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

[Tao Cui <cui.tao@linux.dev>]

在 2026/7/14 19:37, David Laight 写道:
> On Tue, 14 Jul 2026 18:35:52 +0800
> Tao Cui <cui.tao@linux.dev> 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.
>>
>> 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);
> 
> The LHS casts look horrid - there has to be a nicer way to do that.
> 
> And you don't need min_t() a plain min() will be fine.
> 
Hi David,

Both done in v2 — introduced a void *field local so the writes read
*(u64 *)field / *(unsigned int *)field, and switched to
min(v, (u64)UINT_MAX).

Thanks,
Tao
> 	David
> 
> 
> 
>>  
>>  	tg_conf_updated(tg, false);
>>  	ret = 0;
>