[PATCH bpf-next RFC v1 4/8] libbpf: Resolve invalid kfunc calls with imm = 0, off = 0

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

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.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 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