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: Tue, 10 May 2022 16:19:42 -0700	[thread overview]
Message-ID: <CAEf4BzZuLHc=LFV8Gpp7KQ8dGB8gi4TjOBznqo2b9mLq4QENPQ@mail.gmail.com> (raw)
In-Reply-To: <7246362c-8eca-027e-d43d-8d4955ad5bdd@fb.com>

On Tue, May 10, 2022 at 3:14 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 5/9/22 3:37 PM, Andrii Nakryiko wrote:
> > 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
>
> Okay, will do.
>
> >
> > but also in general for non-ldimm64 instructions we need to check that
> > new value fits in 32 bits
>
> The real 64-bit value can only be retrieved for ldimm64 insn, so I
> suppose it should be fine here. But let me double check.

So, technically (I don't think that happens in practice, though), you
can have ALU operation with a local 32-bit enum with some reasonable
value, which in the kernel is actually ENUM64 with huge value.

>
> >
> > [...]
> >
> >> @@ -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
>
> Will do.
>
> >
> >
> >> -               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);
> [...]