[PATCH v4] blk-throttle: fix divide-by-zero on legacy iops limit of 0

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

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>

---
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