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Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] block: Detect multiplication o
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] block: Detect multiplication overflow in bdrv_getlength |
Date: |
Fri, 15 May 2015 10:10:47 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux) |
Fam Zheng <address@hidden> writes:
> Bogus image may have a large total_sectors that will overflow the
> multiplication. For cleanness, fix the return code so the error message
> will be meaningful.
>
> Reported-by: Richard W.M. Jones <address@hidden>
> Signed-off-by: Fam Zheng <address@hidden>
> ---
> block.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/block.c b/block.c
> index 7904098..5ee3fdf 100644
> --- a/block.c
> +++ b/block.c
> @@ -2330,6 +2330,7 @@ int64_t bdrv_getlength(BlockDriverState *bs)
> {
> int64_t ret = bdrv_nb_sectors(bs);
>
> + ret = (int64_t)(ret * BDRV_SECTOR_SIZE) < 0 ? -EFBIG : ret;
> return ret < 0 ? ret : ret * BDRV_SECTOR_SIZE;
> }
Signed integer overflow is undefined behavior. Your code works just
fine on any remotely sane machine, *except* when the optimizer decides
to use its undefined behavior license to mess with you.
A more prudent way to test for overflow would be something like
ret > INT64_MAX / BDRV_SECTOR_SIZE