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Date: Fri, 11 Feb 2022 16:42:38 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Mauricio Vásquez <mauricio@...volk.io>
Cc: Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Quentin Monnet <quentin@...valent.com>,
Rafael David Tinoco <rafaeldtinoco@...il.com>,
Lorenzo Fontana <lorenzo.fontana@...stic.co>,
Leonardo Di Donato <leonardo.didonato@...stic.co>
Subject: Re: [PATCH bpf-next v6 5/7] bpftool: Implement btfgen_get_btf()
On Wed, Feb 9, 2022 at 2:27 PM Mauricio Vásquez <mauricio@...volk.io> wrote:
>
> The last part of the BTFGen algorithm is to create a new BTF object with
> all the types that were recorded in the previous steps.
>
> This function performs two different steps:
> 1. Add the types to the new BTF object by using btf__add_type(). Some
> special logic around struct and unions is implemented to only add the
> members that are really used in the field-based relocations. The type
> ID on the new and old BTF objects is stored on a map.
> 2. Fix all the type IDs on the new BTF object by using the IDs saved in
> the previous step.
>
> Signed-off-by: Mauricio Vásquez <mauricio@...volk.io>
> Signed-off-by: Rafael David Tinoco <rafael.tinoco@...asec.com>
> Signed-off-by: Lorenzo Fontana <lorenzo.fontana@...stic.co>
> Signed-off-by: Leonardo Di Donato <leonardo.didonato@...stic.co>
> ---
> tools/bpf/bpftool/gen.c | 136 +++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 135 insertions(+), 1 deletion(-)
>
> diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
> index c3e34db2ec8a..1efc7f3c64b2 100644
> --- a/tools/bpf/bpftool/gen.c
> +++ b/tools/bpf/bpftool/gen.c
> @@ -1481,10 +1481,144 @@ static int btfgen_record_obj(struct btfgen_info *info, const char *obj_path)
> return err;
> }
>
> +static unsigned int btfgen_get_id(struct hashmap *ids, unsigned int old)
> +{
> + uintptr_t new;
> +
> + if (!hashmap__find(ids, uint_as_hash_key(old), (void **)&new))
> + /* return id for BTF_KIND_VOID as it's possible that the
> + * ID we're looking for is the type of a pointer that
> + * we're not adding.
> + */
> + return 0;
> +
> + return (unsigned int)(uintptr_t)new;
> +}
> +
> +static int btfgen_add_id(struct hashmap *ids, unsigned int old, unsigned int new)
> +{
> + return hashmap__add(ids, uint_as_hash_key(old), uint_as_hash_key(new));
> +}
> +
> +static int btfgen_remap_id(__u32 *type_id, void *ctx)
> +{
> + struct hashmap *ids = ctx;
> +
> + *type_id = btfgen_get_id(ids, *type_id);
> +
> + return 0;
> +}
> +
> /* Generate BTF from relocation information previously recorded */
> static struct btf *btfgen_get_btf(struct btfgen_info *info)
> {
> - return ERR_PTR(-EOPNOTSUPP);
> + struct btf *btf_new = NULL;
> + struct hashmap *ids = NULL;
> + unsigned int i;
> + int err = 0;
> +
> + btf_new = btf__new_empty();
> + if (!btf_new) {
> + err = -errno;
> + goto err_out;
> + }
> +
> + ids = hashmap__new(btfgen_hash_fn, btfgen_equal_fn, NULL);
> + if (IS_ERR(ids)) {
> + err = PTR_ERR(ids);
> + goto err_out;
> + }
> +
> + /* first pass: add all marked types to btf_new and add their new ids to the ids map */
> + for (i = 1; i < btf__type_cnt(info->marked_btf); i++) {
small nit: why keep calling btf__type_cnt() on each iteration? store
it as n = btf__type_cnt(...) and do i < n ?
