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

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