netdev - Re: [PATCH v2 bpf-next 04/14] libbpf: make RELO_CALL work for multi-prog sections and sub-program calls

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Date:   Tue, 1 Sep 2020 22:36:28 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...com,
        daniel@...earbox.net, andrii.nakryiko@...il.com, kernel-team@...com
Subject: Re: [PATCH v2 bpf-next 04/14] libbpf: make RELO_CALL work for
 multi-prog sections and sub-program calls

On Mon, Aug 31, 2020 at 06:49:53PM -0700, Andrii Nakryiko wrote:
> +
> +static int
> +bpf_object__reloc_code(struct bpf_object *obj, struct bpf_program *main_prog,
> +		       struct bpf_program *prog)
> +{
> +	size_t sub_insn_idx, insn_idx, new_cnt;
> +	struct bpf_program *subprog;
> +	struct bpf_insn *insns, *insn;
> +	struct reloc_desc *relo;
> +	int err;
> +
> +	err = reloc_prog_func_and_line_info(obj, main_prog, prog);
> +	if (err)
> +		return err;
> +
> +	for (insn_idx = 0; insn_idx < prog->sec_insn_cnt; insn_idx++) {
> +		insn = &main_prog->insns[prog->sub_insn_off + insn_idx];
> +		if (!insn_is_subprog_call(insn))
> +			continue;
> +
> +		relo = find_prog_insn_relo(prog, insn_idx);
> +		if (relo && relo->type != RELO_CALL) {
> +			pr_warn("prog '%s': unexpected relo for insn #%zu, type %d\n",
> +				prog->name, insn_idx, relo->type);
> +			return -LIBBPF_ERRNO__RELOC;
> +		}
> +		if (relo) {
> +			/* sub-program instruction index is a combination of
> +			 * an offset of a symbol pointed to by relocation and
> +			 * call instruction's imm field; for global functions,
> +			 * call always has imm = -1, but for static functions
> +			 * relocation is against STT_SECTION and insn->imm
> +			 * points to a start of a static function
> +			 */
> +			sub_insn_idx = relo->sym_off / BPF_INSN_SZ + insn->imm + 1;
> +		} else {
> +			/* if subprogram call is to a static function within
> +			 * the same ELF section, there won't be any relocation
> +			 * emitted, but it also means there is no additional
> +			 * offset necessary, insns->imm is relative to
> +			 * instruction's original position within the section
> +			 */

Great two comments. Thanks.

> +			sub_insn_idx = prog->sec_insn_off + insn_idx + insn->imm + 1;
> +		}
> +
> +		/* we enforce that sub-programs should be in .text section */
> +		subprog = find_prog_by_sec_insn(obj, obj->efile.text_shndx, sub_insn_idx);
> +		if (!subprog) {
> +			pr_warn("prog '%s': no .text section found yet sub-program call exists\n",
> +				prog->name);
> +			return -LIBBPF_ERRNO__RELOC;
> +		}
> +
> +		/* if subprogram hasn't been used in current main program,
> +		 * relocate it and append at the end of main program code
> +		 */

This one is quite confusing.
"hasn't been used" isn't right.
This subprog was used, but wasn't appeneded yet. That's what sub_insn_off is tracking.
Also "relocate and append it" is not right either.
It's "append and start relocating".
Probably shouldn't call it 'main' and 'subprog'.
It equally applies to 'subprog' and 'another subprog'.

> +		if (subprog->sub_insn_off == 0) {
> +			subprog->sub_insn_off = main_prog->insns_cnt;
> +
> +			new_cnt = main_prog->insns_cnt + subprog->insns_cnt;
> +			insns = libbpf_reallocarray(main_prog->insns, new_cnt, sizeof(*insns));
> +			if (!insns) {
> +				pr_warn("prog '%s': failed to realloc prog code\n", main_prog->name);
> +				return -ENOMEM;
> +			}
> +			main_prog->insns = insns;
> +			main_prog->insns_cnt = new_cnt;
> +
> +			memcpy(main_prog->insns + subprog->sub_insn_off, subprog->insns,
> +			       subprog->insns_cnt * sizeof(*insns));
> +
> +			pr_debug("prog '%s': added %zu insns from sub-prog '%s'\n",
> +				 main_prog->name, subprog->insns_cnt, subprog->name);
> +
> +			err = bpf_object__reloc_code(obj, main_prog, subprog);
> +			if (err)
> +				return err;
> +		}
> +
> +		/* main_prog->insns memory could have been re-allocated, so
> +		 * calculate pointer again
> +		 */
> +		insn = &main_prog->insns[prog->sub_insn_off + insn_idx];
> +		/* calculate correct instruction position within main prog */

may be: "calculate position within the prog being relocated?"

> +		insn->imm = subprog->sub_insn_off - (prog->sub_insn_off + insn_idx) - 1;

I think the algorithm is sound.
Could you add a better description of it?
May be some small diagram to illustrate how it recursively relocates?
That it starts with main, walks some number of insn, when it sees pseudo_call to
not yet appended subprog, it adds it to the end and recursively starts relocating it.
That subprog can have relos too. If they're pointing to not yet appended subprog it will be
added again and that 2nd subprog will start relocating while the main and 1st subprog
will be pending.
The algorithm didn't have to be recursive, but I guess it's fine to keep this way.
It's simple enough. I haven't thought through how it can look without recursion.
Probably a bunch of book keeping of things to relocate would have been necessary.