Re: NULL pointer dereference in ext4_ext_remove_space on 3.5.1

Theodore Ts'o
2012-08-16T15:25:21Z

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);