Re: [PATCH bpf-next v3 1/3] libbpf: split bpf_core_apply_relo()

From: "Mauricio Vásquez Bernal" <mauricio@kinvolk.io>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Quentin Monnet <quentin@isovalent.com>,
	Rafael David Tinoco <rafaeldtinoco@gmail.com>,
	Lorenzo Fontana <lorenzo.fontana@elastic.co>,
	Leonardo Di Donato <leonardo.didonato@elastic.co>
Subject: Re: [PATCH bpf-next v3 1/3] libbpf: split bpf_core_apply_relo()
Date: Wed, 12 Jan 2022 09:26:52 -0500	[thread overview]
Message-ID: <CAHap4ztB7BWxXX3DerY2AVvV54vdhi+4wgTrrM9RzbiQ9KjhrQ@mail.gmail.com> (raw)
In-Reply-To: <CAEf4BzZw2RBPSxE0j8uQd8-75qOfq=iPnhB73ONErsHYUaF+pg@mail.gmail.com>

> > -static int bpf_core_apply_relo(struct bpf_program *prog,
> > -                              const struct bpf_core_relo *relo,
> > -                              int relo_idx,
> > -                              const struct btf *local_btf,
> > -                              struct hashmap *cand_cache)
> > +static int bpf_core_calc_relo_res(struct bpf_program *prog,
>
> bpf_core_calc_relo_res is almost indistinguishable from
> bpf_core_calc_relo... Let's call this one bpf_core_resolve_relo()?
>

That's a much better name! Deciding the name of that function was
probably the most complicated part of this patch.

> > @@ -5636,12 +5627,31 @@ bpf_object__relocate_core(struct bpf_object *obj, const char *targ_btf_path)
> >                         if (!prog->load)
> >                                 continue;
> >
> > -                       err = bpf_core_apply_relo(prog, rec, i, obj->btf, cand_cache);
> > +                       err = bpf_core_calc_relo_res(prog, rec, i, obj->btf, cand_cache, &targ_res);
> >                         if (err) {
> >                                 pr_warn("prog '%s': relo #%d: failed to relocate: %d\n",
> >                                         prog->name, i, err);
> >                                 goto out;
> >                         }
> > +
> > +                       if (rec->insn_off % BPF_INSN_SZ)
> > +                               return -EINVAL;
> > +                       insn_idx = rec->insn_off / BPF_INSN_SZ;
> > +                       /* adjust insn_idx from section frame of reference to the local
> > +                        * program's frame of reference; (sub-)program code is not yet
> > +                        * relocated, so it's enough to just subtract in-section offset
> > +                        */
> > +                       insn_idx = insn_idx - prog->sec_insn_off;
> > +                       if (insn_idx >= prog->insns_cnt)
> > +                               return -EINVAL;
> > +                       insn = &prog->insns[insn_idx];
>
> this is sort of like sanity checks, let's do them before the core_calc
> step, so after that it's a clean sequence of calc_relo + pathc_insn?
>

Makes sense.

> > @@ -1177,18 +1152,18 @@ static void bpf_core_dump_spec(const char *prog_name, int level, const struct bp
> >   *    between multiple relocations for the same type ID and is updated as some
> >   *    of the candidates are pruned due to structural incompatibility.
> >   */
> > -int bpf_core_apply_relo_insn(const char *prog_name, struct bpf_insn *insn,
> > -                            int insn_idx,
> > -                            const struct bpf_core_relo *relo,
> > -                            int relo_idx,
> > -                            const struct btf *local_btf,
> > -                            struct bpf_core_cand_list *cands,
> > -                            struct bpf_core_spec *specs_scratch)
> > +int bpf_core_calc_relo_insn(const char *prog_name,
>
> please update the comment for this function, it's not "CO-RE relocate
> single instruction" anymore, it's more like "Calculate CO-RE
> relocation target result" or something along those lines.
>

Updated with your suggestion.

> > @@ -1223,12 +1198,12 @@ int bpf_core_apply_relo_insn(const char *prog_name, struct bpf_insn *insn,
> >         /* TYPE_ID_LOCAL relo is special and doesn't need candidate search */
> >         if (relo->kind == BPF_CORE_TYPE_ID_LOCAL) {
> >                 /* bpf_insn's imm value could get out of sync during linking */
> > -               memset(&targ_res, 0, sizeof(targ_res));
> > -               targ_res.validate = false;
> > -               targ_res.poison = false;
> > -               targ_res.orig_val = local_spec->root_type_id;
> > -               targ_res.new_val = local_spec->root_type_id;
> > -               goto patch_insn;
> > +               memset(targ_res, 0, sizeof(*targ_res));
> > +               targ_res->validate = true;
>
> hm.. original code sets it to false here, please don't regress the logic
>

ops, I introduced this by mistake while rebasing.