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Re: [Qemu-trivial] [PATCH] dmg: fix binary search
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-trivial] [PATCH] dmg: fix binary search |
Date: |
Wed, 2 Jan 2019 10:46:59 +0000 |
User-agent: |
Mutt/1.10.1 (2018-07-13) |
On Fri, Dec 21, 2018 at 09:58:03PM +0800, yuchenlin wrote:
> There is a possible hang in original binary searsh implemtation. 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(-)
Do you have an example dmg file that triggers this bug?
> 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;
What if chunk1 = 0 and chunk2 = 1? chunk3 = 0 and the new chunk2 value
would be 0xffffffff so the loop continues with out-of-bounds sectors[]
accesses!
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