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

Tao Cui posted 1 patch an hour ago
There is a newer version of this series
block/blk-throttle.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
[PATCH v5] blk-throttle: fix divide-by-zero on legacy iops limit of 0
Posted by Tao Cui an hour ago 
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 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@kylinos.cn>
Reviewed-by: Yu Kuai <yukuai@huawei.com>

---
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@linux.dev

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;
-- 
2.43.0
Re: [PATCH v5] blk-throttle: fix divide-by-zero on legacy iops limit of 0
Posted by yu kuai an hour ago 
Hi,

在 2026/9/4 11:06, Tao Cui 写道:
> 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 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@kylinos.cn>
> Reviewed-by: Yu Kuai <yukuai@huawei.com>
This tag is wrong, please use yukuai@fygo.io.
>
> ---
> 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@linux.dev
>
> 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;

-- 
Thanks,
Kuai
Re: [PATCH v5] blk-throttle: fix divide-by-zero on legacy iops limit of 0
Posted by Tao Cui an hour ago 

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@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 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@kylinos.cn>
>> Reviewed-by: Yu Kuai <yukuai@huawei.com>
> This tag is wrong, please use yukuai@fygo.io.

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@linux.dev
>>
>> 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;
>

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