Re: [PATCH v5] blk-throttle: fix divide-by-zero on legacy iops limit of 0

[Tao Cui]

在 2026/9/4 11:12, yu kuai 写道:
> Hi,
> 
> 在 2026/9/4 11:06, Tao Cui 写道:
>> 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 tg_set_conf() by clamping the value to UINT_MAX, consistent
>> with tg_set_limit(). This closes the truncation root cause: with 0 no
>> longer reachable as a stored limit, the HZ / iops_limit divide is never
>> hit.
>>
>> Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
>> Reviewed-by: Yu Kuai <yukuai@xxxxxxxxxx>
> This tag is wrong, please use yukuai@xxxxxxx.

Sorry, will update right away.

>>
>> ---
>> Changes in v5:
>> - Rebase onto current linux-next head (no code change, context shifted only).
>> - Add the Reviewed-by tag collected on v4.
>> - Link to v4: https://lore.kernel.org/r/20260722102459.253189-1-cui.tao@xxxxxxxxx
>>
>> Changes in v4:
>> - Drop the defensive "iops_limit == 0" check in tg_dispatch_iops_time():
>>    with the tg_set_conf() clamp in place, 0 can never be stored as a limit,
>>    so the runtime check only guards an unreachable state. (Yu Kuai)
>> - Drop the Fixes: tag: the unclamped write -- and the iops=0 behavior it
>>    can produce (calculate_io_allowed() returns 0, so no IO is issued) --
>>    long predates the commit that added the HZ / iops_limit divide, so
>>    attributing it there was incorrect. (Yu Kuai)
>>
>> Changes in v3:
>> - Drop the (u64) cast on UINT_MAX: the kernel's type-checked min() accepts
>>    two unsigned types of different width (both >= 4 bytes), so
>>    min(v, UINT_MAX) compiles clean. (David Laight)
>>
>> Changes in v2:
>> - Use a "void *field" local for the config write so the assignment reads
>>    *(u64 *)field / *(unsigned int *)field instead of the
>>    (type *)((void *)tg + of_cft(of)->private) casts.
>> - Use min(v, UINT_MAX) instead of min_t(u64, v, UINT_MAX).
>> ---
>>   block/blk-throttle.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
>> index ffc3b70065d4..97ad7959d006 100644
>> --- a/block/blk-throttle.c
>> +++ b/block/blk-throttle.c
>> @@ -1383,10 +1383,12 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of,
>>   	tg = blkg_to_tg(ctx.blkg);
>>   	tg_update_carryover(tg);
>>   
>> +	void *field = (void *)tg + of_cft(of)->private;
>> +
>>   	if (is_u64)
>> -		*(u64 *)((void *)tg + of_cft(of)->private) = v;
>> +		*(u64 *)field = v;
>>   	else
>> -		*(unsigned int *)((void *)tg + of_cft(of)->private) = v;
>> +		*(unsigned int *)field = min(v, UINT_MAX);
>>   
>>   	tg_conf_updated(tg, false);
>>   	ret = 0;
>