> + const struct btf_type *cloned_type, *btf_type;
> + int new_id;
> +
> + cloned_type = btf__type_by_id(info->marked_btf, i);
> +
> + if (cloned_type->name_off != MARKED)
> + continue;
see, if you did
#define MARKED (1<<31)
and did
t->name_off |= MARKED
everywhere, then you wouldn't need src_btf anymore, as you'd just
restore original name_off right here with t->name_off &= ~MARKED.
But it's fine, just wanted to point out why I wanted to use one bit,
so that original values are still available.
> +
> + btf_type = btf__type_by_id(info->src_btf, i);
> +
> + /* add members for struct and union */
> + if (btf_is_struct(btf_type) || btf_is_union(btf_type)) {
btf_is_composite(btf_type)
> + struct btf_type *btf_type_cpy;
> + int nmembers = 0, idx_dst, idx_src;
> + size_t new_size;
> +
> + /* calculate nmembers */
> + for (idx_src = 0; idx_src < btf_vlen(cloned_type); idx_src++) {
> + struct btf_member *cloned_m = btf_members(cloned_type) + idx_src;
a bit nicer pattern is:
struct btf_member *m = btf_members(cloned_type);
int vlen = btf_vlen(cloned_type)
for (i = 0; i < vlen; i++, m++) {
}
That way you don't have to re-calculate member
> +
> + if (cloned_m->name_off == MARKED)
> + nmembers++;
> + }
> +
> + new_size = sizeof(struct btf_type) + nmembers * sizeof(struct btf_member);
> +
> + btf_type_cpy = malloc(new_size);
> + if (!btf_type_cpy)
> + goto err_out;
> +
> + /* copy btf type */
> + *btf_type_cpy = *btf_type;
> +
> + idx_dst = 0;
> + for (idx_src = 0; idx_src < btf_vlen(cloned_type); idx_src++) {
> + struct btf_member *btf_member_src, *btf_member_dst;
> + struct btf_member *cloned_m = btf_members(cloned_type) + idx_src;
> +
> + /* copy only members that are marked as used */
> + if (cloned_m->name_off != MARKED)
> + continue;
> +
> + btf_member_src = btf_members(btf_type) + idx_src;
> + btf_member_dst = btf_members(btf_type_cpy) + idx_dst;
> +
> + *btf_member_dst = *btf_member_src;
> +
> + idx_dst++;
> + }
> +
> + /* set new vlen */
> + btf_type_cpy->info = btf_type_info(btf_kind(btf_type_cpy), nmembers,
> + btf_kflag(btf_type_cpy));
> +
> + err = btf__add_type(btf_new, info->src_btf, btf_type_cpy);
> + free(btf_type_cpy);
hmm.. this malloc and the rest still feels clunky... why not do it
explicitly with btf__add_struct()/btf__add_union() and then
btf__add_field() for each marked field? You also won't need to
pre-calculate the number of members (libbpf will adjust number of
members automatically, it's pretty nice API, try it).
You can also use err = err ?: btf__add_xxx() pattern to minimize error
handling conditionals
> + } else {
> + err = btf__add_type(btf_new, info->src_btf, btf_type);
> + }
> +
> + if (err < 0)
> + goto err_out;
> +
> + new_id = err;
> +
> + /* add ID mapping */
> + err = btfgen_add_id(ids, i, new_id);
Why using clunky hashmap API if we are talking about mapping
sequential integers? Just allocate an array of btf__type_cnt()
integers and use that as a straightforward map?
> + if (err)
> + goto err_out;
> + }
> +
> + /* second pass: fix up type ids */
> + for (i = 1; i < btf__type_cnt(btf_new); i++) {
> + struct btf_type *btf_type = (struct btf_type *) btf__type_by_id(btf_new, i);
> +
> + err = btf_type_visit_type_ids(btf_type, btfgen_remap_id, ids);
> + if (err)
> + goto err_out;
> + }
> +
> + hashmap__free(ids);
> + return btf_new;
> +
> +err_out:
> + btf__free(btf_new);
> + hashmap__free(ids);
> + errno = -err;
> + return NULL;
> }
>
> /* Create minimized BTF file for a set of BPF objects.
> --
> 2.25.1
>
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