netdev - Re: [PATCH v2 bpf-next 07/11] libbpf: add BPF static linker BTF and BTF.ext support

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Date:   Tue, 16 Mar 2021 22:25:40 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii@...nel.org>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...com,
        daniel@...earbox.net, kernel-team@...com
Subject: Re: [PATCH v2 bpf-next 07/11] libbpf: add BPF static linker BTF and
 BTF.ext support

On Sat, Mar 13, 2021 at 11:35:33AM -0800, Andrii Nakryiko wrote:
> +		for (j = 0; j < n; j++, src_var++) {
> +			void *sec_vars = dst_sec->sec_vars;
> +
> +			sec_vars = libbpf_reallocarray(sec_vars,
> +						       dst_sec->sec_var_cnt + 1,
> +						       sizeof(*dst_sec->sec_vars));
> +			if (!sec_vars)
> +				return -ENOMEM;
> +
> +			dst_sec->sec_vars = sec_vars;
> +			dst_sec->sec_var_cnt++;
> +
> +			dst_var = &dst_sec->sec_vars[dst_sec->sec_var_cnt - 1];
> +			dst_var->type = obj->btf_type_map[src_var->type];
> +			dst_var->size = src_var->size;
> +			dst_var->offset = src_sec->dst_off + src_var->offset;
> +		}
> +	}
> +
> +	return 0;
> +}
> +
> +static void *add_btf_ext_rec(struct btf_ext_sec_data *ext_data, const void *src_rec)
> +{
> +	size_t new_sz = (ext_data->rec_cnt + 1) * ext_data->rec_sz;
> +	void *tmp;
> +
> +	tmp = realloc(ext_data->recs, new_sz);
> +	if (!tmp)
> +		return NULL;
> +
> +	ext_data->recs = tmp;
> +	ext_data->rec_cnt++;
> +
> +	tmp += new_sz - ext_data->rec_sz;
> +	memcpy(tmp, src_rec, ext_data->rec_sz);

while reading this and previous patch the cnt vs sz difference was
constantly throwing me off. Not a big deal, of course.
Did you consider using _cnt everywhere and use finalize method
to convert everything to size?
Like in this function libbpf_reallocarray() instead of realloc() would
probably be easier to read and more consistent, since btf_ext_sec_data
is measuring things in _cnt.
In the previous patch the section is in _sz which I guess is necessary
because sections can contain differently sized objects?

btw, strset abstraction is really nice. It made the patches much easier
to read.