Re: [PATCH] blk-throttle: fix divide-by-zero on legacy iops limit of 0

[Haris Iqbal <haris.iqbal@linux.dev>]

On 7/14/26 12:35, Tao Cui wrote:
> From: Tao Cui <cuitao@kylinos.cn>
> 
> Writing a multiple of 2^32 (e.g. 4294967296) to a legacy cgroup v1
> throttle iops file (blkio.throttle.{read,write}_iops_device) silently
> truncates to 0: tg_set_conf() stores the sscanf-parsed u64 value into
> an unsigned int field with no clamping. The cgroup v2 path,
> tg_set_limit(), already clamps the same kind of value with
> min_t(u64, val, UINT_MAX), but the legacy path never did. Note that
> the "!v -> U64_MAX" mapping only catches an explicit zero and does not
> catch a value that truncates to zero.
> 
> With iops stored as 0, tg_update_has_rules() sets has_rules_iops[] and
> the next IO reaches tg_within_iops_limit(), which computes
> 
>      jiffy_wait = max(jiffy_wait, HZ / iops_limit + 1);
> 
> triggering a divide-by-zero oops.
> 
> Fix it in two places:
> 
>    * tg_set_conf(): clamp the value to UINT_MAX, consistent with
>      tg_set_limit(). This closes the truncation root cause (and the
>      general silent truncation for any value above UINT_MAX).
> 
>    * tg_dispatch_iops_time(): treat iops_limit == 0 as unlimited so the
>      divide in tg_within_iops_limit() is never reached, defending
>      against any future path that could produce a zero limit.
> 

Does this need a "Fixes:" tag.

> Signed-off-by: Tao Cui <cuitao@kylinos.cn>
> ---
>   block/blk-throttle.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index ffc3b70065d4..3f3c1374f4b2 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -883,7 +883,12 @@ static unsigned long tg_dispatch_iops_time(struct throtl_grp *tg, struct bio *bi
>   	u32 iops_limit = tg_iops_limit(tg, rw);
>   	unsigned long iops_wait;
>   
> -	if (iops_limit == UINT_MAX || tg->flags & THROTL_TG_CANCELING)
> +	/*
> +	 * iops_limit == 0 is not a valid limit. Treat it as unlimited so we
> +	 * never reach the HZ / iops_limit divide in tg_within_iops_limit().
> +	 */
> +	if (iops_limit == UINT_MAX || iops_limit == 0 ||
> +	    tg->flags & THROTL_TG_CANCELING)
>   		return 0;
>   
>   	tg_update_slice(tg, rw);
> @@ -1386,7 +1391,8 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of,
>   	if (is_u64)
>   		*(u64 *)((void *)tg + of_cft(of)->private) = v;
>   	else
> -		*(unsigned int *)((void *)tg + of_cft(of)->private) = v;
> +		*(unsigned int *)((void *)tg + of_cft(of)->private) =
> +			min_t(u64, v, UINT_MAX);
>   
>   	tg_conf_updated(tg, false);
>   	ret = 0;