On Mon, Aug 30, 2021 at 10:34 AM Kumar Kartikeya Dwivedi
<memxor@xxxxxxxxx> wrote:
>
> Preserve these calls as it allows verifier to succeed in loading the
> program if they are determined to be unreachable after dead code
> elimination during program load. If not, the verifier will fail at
> runtime.
>
This should be controlled by whether extern for func is weak or not,
just like we do for variables (see
bpf_object__resolve_ksym_var_btf_id()).
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> ---
> tools/lib/bpf/libbpf.c | 20 ++++++++++++++++----
> 1 file changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index c4677ef97caa..9df90098f111 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -6736,9 +6736,14 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj,
> kfunc_id = find_ksym_btf_id(obj, ext->name, BTF_KIND_FUNC,
> &kern_btf, &kern_btf_fd);
> if (kfunc_id < 0) {
> - pr_warn("extern (func ksym) '%s': not found in kernel BTF\n",
> + pr_warn("extern (func ksym) '%s': not found in kernel BTF, encoding btf_id as 0\n",
> ext->name);
> - return kfunc_id;
> + /* keep invalid kfuncs, so that verifier can load the program if
> + * they get removed during DCE pass in the verifier.
> + * The encoding must be insn->imm = 0, insn->off = 0.
> + */
> + kfunc_id = kern_btf_fd = 0;
> + goto resolve;
> }
>
> if (kern_btf != obj->btf_vmlinux) {
> @@ -6798,11 +6803,18 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj,
> return -EINVAL;
> }
>
> +resolve:
> ext->is_set = true;
> ext->ksym.kernel_btf_obj_fd = kern_btf_fd;
> ext->ksym.kernel_btf_id = kfunc_id;
> - pr_debug("extern (func ksym) '%s': resolved to kernel [%d]\n",
> - ext->name, kfunc_id);
> + if (kfunc_id) {
> + pr_debug("extern (func ksym) '%s': resolved to kernel [%d]\n",
> + ext->name, kfunc_id);
> + } else {
> + ext->ksym.offset = 0;
> + pr_debug("extern (func ksym) '%s': added special invalid kfunc with imm = 0\n",
> + ext->name);
> + }
>
> return 0;
> }
> --
> 2.33.0
>
