On Jun 17, 2020, at 1:19 PM, Eric Sandeen <sandeen@xxxxxxxxxx> 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@xxxxxxxxxx>
Reviewed-by: Andreas Dilger <adilger@xxxxxxxxx>
> ---
>
> 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;
> +
> hash2 = map[split].hash;
> continued = hash2 == map[split - 1].hash;
> dxtrace(printk(KERN_INFO "Split block %lu at %x, %i/%i\n",
>
>
Cheers, Andreas
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