Re: [PATCH bpf v2 3/5] libbpf: Skip adjust mem size for load pointer in 32-bit arch in CO_RE

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Yang Jihong <yangjihong1@huawei.com>
Cc: ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org,
	martin.lau@linux.dev, song@kernel.org, yhs@fb.com,
	john.fastabend@gmail.com, kpsingh@kernel.org, sdf@google.com,
	haoluo@google.com, jolsa@kernel.org, illusionist.neo@gmail.com,
	linux@armlinux.org.uk, davem@davemloft.net, edumazet@google.com,
	kuba@kernel.org, pabeni@redhat.com, mykolal@fb.com,
	shuah@kernel.org, benjamin.tissoires@redhat.com,
	memxor@gmail.com, asavkov@redhat.com, delyank@fb.com,
	bpf@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
	linux-kselftest@vger.kernel.org
Subject: Re: [PATCH bpf v2 3/5] libbpf: Skip adjust mem size for load pointer in 32-bit arch in CO_RE
Date: Mon, 7 Nov 2022 17:22:30 -0800	[thread overview]
Message-ID: <CAEf4BzZd+hzeRhLD6DaDVx67fySd+KaTP6eOJid-u9mqnQwigg@mail.gmail.com> (raw)
In-Reply-To: <20221107092032.178235-4-yangjihong1@huawei.com>

On Mon, Nov 7, 2022 at 1:23 AM Yang Jihong <yangjihong1@huawei.com> wrote:
>
> bpf_core_patch_insn modifies load's mem size from 8 bytes to 4 bytes.
> As a result, the bpf check fails, we need to skip adjust mem size to fit
> the verifier.
>
> Signed-off-by: Yang Jihong <yangjihong1@huawei.com>
> ---
>  tools/lib/bpf/libbpf.c | 34 +++++++++++++++++++++++++++++-----
>  1 file changed, 29 insertions(+), 5 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 184ce1684dcd..e1c21b631a0b 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -5634,6 +5634,28 @@ static int bpf_core_resolve_relo(struct bpf_program *prog,
>                                        targ_res);
>  }
>
> +static bool
> +bpf_core_patch_insn_skip(const struct btf *local_btf, const struct bpf_insn *insn,
> +                        const struct bpf_core_relo_res *res)
> +{
> +       __u8 class;
> +       const struct btf_type *orig_t;
> +
> +       class = BPF_CLASS(insn->code);
> +       orig_t = btf_type_by_id(local_btf, res->orig_type_id);
> +
> +       /*
> +        * verifier has to see a load of a pointer as a 8-byte load,
> +        * CO_RE should not screws up access, bpf_core_patch_insn modifies
> +        * load's mem size from 8 bytes to 4 bytes in 32-bit arch,
> +        * so we skip adjust mem size.
> +        */

Nope, this is only for BPF UAPI context types like __sk_buff (right
now). fentry/fexit/raw_tp_btf programs traversing kernel types and
following pointers actually need this to work correctly. Don't do
this.

> +       if (class == BPF_LDX && btf_is_ptr(orig_t))
> +               return true;
> +
> +       return false;
> +}
> +
>  static int
>  bpf_object__relocate_core(struct bpf_object *obj, const char *targ_btf_path)
>  {
> @@ -5730,11 +5752,13 @@ bpf_object__relocate_core(struct bpf_object *obj, const char *targ_btf_path)
>                                 goto out;
>                         }
>
> -                       err = bpf_core_patch_insn(prog->name, insn, insn_idx, rec, i, &targ_res);
> -                       if (err) {
> -                               pr_warn("prog '%s': relo #%d: failed to patch insn #%u: %d\n",
> -                                       prog->name, i, insn_idx, err);
> -                               goto out;
> +                       if (!bpf_core_patch_insn_skip(obj->btf, insn, &targ_res)) {
> +                               err = bpf_core_patch_insn(prog->name, insn, insn_idx, rec, i, &targ_res);
> +                               if (err) {
> +                                       pr_warn("prog '%s': relo #%d: failed to patch insn #%u: %d\n",
> +                                               prog->name, i, insn_idx, err);
> +                                       goto out;
> +                               }
>                         }
>                 }
>         }
> --
> 2.30.GIT
>