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

Andrii Nakryiko
2022-11-08T01:22:50Z

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
>