Re: [PATCH 3/3] assoc_array: trim the final shortcut word when skip_to_level is chunk-aligned

[Jarkko Sakkinen <jarkko@kernel.org>]

On Sat, Jul 11, 2026 at 09:45:00PM -0400, Michael Bommarito wrote:
> assoc_array_walk() masks off the bits past shortcut->skip_to_level in
> the final word of a shortcut before testing it, gated on
> round_up(sc_level, chunk_size) > skip_to_level.  Once sc_level is
> word-aligned (every word after the first) round_up() is a no-op and the
> guard never fires for the word that contains skip_to_level, so its stale
> high bits leak into the dissimilarity word and can steer the walk down
> the wrong descendant.
> 
> Test sc_level + ASSOC_ARRAY_KEY_CHUNK_SIZE > skip_to_level directly; an
> exact-multiple skip_to_level ends on the boundary and stays untrimmed.
> 
> Fixes: 3cb989501c26 ("Add a generic associative array implementation.")
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
> ---
>  lib/assoc_array.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/assoc_array.c b/lib/assoc_array.c
> index bcc6e0a013eb8..1de2c337f8fcf 100644
> --- a/lib/assoc_array.c
> +++ b/lib/assoc_array.c
> @@ -255,7 +255,7 @@ assoc_array_walk(const struct assoc_array *array,
>  		sc_segments = shortcut->index_key[sc_level >> ASSOC_ARRAY_KEY_CHUNK_SHIFT];
>  		dissimilarity = segments ^ sc_segments;
>  
> -		if (round_up(sc_level, ASSOC_ARRAY_KEY_CHUNK_SIZE) > shortcut->skip_to_level) {
> +		if (sc_level + ASSOC_ARRAY_KEY_CHUNK_SIZE > shortcut->skip_to_level) {
>  			/* Trim segments that are beyond the shortcut */
>  			int shift = shortcut->skip_to_level & ASSOC_ARRAY_KEY_CHUNK_MASK;
>  			dissimilarity &= ~(ULONG_MAX << shift);
> -- 
> 2.53.0
> 

Ditto.

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

BR, Jarkko