netdev - Re: [PATCH bpf-next v6 03/11] bpf: Populate kfunc BTF ID sets in struct btf

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Date:   Tue, 4 Jan 2022 22:19:11 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, netdev@...r.kernel.org,
        netfilter-devel@...r.kernel.org, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Maxim Mikityanskiy <maximmi@...dia.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Florian Westphal <fw@...len.de>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: [PATCH bpf-next v6 03/11] bpf: Populate kfunc BTF ID sets in
 struct btf

On Sun, Jan 02, 2022 at 09:51:07PM +0530, Kumar Kartikeya Dwivedi wrote:
>  
> +enum btf_kfunc_hook {
> +	BTF_KFUNC_HOOK_XDP,
> +	BTF_KFUNC_HOOK_TC,
> +	BTF_KFUNC_HOOK_STRUCT_OPS,
> +	_BTF_KFUNC_HOOK_MAX,

Why prefix with _ ?

> +enum {
> +	BTF_KFUNC_SET_MAX_CNT = 32,
> +};
...
> +	if (set_cnt + add_set->cnt > BTF_KFUNC_SET_MAX_CNT) {
> +		ret = -E2BIG;
> +		goto end;
> +	}

This artificial limit wouldn't be needed if you didn't insist on sorting.
The later patches don't take advantage of this sorting feature and
I don't see a test for sorting either.

> +
> +	/* Grow set */
> +	set = krealloc(tab->sets[hook][type], offsetof(struct btf_id_set, ids[set_cnt + add_set->cnt]),
> +		       GFP_KERNEL | __GFP_NOWARN);
> +	if (!set) {
> +		ret = -ENOMEM;
> +		goto end;
> +	}
> +
> +	/* For newly allocated set, initialize set->cnt to 0 */
> +	if (!tab->sets[hook][type])
> +		set->cnt = 0;
> +	tab->sets[hook][type] = set;
> +
> +	/* Concatenate the two sets */
> +	memcpy(set->ids + set->cnt, add_set->ids, add_set->cnt * sizeof(set->ids[0]));
> +	set->cnt += add_set->cnt;

Without sorting this function would just assign the pointer.
No need for krealloc and memcpy.

> +
> +	if (sort_set)
> +		sort(set->ids, set->cnt, sizeof(set->ids[0]), btf_id_cmp_func, NULL);

All that looks like extra code for a dubious feature.

> +bool btf_kfunc_id_set_contains(const struct btf *btf,
> +			       enum bpf_prog_type prog_type,
> +			       enum btf_kfunc_type type, u32 kfunc_btf_id)
> +{
> +	enum btf_kfunc_hook hook;
> +
> +	switch (prog_type) {
> +	case BPF_PROG_TYPE_XDP:
> +		hook = BTF_KFUNC_HOOK_XDP;
> +		break;
> +	case BPF_PROG_TYPE_SCHED_CLS:
> +		hook = BTF_KFUNC_HOOK_TC;
> +		break;
> +	case BPF_PROG_TYPE_STRUCT_OPS:
> +		hook = BTF_KFUNC_HOOK_STRUCT_OPS;
> +		break;
> +	default:
> +		return false;
> +	}

So this switch() is necessary only to compress prog_types into smaller hooks
to save memory in the struct btf_kfunc_set_tab, right ?
If so both kfunc_id_set_contains() and register_btf_kfunc() should
probably use prog_type as an argument for symmetry.