blk-throttle: fix divide-by-zero on legacy iops limit of 0
Subsystems (3)
PATCH v4 1/1 blk-throttle: fix divide-by-zero on legacy iops limit of 0
From: Tao Cui <[email protected]>
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.
---
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(-)
block/blk-throttle.c +4 -2
@@ -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 ;
Signature · 1 line
2.43.0
Quoted 40 lines · Tao Cui
From: Tao Cui<[email protected]>
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 withmin_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 reachestg_within_iops_limit(), which computesjiffy_wait = max(jiffy_wait, HZ / iops_limit + 1);triggering a divide-by-zero oops.
Fix it in
tg_set_conf()by clamping the value toUINT_MAX, consistent withtg_set_limit(). This closes the truncation root cause: with 0 no longer reachable as a stored limit, the HZ /iops_limitdivide is never hit.Signed-off-by Tao Cui<[email protected]>---
Changes in v4:
- Drop the defensive "iops_limit== 0" check intg_dispatch_iops_time():
with thetg_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_limitdivide, so attributing it there was incorrect. (Yu Kuai)Changes in v3:
- Drop the (u64) cast onUINT_MAX: the kernel's type-checkedmin()accepts two unsigned types of different width (both >= 4 bytes), somin(v, UINT_MAX)compiles clean. (David Laight)
Quoted 8 lines
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.
- Usemin(v, UINT_MAX)instead ofmin_t(u64, v, UINT_MAX).--- block/blk-throttle.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
Signature · 2 lines
Thanks, Kuai