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Re: [Qemu-devel] [PATCH v2 1/3] dmg: fix binary search
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH v2 1/3] dmg: fix binary search |
Date: |
Wed, 2 Jan 2019 11:49:19 +0000 |
User-agent: |
Mutt/1.10.1 (2018-07-13) |
On Sun, Dec 23, 2018 at 10:59:37AM +0800, yuchenlin wrote:
> There is a possible hang in original binary search implementation. That is
> if chunk1 = 4, chunk2 = 5, chunk3 = 4, and we go else case.
>
> The chunk1 will be still 4, and so on.
>
> Signed-off-by: yuchenlin <address@hidden>
> ---
> block/dmg.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/block/dmg.c b/block/dmg.c
> index 50e91aef6d..0e05702f5d 100644
> --- a/block/dmg.c
> +++ b/block/dmg.c
> @@ -572,14 +572,14 @@ static inline uint32_t search_chunk(BDRVDMGState *s,
> uint64_t sector_num)
> {
> /* binary search */
> uint32_t chunk1 = 0, chunk2 = s->n_chunks, chunk3;
> - while (chunk1 != chunk2) {
> + while (chunk1 <= chunk2) {
> chunk3 = (chunk1 + chunk2) / 2;
> if (s->sectors[chunk3] > sector_num) {
> - chunk2 = chunk3;
> + chunk2 = chunk3 - 1;
Question from the previous email you sent:
What happens when chunk1 = 0, chunk2 = 1, and chunk3 = 0? This would
cause out-of-bounds sectors[] accesses.
Stefan
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