Re: [net-next] bpf: avoid the multi checking

[Daniel Borkmann <daniel@iogearbox.net>]

On 11/21/22 11:05 AM, xiangxia.m.yue@gmail.com wrote:
> From: Tonghao Zhang <xiangxia.m.yue@gmail.com>
> 
> .map_alloc_check checked bpf_attr::max_entries, and if bpf_attr::max_entries
> == 0, return -EINVAL. bpf_htab::n_buckets will not be 0, while -E2BIG is not
> appropriate.
> 
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Andrii Nakryiko <andrii@kernel.org>
> Cc: Martin KaFai Lau <martin.lau@linux.dev>
> Cc: Song Liu <song@kernel.org>
> Cc: Yonghong Song <yhs@fb.com>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: KP Singh <kpsingh@kernel.org>
> Cc: Stanislav Fomichev <sdf@google.com>
> Cc: Hao Luo <haoluo@google.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>

Pls Cc bpf@vger.kernel.org list and $subj line should target bpf-next.

> ---
>   kernel/bpf/hashtab.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 50d254cd0709..22855d6ff6d3 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -500,9 +500,10 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
>   		htab->elem_size += round_up(htab->map.value_size, 8);
>   
>   	err = -E2BIG;
> -	/* prevent zero size kmalloc and check for u32 overflow */
> -	if (htab->n_buckets == 0 ||
> -	    htab->n_buckets > U32_MAX / sizeof(struct bucket))
> +	/* avoid zero size and u32 overflow kmalloc.
> +	 * bpf_attr::max_entries checked in .map_alloc_check().
> +	 */
> +	if (htab->n_buckets > U32_MAX / sizeof(struct bucket))

This looks buggy to remove it, this is to guard against the previous
roundup_pow_of_two() on htab->n_buckets.

>   		goto free_htab;
>   
>   	err = -ENOMEM;
>