RE: [PATCH bpf-next 7/9] libbpf: add BTF writing APIs

[John Fastabend <john.fastabend@gmail.com>]

Andrii Nakryiko wrote:
> Add APIs for appending new BTF types at the end of BTF object.
> 
> Each BTF kind has either one API of the form btf__append_<kind>(). For types
> that have variable amount of additional items (struct/union, enum, func_proto,
> datasec), additional API is provided to emit each such item. E.g., for
> emitting a struct, one would use the following sequence of API calls:
> 
> btf__append_struct(...);
> btf__append_field(...);
> ...
> btf__append_field(...);
> 
> Each btf__append_field() will ensure that the last BTF type is of STRUCT or
> UNION kind and will automatically increment that type's vlen field.
> 
> All the strings are provided as C strings (const char *), not a string offset.
> This significantly improves usability of BTF writer APIs. All such strings
> will be automatically appended to string section or existing string will be
> re-used, if such string was already added previously.
> 
> Each API attempts to do all the reasonable validations, like enforcing
> non-empty names for entities with required names, proper value bounds, various
> bit offset restrictions, etc.
> 
> Type ID validation is minimal because it's possible to emit a type that refers
> to type that will be emitted later, so libbpf has no way to enforce such
> cases. User must be careful to properly emit all the necessary types and
> specify type IDs that will be valid in the finally generated BTF.
> 
> Each of btf__append_<kind>() APIs return new type ID on success or negative
> value on error. APIs like btf__append_field() that emit additional items
> return zero on success and negative value on error.
> 
> Signed-off-by: Andrii Nakryiko <andriin@fb.com>
> ---

[...]

> +	/* deconstruct BTF, if necessary, and invalidate raw_data */
> +	if (btf_ensure_modifiable(btf))
> +		return -ENOMEM;
> +
> +	sz = sizeof(struct btf_type) + sizeof(int);
> +	t = btf_add_type_mem(btf, sz);
> +	if (!t)
> +		return -ENOMEM;
> +
> +	/* if something goes wrong later, we might end up with extra an string,

nit typo, 'with an extra string'

> +	 * but that shouldn't be a problem, because BTF can't be constructed
> +	 * completely anyway and will most probably be just discarded
> +	 */
> +	name_off = btf__add_str(btf, name);
> +	if (name_off < 0)
> +		return name_off;
> +
> +	t->name_off = name_off;
> +	t->info = btf_type_info(BTF_KIND_INT, 0, 0);
> +	t->size = byte_sz;
> +	/* set INT info, we don't allow setting legacy bit offset/size */
> +	*(__u32 *)(t + 1) = (encoding << 24) | (byte_sz * 8);
> +
> +	err = btf_add_type_idx_entry(btf, btf->hdr->type_len);
> +	if (err)
> +		return err;
> +
> +	btf->hdr->type_len += sz;
> +	btf->hdr->str_off += sz;
> +	btf->nr_types++;
> +	return btf->nr_types;
> +}
> +