[PATCH v2] blk-cgroup: avoid 32-bit overflow in root io.stat byte accounting

From: Tao Cui <redacted>

blkcg_fill_root_iostats() converts per-CPU sector counts to bytes with

    tmp.bytes[BLKG_IOSTAT_READ] += cpu_dkstats->sectors[STAT_READ] << 9;

but disk_stats.sectors is `unsigned long`, and the shift is carried out in
that type before the result is promoted to the u64 accumulator.  On 32-bit
kernels (unsigned long is 32 bits) this wraps, so once a per-CPU counter
reaches 2**23 sectors (~4 GiB) the computed byte count is wrong, corrupting
the root cgroup's io.stat.

Every other sector->byte conversion in the tree casts to a wide type first
((loff_t)sectors << SECTOR_SHIFT in bdev.c, (u64)max_sectors << SECTOR_SHIFT
in blk-settings.c); do the same here.

Fixes: ef45fe470e1e5 ("blk-cgroup: show global disk stats in root cgroup io.stat")
Signed-off-by: Tao Cui <redacted>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Tang Yizhou <redacted>

---
Changes in v2:
- Rebase onto current linux-next head (no code change).
- Add the Reviewed-by tags collected on v1 (Bart Van Assche, Tang Yizhou).
- Link to v1: https://lore.kernel.org/r/20260714063118.1153083-1-cui.tao@linux.dev (local)
---
 block/blk-cgroup.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 2b5c29434e42..471cfd2347c9 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c

@@ -1118,11 +1118,11 @@ static void blkcg_fill_root_iostats(void)
 				cpu_dkstats->ios[STAT_DISCARD];
 			// convert sectors to bytes
 			tmp.bytes[BLKG_IOSTAT_READ] +=
-				cpu_dkstats->sectors[STAT_READ] << 9;
+				(u64)cpu_dkstats->sectors[STAT_READ] << SECTOR_SHIFT;
 			tmp.bytes[BLKG_IOSTAT_WRITE] +=
-				cpu_dkstats->sectors[STAT_WRITE] << 9;
+				(u64)cpu_dkstats->sectors[STAT_WRITE] << SECTOR_SHIFT;
 			tmp.bytes[BLKG_IOSTAT_DISCARD] +=
-				cpu_dkstats->sectors[STAT_DISCARD] << 9;
+				(u64)cpu_dkstats->sectors[STAT_DISCARD] << SECTOR_SHIFT;
 		}
 
 		flags = u64_stats_update_begin_irqsave(&blkg->iostat.sync);

-- 
2.43.0