Re: [PATCH] mm/kmemleak: fix checksum computation for per-cpu objects

[Catalin Marinas]

On Fri, Jul 03, 2026 at 09:17:24AM -0700, Breno Leitao wrote:
> The per-cpu object checksum folds each CPU's CRC together with XOR and
> seeds every CRC with 0. Both choices make update_checksum() miss content
> changes:
> 
>   - XOR is self-cancelling, so equal contents on two CPUs cancel out and
>     simultaneous identical changes leave the checksum unchanged.
>   - crc32(0, ...) over all-zero content is 0, so a freshly allocated,
>     zeroed per-cpu area checksums to 0, matching the initial value, and
>     the object is never seen to change.
> 
> See discussions at [0].
> 
> When update_checksum() wrongly reports an actively modified object as
> unchanged, kmemleak stops greying it for an extra scan and can report a
> live per-cpu object as a leak.
> 
> Fold the per-cpu CRC as a single rolling checksum across all CPUs and
> initialise the object checksum to ~0 so the first computed value always
> registers as a change, even for content that hashes to 0.
> reset_checksum() is seeded the same way.
> 
> Link: https://lore.kernel.org/all/akfYImSNDh3OjIfR@xxxxxxxxx [0]
> Co-developed-by: Catalin Marinas <catalin.marinas@xxxxxxx>

Since you added co-developed-by, I think it needs this as well:

Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>

(otherwise I'm fine with suggested-by)

Thanks for the investigation and posting this.

-- 
Catalin