Re: Apparent serious progressive ext4 data corruption bug in 3.6.3(and other stable branches?)

[Eric Sandeen]

On 10/28/12 8:00 PM, Theodore Ts'o wrote:
> On Sat, Oct 27, 2012 at 05:42:07PM -0500, Eric Sandeen wrote:
>>
>> It looks like the inode_bitmap_bh is being modified outside a transaction:
>>
>>                 ret2 = ext4_test_and_set_bit(ino, inode_bitmap_bh->b_data);
>>
>> It needs a get_write_access / handle_dirty_metadata pair around it.
> 
> Oops.   Nice catch!!
> 
> The patch isn't quite right, though.  

Yeah, I knew it wasn't ;)  I did resend 
[PATCH] ext4: fix unjournaled inode bitmap modification
which is a bit more involved.

> We only want to call
> ext4_journal_get_write_access() when we know that there is an available
> bit in the bitmap.  (We could still lose the race, but in that case
> the winner of the race probably grabbed the bitmap block first.)
> 
> Also, we only need to call ext4_handle_dirty_metadata() if we
> successfully grab the bit in the bitmap.
> 
> So I suggest this patch instead:

That'll get_write_access on the same buffer over and over, I suppose
it's ok, but the patch I sent tries to minimize that, and call
ext4_handle_release_buffer if we're not going to use it (which is
a no-op today anyway and not normally used I guess...)

If ext4_handle_release_buffer() is dead code now, and repeated calls
via repeat_in_this_group: are no big deal, then your version looks fine.

-Eric

> commit 087eda81f1ac6a6a0394f781b44f1d555d8f64c6
> Author: Eric Sandeen <sandeen@xxxxxxxxxx>
> Date:   Sun Oct 28 20:59:57 2012 -0400
> 
>     ext4: fix unjournaled inode bitmap modification
>     
>     commit 119c0d4460b001e44b41dcf73dc6ee794b98bd31 modified this function
>     such that the inode bitmap was being modified outside a transaction,
>     which could lead to corruption, and was discovered when journal_checksum
>     found a bad checksum in the journal.
>     
>     Reported-by: Nix <nix@xxxxxxxxxxxxx>
>     Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
>     Signed-off-by: "Theodore Ts'o" <tytso@xxxxxxx>
>     Cc: stable@xxxxxxxxxxxxxxx
> 
> diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
> index 4facdd2..575afac 100644
> --- a/fs/ext4/ialloc.c
> +++ b/fs/ext4/ialloc.c
> @@ -725,6 +725,10 @@ repeat_in_this_group:
>  				   "inode=%lu", ino + 1);
>  			continue;
>  		}
> +		BUFFER_TRACE(inode_bitmap_bh, "get_write_access");
> +		err = ext4_journal_get_write_access(handle, inode_bitmap_bh);
> +		if (err)
> +			goto fail;
>  		ext4_lock_group(sb, group);
>  		ret2 = ext4_test_and_set_bit(ino, inode_bitmap_bh->b_data);
>  		ext4_unlock_group(sb, group);
> @@ -738,6 +742,11 @@ repeat_in_this_group:
>  	goto out;
>  
>  got:
> +	BUFFER_TRACE(inode_bitmap_bh, "call ext4_handle_dirty_metadata");
> +	err = ext4_handle_dirty_metadata(handle, NULL, inode_bitmap_bh);
> +	if (err)
> +		goto fail;
> +
>  	/* We may have to initialize the block bitmap if it isn't already */
>  	if (ext4_has_group_desc_csum(sb) &&
>  	    gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
> --
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