Re: [PATCH bpf-next 02/12] libbpf: Permit 64bit relocation value

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Yonghong Song <yhs@fb.com>
Cc: bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH bpf-next 02/12] libbpf: Permit 64bit relocation value
Date: Mon, 9 May 2022 15:37:33 -0700	[thread overview]
Message-ID: <CAEf4BzYyUUjVYEcDJ75DWyg4HoOm4YbFSy84OY01WgENdWrh8A@mail.gmail.com> (raw)
In-Reply-To: <20220501190012.2577087-1-yhs@fb.com>

On Sun, May 1, 2022 at 12:00 PM Yonghong Song <yhs@fb.com> wrote:
>
> Currently, the libbpf limits the relocation value to be 32bit
> since all current relocations have such a limit. But with
> BTF_KIND_ENUM64 support, the enum value could be 64bit.
> So let us permit 64bit relocation value in libbpf.
>
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
>  tools/lib/bpf/relo_core.c | 24 ++++++++++++------------
>  tools/lib/bpf/relo_core.h |  4 ++--
>  2 files changed, 14 insertions(+), 14 deletions(-)
>

[...]

> @@ -929,7 +929,7 @@ int bpf_core_patch_insn(const char *prog_name, struct bpf_insn *insn,
>                         int insn_idx, const struct bpf_core_relo *relo,
>                         int relo_idx, const struct bpf_core_relo_res *res)
>  {
> -       __u32 orig_val, new_val;
> +       __u64 orig_val, new_val;
>         __u8 class;
>
>         class = BPF_CLASS(insn->code);
> @@ -954,14 +954,14 @@ int bpf_core_patch_insn(const char *prog_name, struct bpf_insn *insn,
>                 if (BPF_SRC(insn->code) != BPF_K)
>                         return -EINVAL;
>                 if (res->validate && insn->imm != orig_val) {
> -                       pr_warn("prog '%s': relo #%d: unexpected insn #%d (ALU/ALU64) value: got %u, exp %u -> %u\n",
> +                       pr_warn("prog '%s': relo #%d: unexpected insn #%d (ALU/ALU64) value: got %u, exp %llu -> %llu\n",
>                                 prog_name, relo_idx,
>                                 insn_idx, insn->imm, orig_val, new_val);

%llu is not valid formatter for __u64 on all architectures, please add
explicit (unsigned long long) cast

but also in general for non-ldimm64 instructions we need to check that
new value fits in 32 bits

[...]

> @@ -1026,7 +1026,7 @@ int bpf_core_patch_insn(const char *prog_name, struct bpf_insn *insn,
>
>                 imm = insn[0].imm + ((__u64)insn[1].imm << 32);
>                 if (res->validate && imm != orig_val) {
> -                       pr_warn("prog '%s': relo #%d: unexpected insn #%d (LDIMM64) value: got %llu, exp %u -> %u\n",
> +                       pr_warn("prog '%s': relo #%d: unexpected insn #%d (LDIMM64) value: got %llu, exp %llu -> %llu\n",
>                                 prog_name, relo_idx,
>                                 insn_idx, (unsigned long long)imm,
>                                 orig_val, new_val);
> @@ -1035,7 +1035,7 @@ int bpf_core_patch_insn(const char *prog_name, struct bpf_insn *insn,
>
>                 insn[0].imm = new_val;
>                 insn[1].imm = 0; /* currently only 32-bit values are supported */

as Dave mentioned, not anymore, so this should take higher 32-bit of new_val


> -               pr_debug("prog '%s': relo #%d: patched insn #%d (LDIMM64) imm64 %llu -> %u\n",
> +               pr_debug("prog '%s': relo #%d: patched insn #%d (LDIMM64) imm64 %llu -> %llu\n",
>                          prog_name, relo_idx, insn_idx,
>                          (unsigned long long)imm, new_val);
>                 break;
> @@ -1261,7 +1261,7 @@ int bpf_core_calc_relo_insn(const char *prog_name,
>                          * decision and value, otherwise it's dangerous to
>                          * proceed due to ambiguity
>                          */
> -                       pr_warn("prog '%s': relo #%d: relocation decision ambiguity: %s %u != %s %u\n",
> +                       pr_warn("prog '%s': relo #%d: relocation decision ambiguity: %s %llu != %s %llu\n",
>                                 prog_name, relo_idx,
>                                 cand_res.poison ? "failure" : "success", cand_res.new_val,
>                                 targ_res->poison ? "failure" : "success", targ_res->new_val);
> diff --git a/tools/lib/bpf/relo_core.h b/tools/lib/bpf/relo_core.h
> index 073039d8ca4f..7df0da082f2c 100644
> --- a/tools/lib/bpf/relo_core.h
> +++ b/tools/lib/bpf/relo_core.h
> @@ -46,9 +46,9 @@ struct bpf_core_spec {
>
>  struct bpf_core_relo_res {
>         /* expected value in the instruction, unless validate == false */
> -       __u32 orig_val;
> +       __u64 orig_val;
>         /* new value that needs to be patched up to */
> -       __u32 new_val;
> +       __u64 new_val;
>         /* relocation unsuccessful, poison instruction, but don't fail load */
>         bool poison;
>         /* some relocations can't be validated against orig_val */
> --
> 2.30.2
>