[PATCH v3] 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. The unclamped write in tg_set_conf()
is long-standing, but it only became a crash once the HZ / iops_limit
divide was added.

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.

Fixes: 1beabab88ece ("blk-throttle: fix lower control under super low iops limit")
Signed-off-by: Tao Cui <cuitao@kylinos.cn>

---
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)
- Add a Fixes: tag pointing at the commit that introduced the
  HZ / iops_limit divide, which is also where the oops became reachable.

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 | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index ffc3b70065d4..e894852c3142 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -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, UINT_MAX);
 
 	tg_conf_updated(tg, false);
 	ret = 0;
-- 
2.43.0