From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Date: Sat, 2 Oct 2021 06:47:55 +0530
Subject: libbpf: Resolve invalid weak kfunc calls with imm = 0, off = 0
Patch-mainline: v5.16-rc1
Git-commit: 466b2e13971ef65cd7b621ca3044be14028b002b
References: jsc#PED-1377

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 is done for ext->is_weak symbols similar to the case for
variable ksyms.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211002011757.311265-8-memxor@gmail.com
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
---
 tools/lib/bpf/libbpf.c |   24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -3416,11 +3416,6 @@ static int bpf_object__collect_externs(s
 				return -ENOTSUP;
 			}
 		} else if (strcmp(sec_name, KSYMS_SEC) == 0) {
-			if (btf_is_func(t) && ext->is_weak) {
-				pr_warn("extern weak function %s is unsupported\n",
-					ext->name);
-				return -ENOTSUP;
-			}
 			ksym_sec = sec;
 			ext->type = EXT_KSYM;
 			skip_mods_and_typedefs(obj->btf, t->type,
@@ -5393,8 +5388,13 @@ bpf_object__relocate_data(struct bpf_obj
 		case RELO_EXTERN_FUNC:
 			ext = &obj->externs[relo->sym_off];
 			insn[0].src_reg = BPF_PSEUDO_KFUNC_CALL;
-			insn[0].imm = ext->ksym.kernel_btf_id;
-			insn[0].off = ext->ksym.btf_fd_idx;
+			if (ext->is_set) {
+				insn[0].imm = ext->ksym.kernel_btf_id;
+				insn[0].off = ext->ksym.btf_fd_idx;
+			} else { /* unresolved weak kfunc */
+				insn[0].imm = 0;
+				insn[0].off = 0;
+			}
 			break;
 		case RELO_SUBPROG_ADDR:
 			if (insn[0].src_reg != BPF_PSEUDO_FUNC) {
@@ -6743,9 +6743,9 @@ static int bpf_object__resolve_ksym_var_
 	int id, err;
 
 	id = find_ksym_btf_id(obj, ext->name, BTF_KIND_VAR, &btf, &mod_btf);
-	if (id == -ESRCH && ext->is_weak) {
-		return 0;
-	} else if (id < 0) {
+	if (id < 0) {
+		if (id == -ESRCH && ext->is_weak)
+			return 0;
 		pr_warn("extern (var ksym) '%s': not found in kernel BTF\n",
 			ext->name);
 		return id;
@@ -6798,7 +6798,9 @@ static int bpf_object__resolve_ksym_func
 
 	kfunc_id = find_ksym_btf_id(obj, ext->name, BTF_KIND_FUNC, &kern_btf, &mod_btf);
 	if (kfunc_id < 0) {
-		pr_warn("extern (func ksym) '%s': not found in kernel BTF\n",
+		if (kfunc_id == -ESRCH && ext->is_weak)
+			return 0;
+		pr_warn("extern (func ksym) '%s': not found in kernel or module BTFs\n",
 			ext->name);
 		return kfunc_id;
 	}