linux-kernel - Re: NULL pointer dereference in ext4_ext_remove

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Message-ID: <CANP3RGdMfM0tZbaJS9dFetRhXoGtyS0Nx4hZER_Qv0a061X_dQ@mail.gmail.com>
Date:	Thu, 16 Aug 2012 13:21:12 -0700
From:	Maciej Żenczykowski <maze@...gle.com>
To:	"Theodore Ts'o" <tytso@....edu>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Marti Raudsepp <marti@...fo.org>,
	Kernel hackers <linux-kernel@...r.kernel.org>,
	ext4 hackers <linux-ext4@...r.kernel.org>, maze@...gle.com
Subject: Re: NULL pointer dereference in ext4_ext_remove_space on 3.5.1

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@....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
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