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 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.

---
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, (u64)UINT_MAX) instead of min_t(u64, v, UINT_MAX).

---
 block/blk-throttle.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

index ffc3b70065d4..9900443b0ad5 100644
@@ -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);

@@ -1383,10 +1388,12 @@ 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, (u64)UINT_MAX);
 
 	tg_conf_updated(tg, false);
 	ret = 0;

2.43.0