Re: [PATCH bpf-next v8 2/2] bpf: htab: Reduce elem_size by 8 bytes for small key sizes

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Fri, Oct 02, 2026 at 09:17 AM T.J. Mercier <tjmercier@google.com> wrote:
> +static __always_inline struct htab_elem *
> +__lookup_elem_raw(struct bpf_htab *htab, struct hlist_nulls_head *head,
> +		  u32 hash, void *key, u32 key_size,
> +		  struct hlist_nulls_node **out_n)
>  {
>  	struct hlist_nulls_node *n;
>  	struct htab_elem *l;
>
> -	hlist_nulls_for_each_entry_rcu(l, n, head, hash_node)
> -		if (l->hash == hash && !memcmp(&l->key, key, key_size))
> -			return l;
> +	if (htab_has_hash(htab)) {

[...]

> +		unsigned long k = 0;
> +
> +		memcpy(&k, key, key_size);

My bot is saying that this will break 32-bit builds with FORTIFY_SOURCE.
__htab_map_lookup_elem_u64() inlines all of it with key_size == 8.
has_hash is a run-time load, so the compiler has to keep this branch
and it's a memcpy of 8 bytes into 4 byte 'k'.
fortify_memcpy_chk() will hit __write_overflow_field().
That's a build error in i386 and arm allmodconfig.

has_hash is a compile time constant in __htab_map_lookup_elem_u32()
and _u64(), just like key_size.
Only htab_map_gen_lookup() emits them, so the map is BPF_MAP_TYPE_HASH,
never LRU.
Don't read it from htab in these two.

pw-bot: cr