On Mon, 2024-06-03 at 16:17 -0700, Andrii Nakryiko wrote:
> Implement iterator-based type ID and string offset BTF field iterator.
> This is used extensively in BTF-handling code and BPF linker code for
> various sanity checks, rewriting IDs/offsets, etc. Currently this is
> implemented as visitor pattern calling custom callbacks, which makes the
> logic (especially in simple cases) unnecessarily obscure and harder to
> follow.
>
> Having equivalent functionality using iterator pattern makes for simpler
> to understand and maintain code. As we add more code for BTF processing
> logic in libbpf, it's best to switch to iterator pattern before adding
> more callback-based code.
>
> The idea for iterator-based implementation is to record offsets of
> necessary fields within fixed btf_type parts (which should be iterated
> just once), and, for kinds that have multiple members (based on vlen
> field), record where in each member necessary fields are located.
>
> Generic iteration code then just keeps track of last offset that was
> returned and handles N members correctly. Return type is just u32
> pointer, where NULL is returned when all relevant fields were already
> iterated.
>
> Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> ---
Acked-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
[...]
> +__u32 *btf_field_iter_next(struct btf_field_iter *it)
> +{
> + if (!it->p)
> + return NULL;
> +
> + if (it->m_idx < 0) {
> + if (it->off_idx < it->desc.t_cnt)
> + return it->p + it->desc.t_offs[it->off_idx++];
> + /* move to per-member iteration */
> + it->m_idx = 0;
> + it->p += sizeof(struct btf_type);
> + it->off_idx = 0;
> + }
> +
> + /* if type doesn't have members, stop */
> + if (it->desc.m_sz == 0) {
> + it->p = NULL;
> + return NULL;
> + }
> +
> + if (it->off_idx >= it->desc.m_cnt) {
> + /* exhausted this member's fields, go to the next member */
> + it->m_idx++;
> + it->p += it->desc.m_sz;
> + it->off_idx = 0;
> + }
> +
> + if (it->m_idx < it->vlen)
> + return it->p + it->desc.m_offs[it->off_idx++];
Nit: it is a bit confusing that for two 'if' statements above
m_idx is guarded by vlen and off_idx is guarded by m_cnt :)
> +
> + it->p = NULL;
> + return NULL;
> +}
> +
[...]
