Re: [RFC PATCH bpf-next] bpf: Check percpu map value size first

[Hou Tao]

Hi,

On 9/6/2024 1:14 AM, Tao Chen wrote:
> Percpu map is often used, but the map value size limit often ignored,
> like issue: https://github.com/iovisor/bcc/issues/2519. Actually,
> percpu map value size is bound by PCPU_MIN_UNIT_SZIE, so we

s/SZIE/SIZE
> can check the value size whether it exceeds PCPU_MIN_UNIT_SZIE first,

The same typo here.
> like percpu map of local_storage. Maybe the error message seems clearer
> compared with "cannot allocate memory".
>
> the test case we create a percpu map with large value like:
> struct test_t {
>         int a[100000];
> };
> struct {
>         __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
>         __uint(max_entries, 1);
>         __type(key, u32);
>         __type(value, struct test_t);
> } start SEC(".maps");
>
> test on ubuntu24.04 vm:
> libbpf: Error in bpf_create_map_xattr(start):Argument list too long(-7).
> Retrying without BTF.

Could you please convert it into a separated bpf selftest patch ?
>
> Signed-off-by: Tao Chen <chen.dylane@xxxxxxxxx>
> ---
>  kernel/bpf/arraymap.c | 3 +++
>  kernel/bpf/hashtab.c  | 3 +++
>  2 files changed, 6 insertions(+)
>
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index a43e62e2a8bb..7c3c32f156ff 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -73,6 +73,9 @@ int array_map_alloc_check(union bpf_attr *attr)
>  	/* avoid overflow on round_up(map->value_size) */
>  	if (attr->value_size > INT_MAX)
>  		return -E2BIG;
> +	/* percpu map value size is bound by PCPU_MIN_UNIT_SIZE */
> +	if (percpu && attr->value_size > PCPU_MIN_UNIT_SIZE)
> +		return -E2BIG;
>  
>  	return 0;
>  }

Make sense. However because the map passes round_up(attr->value_size, 8)
to bpf_map_alloc_percpu(). Is it more reasonable to check
round_up(value_size) > PCPU_MIN_UNIT_SIZE instead ?
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 45c7195b65ba..16d590fe1ffb 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -462,6 +462,9 @@ static int htab_map_alloc_check(union bpf_attr *attr)
>  		 * kmalloc-able later in htab_map_update_elem()
>  		 */
>  		return -E2BIG;
> +	/* percpu map value size is bound by PCPU_MIN_UNIT_SIZE */
> +	if (percpu && attr->value_size > PCPU_MIN_UNIT_SIZE)
> +		return -E2BIG;
>  

The percpu allocation logic is the same as the array map:
round_up(value_size, 8) is used.
>  	return 0;
>  }