Re: [Qemu-block] [Qemu-devel] [PATCH] throttle: Assert that bkt->max is valid in throttle_compute_wait()

[Philippe Mathieu-Daudé]

Hi Alberto,

On 09/13/2017 05:28 AM, Alberto Garcia wrote:
> If bkt->max == 0 and bkt->burst_length > 1 then we could have a
> division by 0 in throttle_do_compute_wait(). That configuration is
> however not permitted and is already detected by throttle_is_valid(),
> but let's assert it in throttle_compute_wait() to make it explicit.

This is correct but I'm not sure this is enough, as 
throttle_compute_wait() is exported/public, however it seems testing is 
the only reason to export it.

Also I spent 10min looking at it thinking about how bkt->max is used, 
before to realize there should be a simpler way to write this (KISS).

Anyway I'm queuing your patch in my /static-analysis branch I plan to 
this week where I use a self-explanatory macro instead of assert() and 
will see if I can simplify it (note that I'm not a maintainer!).

Regards,

Phil.

> Found by Coverity (CID: 1381016).
>
> Signed-off-by: Alberto Garcia <address@hidden>
> ---
>   util/throttle.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/util/throttle.c b/util/throttle.c
> index 06bf916adc..b38e742da5 100644
> --- a/util/throttle.c
> +++ b/util/throttle.c
> @@ -124,6 +124,7 @@ int64_t throttle_compute_wait(LeakyBucket *bkt)
>       /* If the main bucket is not full yet we still have to check the
>        * burst bucket in order to enforce the burst limit */
>       if (bkt->burst_length > 1) {
> +        assert(bkt->max > 0); /* see throttle_is_valid() */
>           extra = bkt->burst_level - burst_bucket_size;
>           if (extra > 0) {
>               return throttle_do_compute_wait(bkt->max, extra);