Re: NULL pointer dereference in ext4_ext_remove_space on 3.5.1

Maciej Żenczykowski
2012-08-16T20:21:15Z

This would probably be much more readable code if the 'i=0' init was
before path=kzalloc.

On Thu, Aug 16, 2012 at 8:25 AM, Theodore Ts'o <tytso@mit.edu> wrote:
> On Thu, Aug 16, 2012 at 07:10:51PM +0800, Fengguang Wu wrote:
>>
>> Here is the dmesg. BTW, it seems 3.5.0 don't have this issue.
>
> Fengguang,
>
> It sounds like you have a (at least fairly) reliable reproduction for
> this problem? Is it something you can share? It would be good to get
> this into our test suites, since it was _not_ something that was
> caught by xfstests, apparently.
>
> Can you see if this patch addresses it? (The first two patch hunks
> are the same debugging additions I had posted before.)
>
> It looks like the responsible commit is 968dee7722: "ext4: fix hole
> punch failure when depth is greater than 0". I had thought this patch
> was low risk if you weren't using the new punch ioctl, but it turns
> out it did make a critical change in the non-punch (i.e., truncate)
> code path, which is what the addition of "i = 0;" in the patch below
> addresses.
>
> Regards,
>
> - Ted
>
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 769151d..fa829dc 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -2432,6 +2432,10 @@ ext4_ext_rm_leaf(handle_t *handle, struct inode *inode,
>
> /* the header must be checked already in ext4_ext_remove_space() */
> ext_debug("truncate since %u in leaf to %u\n", start, end);
> + if (!path[depth].p_hdr && !path[depth].p_bh) {
> + EXT4_ERROR_INODE(inode, "depth %d", depth);
> + BUG_ON(1);
> + }
> if (!path[depth].p_hdr)
> path[depth].p_hdr = ext_block_hdr(path[depth].p_bh);
> eh = path[depth].p_hdr;
> @@ -2730,6 +2734,10 @@ cont:
> /* this is index block */
> if (!path[i].p_hdr) {
> ext_debug("initialize header\n");
> + if (!path[i].p_hdr && !path[i].p_bh) {
> + EXT4_ERROR_INODE(inode, "i=%d", i);
> + BUG_ON(1);
> + }
> path[i].p_hdr = ext_block_hdr(path[i].p_bh);
> }
>
> @@ -2828,6 +2836,7 @@ out:
> kfree(path);
> if (err == -EAGAIN) {
> path = NULL;
> + i = 0;
> goto again;
> }
> ext4_journal_stop(handle);

--
Maciej A. Żenczykowski
Kernel Networking Developer @ Google
1600 Amphitheatre Parkway, Mountain View, CA 94043
tel: +1 (650) 253-0062