Jan 25 00:03:58 2002
+++ linux-2.5.3-pre5/fs/ext2/super.c	Fri Jan 25 00:35:12 2002
@@ -155,6 +155,7 @@
 	ei = (struct ext2_inode_info *)kmem_cache_alloc(ext2_inode_cachep, SLAB_KERNEL);
 	if (!ei)
 		return NULL;
+	memset(ei, 0, offsetof(struct ext2_inode_info, vfs_inode));
 	return &ei->vfs_inode;
 }