Re: [PATCH bpf-next 1/2] bpf: add bpf_dynptr_memset() kfunc

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

On 6/18/25 23:33, Ihor Solodrai wrote:
> Currently there is no straightforward way to fill dynptr memory with a
> value (most commonly zero). One can do it with bpf_dynptr_write(), but
> a temporary buffer is necessary for that.
>
> Implement bpf_dynptr_memset() - an analogue of memset() from libc.
>
> Signed-off-by: Ihor Solodrai <isolodrai@meta.com>
> ---
>   kernel/bpf/helpers.c | 28 ++++++++++++++++++++++++++++
>   1 file changed, 28 insertions(+)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index b71e428ad936..dfd04628a522 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2906,6 +2906,33 @@ __bpf_kfunc int bpf_dynptr_copy(struct bpf_dynptr *dst_ptr, u32 dst_off,
>   	return 0;
>   }
>   
> +/**
> + * bpf_dynptr_memset() - Fill dynptr memory with a constant byte.
> + * @ptr: Destination dynptr - where data will be filled
> + * @val: Constant byte to fill the memory with
> + * @n: Number of bytes to fill
> + *
> + * Fills the first n bytes of the memory area pointed to by ptr
> + * with the constant byte val.
> + * Returns 0 on success; negative error, otherwise.
> + */
> + __bpf_kfunc int bpf_dynptr_memset(struct bpf_dynptr *ptr, u8 val, u32 n)
> + {
> +	struct bpf_dynptr_kern *p = (struct bpf_dynptr_kern *)ptr;
> +	int err;
> +
> +	if (__bpf_dynptr_is_rdonly(p))
> +		return -EINVAL;
> +
> +	err = bpf_dynptr_check_off_len(p, 0, n);
> +	if (err)
> +		return err;
> +
> +	memset(p->data + p->offset, val, n);
Do we need to handle non-contiguous buffers, similarly to 
bpf_dynptr_write (BPF_DYNPTR_TYPE_XDP case)?
> +
> +	return 0;
> +}
> +
>   __bpf_kfunc void *bpf_cast_to_kern_ctx(void *obj)
>   {
>   	return obj;
> @@ -3364,6 +3391,7 @@ BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
>   BTF_ID_FLAGS(func, bpf_dynptr_size)
>   BTF_ID_FLAGS(func, bpf_dynptr_clone)
>   BTF_ID_FLAGS(func, bpf_dynptr_copy)
> +BTF_ID_FLAGS(func, bpf_dynptr_memset)
>   #ifdef CONFIG_NET
>   BTF_ID_FLAGS(func, bpf_modify_return_test_tp)
>   #endif