Re: [PATCH 1/1] ext4: fix potential negative array index in do_split()

[Lukas Czerner <lczerner@redhat.com>]

On Wed, Jun 17, 2020 at 02:19:04PM -0500, Eric Sandeen wrote:
> If for any reason a directory passed to do_split() does not have enough
> active entries to exceed half the size of the block, we can end up
> iterating over all "count" entries without finding a split point.
> 
> In this case, count == move, and split will be zero, and we will
> attempt a negative index into map[].
> 
> Guard against this by detecting this case, and falling back to
> split-to-half-of-count instead; in this case we will still have
> plenty of space (> half blocksize) in each split block.
> 
> Fixes: ef2b02d3e617 ("ext34: ensure do_split leaves enough free space in both blocks")
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> ---
> 
> diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> index a8aca4772aaa..8b60881f07ee 100644
> --- a/fs/ext4/namei.c
> +++ b/fs/ext4/namei.c
> @@ -1858,7 +1858,7 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
>  			     blocksize, hinfo, map);
>  	map -= count;
>  	dx_sort_map(map, count);
> -	/* Split the existing block in the middle, size-wise */
> +	/* Ensure that neither split block is over half full */
>  	size = 0;
>  	move = 0;
>  	for (i = count-1; i >= 0; i--) {
> @@ -1868,8 +1868,18 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
>  		size += map[i].size;
>  		move++;
>  	}
> -	/* map index at which we will split */
> -	split = count - move;
> +	/*
> +	 * map index at which we will split
> +	 *
> +	 * If the sum of active entries didn't exceed half the block size, just
> +	 * split it in half by count; each resulting block will have at least
> +	 * half the space free.
> +	 */
> +	if (i > 0)
> +		split = count - move;
> +	else
> +		split = count/2;

Won't we have exactly the same problem as we did before your commit
ef2b02d3e617cb0400eedf2668f86215e1b0e6af ? Since we do not know how much
space we actually moved we might have not made enough space for the new
entry ?

Also since we have the move == count when the problem appears then it's
clear that we never hit the condition

1865 →       →       /* is more than half of this entry in 2nd half of the block? */
1866 →       →       if (size + map[i].size/2 > blocksize/2)
1867 →       →       →       break;

in the loop. This is surprising but it means the the entries must have
gaps between them that are small enough that we can't fit the entry
right in ? Should not we try to compact it before splitting, or is it
the case that this should have been done somewhere else ?

If we really want ot be fair and we want to split it right in the middle
of the entries size-wise then we need to keep track of of sum of the
entries and decide based on that, not blocksize/2. But maybe the problem
could be solved by compacting the entries together because the condition
seems to rely on that.

-Lukas

> +
>  	hash2 = map[split].hash;
>  	continued = hash2 == map[split - 1].hash;
>  	dxtrace(printk(KERN_INFO "Split block %lu at %x, %i/%i\n",
> 
>