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

[Tao Cui]

于 2026年7月16日 GMT+08:00 00:13:06，Haris Iqbal <haris.iqbal@xxxxxxxxx> 写道：
>
>
>On 7/14/26 12:35, Tao Cui wrote:
>> From: Tao Cui <cuitao@xxxxxxxxxx>
>> 
>> 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.
>
Noted, I will address this in the next version.

Thanks,
Tao

>> Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
>> ---
>>   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;
>