Re: [PATCH] bpf: hashtab: fix 32-bit overflow in memory usage calculation

[Yafang Shao]

On Fri, Nov 7, 2025 at 4:59 AM Alexei Safin <a.safin@xxxxxxx> wrote:
>
> The intermediate product value_size * num_possible_cpus() is evaluated
> in 32-bit arithmetic and only then promoted to 64 bits. On systems with
> large value_size and many possible CPUs this can overflow and lead to
> an underestimated memory usage.
>
> Cast value_size to u64 before multiplying.
>
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
>
> Fixes: 304849a27b34 ("bpf: hashtab memory usage")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Alexei Safin <a.safin@xxxxxxx>
> ---
>  kernel/bpf/hashtab.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 570e2f723144..7ad6b5137ba1 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -2269,7 +2269,7 @@ static u64 htab_map_mem_usage(const struct bpf_map *map)
>                 usage += htab->elem_size * num_entries;
>
>                 if (percpu)
> -                       usage += value_size * num_possible_cpus() * num_entries;
> +                       usage += (u64)value_size * num_possible_cpus() * num_entries;
>                 else if (!lru)
>                         usage += sizeof(struct htab_elem *) * num_possible_cpus();
>         } else {
> @@ -2281,7 +2281,7 @@ static u64 htab_map_mem_usage(const struct bpf_map *map)
>                 usage += (htab->elem_size + LLIST_NODE_SZ) * num_entries;
>                 if (percpu) {
>                         usage += (LLIST_NODE_SZ + sizeof(void *)) * num_entries;
> -                       usage += value_size * num_possible_cpus() * num_entries;
> +                       usage += (u64)value_size * num_possible_cpus() * num_entries;
>                 }
>         }
>         return usage;
> --
> 2.50.1 (Apple Git-155)
>

Thanks for the fix. What do you think about this change?

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 4a9eeb7aef85..f9084158bfe2 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -2251,7 +2251,7 @@ static long bpf_for_each_hash_elem(struct
bpf_map *map, bpf_callback_t callback_
 static u64 htab_map_mem_usage(const struct bpf_map *map)
 {
        struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
-       u32 value_size = round_up(htab->map.value_size, 8);
+       u64 value_size = round_up(htab->map.value_size, 8);
        bool prealloc = htab_is_prealloc(htab);
        bool percpu = htab_is_percpu(htab);
        bool lru = htab_is_lru(htab);


-- 
Regards
Yafang