linux-ext4 - Re: [PATCH 01/54] libext2fs: Read and write full size inodes

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Date:	Fri, 9 Mar 2012 18:36:31 -0500
From:	Ted Ts'o <tytso@....edu>
To:	"Darrick J. Wong" <djwong@...ibm.com>
Cc:	Andreas Dilger <adilger.kernel@...ger.ca>,
	Sunil Mushran <sunil.mushran@...cle.com>,
	Amir Goldstein <amir73il@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	Mingming Cao <cmm@...ibm.com>,
	Joel Becker <jlbec@...lplan.org>, linux-ext4@...r.kernel.org,
	Coly Li <colyli@...il.com>
Subject: Re: [PATCH 01/54] libext2fs: Read and write full size inodes

On Tue, Mar 06, 2012 at 03:57:27PM -0800, Darrick J. Wong wrote:
> Change libext2fs to read and write full-size inodes in preparation for the
> metadata checksumming patchset, which will require this.  Due to ABI
> compatibility requirements, this change must be hidden from client programs.
> 
> Signed-off-by: Darrick J. Wong <djwong@...ibm.com>

After applying this first patch, the e2fsprogs regression test suite
blew up spectacularly caused by the malloc-managed free lists pointers
getting corrupted:

*** glibc detected *** /usr/projects/e2fsprogs/e2fsprogs/build/debugfs/debugfs: free(): invalid next size (fast): 0x000000000063f9d0 ***
======= Backtrace: =========
/lib/libc.so.6(+0x77806)[0x7ffff6e4a806]
/lib/libc.so.6(cfree+0x73)[0x7ffff6e510d3]
/usr/projects/e2fsprogs/e2fsprogs/build/lib/libext2fs.so.2(ext2fs_free_mem+0x30)[0x7ffff7bb562f]

Interestingly, valgrind was *not* useful in finding the problem;
apparently it was getting confused by the ext2fs_get_mem()
abstraction, which is unfortunate.  I'll have to look into that at
some point.

Anyway, the problem was in ext2fs_write_inode_full(), and it could be
replicated by simply writing to an inode, i.e.

   mke2fs  -F -O resize_inode -o Linux -b 1024 /tmp/image  16384
   debugfs -w -R "set_inode_field <7> mtime now" /tmp/image

is enough to trigger it.  The problem is with a 128 byte ext2 file
system, ext2fs_write_inode_full() is passed a large inode and so
bufsize is 156, but EXT2_INODE_SIZE(fs->super) is 128.  So at
lib/ext2fs/inode.c:698:

	memcpy(w_inode, inode, bufsize);

you end up writing 156 bytes into a memory buffer that was allocated
to a size of 128 bytes.  Hilarity ensues.

The fix is relatively simple:

	memcpy(w_inode, inode, (bufsize > length) ? length : bufsize);

Anyway, this is *why* running the regression tests are important.
(And why projects which don't have regression test suites are just
asking for trouble.)

They catch all sorts of interesting oversights like this....

     	       	     		    	       - Ted
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