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Message-ID: <CAEf4BzZofcwskPQXRpV4ZEiVbrzg296t+fSpezFxDLF3ueQBWg@mail.gmail.com>
Date: Mon, 26 Apr 2021 10:29:09 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: "David S. Miller" <davem@...emloft.net>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 12/16] libbpf: Change the order of data and
text relocations.
On Thu, Apr 22, 2021 at 5:27 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> From: Alexei Starovoitov <ast@...nel.org>
>
> In order to be able to generate loader program in the later
> patches change the order of data and text relocations.
> Also improve the test to include data relos.
>
> If the kernel supports "FD array" the map_fd relocations can be processed
> before text relos since generated loader program won't need to manually
> patch ld_imm64 insns with map_fd.
> But ksym and kfunc relocations can only be processed after all calls
> are relocated, since loader program will consist of a sequence
> of calls to bpf_btf_find_by_name_kind() followed by patching of btf_id
> and btf_obj_fd into corresponding ld_imm64 insns. The locations of those
> ld_imm64 insns are specified in relocations.
> Hence process all data relocations (maps, ksym, kfunc) together after call relos.
>
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> ---
> tools/lib/bpf/libbpf.c | 86 +++++++++++++++----
> .../selftests/bpf/progs/test_subprogs.c | 13 +++
> 2 files changed, 80 insertions(+), 19 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 17cfc5b66111..c73a85b97ca5 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -6379,11 +6379,15 @@ bpf_object__relocate_data(struct bpf_object *obj, struct bpf_program *prog)
> insn[0].imm = ext->ksym.kernel_btf_id;
> break;
> case RELO_SUBPROG_ADDR:
> - insn[0].src_reg = BPF_PSEUDO_FUNC;
> - /* will be handled as a follow up pass */
> + if (insn[0].src_reg != BPF_PSEUDO_FUNC) {
> + pr_warn("prog '%s': relo #%d: bad insn\n",
> + prog->name, i);
> + return -EINVAL;
> + }
given SUBPROG_ADDR is now handled similarly to RELO_CALL in a
different place, I'd probably drop this error check and just combine
RELO_SUBPROG_ADDR and RELO_CALL cases with just a /* handled already
*/ comment.
> + /* handled already */
> break;
> case RELO_CALL:
> - /* will be handled as a follow up pass */
> + /* handled already */
> break;
> default:
> pr_warn("prog '%s': relo #%d: bad relo type %d\n",
> @@ -6552,6 +6556,31 @@ static struct reloc_desc *find_prog_insn_relo(const struct bpf_program *prog, si
> sizeof(*prog->reloc_desc), cmp_relo_by_insn_idx);
> }
>
> +static int append_subprog_relos(struct bpf_program *main_prog, struct bpf_program *subprog)
> +{
> + int new_cnt = main_prog->nr_reloc + subprog->nr_reloc;
> + struct reloc_desc *relos;
> + size_t off = subprog->sub_insn_off;
> + int i;
> +
> + if (main_prog == subprog)
> + return 0;
> + relos = libbpf_reallocarray(main_prog->reloc_desc, new_cnt, sizeof(*relos));
> + if (!relos)
> + return -ENOMEM;
> + memcpy(relos + main_prog->nr_reloc, subprog->reloc_desc,
> + sizeof(*relos) * subprog->nr_reloc);
> +
> + for (i = main_prog->nr_reloc; i < new_cnt; i++)
> + relos[i].insn_idx += off;
nit: off is used only here, so there is little point in having it as a
separate var, inline?
> + /* After insn_idx adjustment the 'relos' array is still sorted
> + * by insn_idx and doesn't break bsearch.
> + */
> + main_prog->reloc_desc = relos;
> + main_prog->nr_reloc = new_cnt;
> + return 0;
> +}
> +
> static int
> bpf_object__reloc_code(struct bpf_object *obj, struct bpf_program *main_prog,
> struct bpf_program *prog)
> @@ -6560,18 +6589,32 @@ bpf_object__reloc_code(struct bpf_object *obj, struct bpf_program *main_prog,
> struct bpf_program *subprog;
> struct bpf_insn *insns, *insn;
> struct reloc_desc *relo;
> - int err;
> + int err, i;
>
> err = reloc_prog_func_and_line_info(obj, main_prog, prog);
> if (err)
> return err;
>
> + for (i = 0; i < prog->nr_reloc; i++) {
> + relo = &prog->reloc_desc[i];
> + insn = &main_prog->insns[prog->sub_insn_off + relo->insn_idx];
> +
> + if (relo->type == RELO_SUBPROG_ADDR)
> + /* mark the insn, so it becomes insn_is_pseudo_func() */
> + insn[0].src_reg = BPF_PSEUDO_FUNC;
> + }
> +
This will do the same work over and over each time we append a subprog
to main_prog. This should logically follow append_subprog_relos(), but
you wanted to do it for main_prog with the same code, right?
How about instead doing this before we start appending subprogs to
main_progs? I.e., do it explicitly in bpf_object__relocate() before
you start code relocation loop.
> 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) && !insn_is_pseudo_func(insn))
> continue;
>
> relo = find_prog_insn_relo(prog, insn_idx);
> + if (relo && relo->type == RELO_EXTERN_FUNC)
> + /* kfunc relocations will be handled later
> + * in bpf_object__relocate_data()
> + */
> + continue;
> if (relo && relo->type != RELO_CALL && relo->type != RELO_SUBPROG_ADDR) {
> pr_warn("prog '%s': unexpected relo for insn #%zu, type %d\n",
> prog->name, insn_idx, relo->type);
[...]
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