3 files changed, 96 insertions, 7 deletions

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index cb106e8c42cb..ff3c0ee79d85 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -5277,11 +5277,11 @@ bpf_object__relocate_data(struct bpf_object *obj, struct bpf_program *prog)
 				}
 				insn[1].imm = ext->kcfg.data_off;
 			} else /* EXT_KSYM */ {
-				if (ext->ksym.type_id) { /* typed ksyms */
+				if (ext->ksym.type_id && ext->is_set) { /* typed ksyms */
 					insn[0].src_reg = BPF_PSEUDO_BTF_ID;
 					insn[0].imm = ext->ksym.kernel_btf_id;
 					insn[1].imm = ext->ksym.kernel_btf_obj_fd;
-				} else { /* typeless ksyms */
+				} else { /* typeless ksyms or unresolved typed ksyms */
 					insn[0].imm = (__u32)ext->ksym.addr;
 					insn[1].imm = ext->ksym.addr >> 32;
 				}
@@ -6608,11 +6608,8 @@ static int find_ksym_btf_id(struct bpf_object *obj, const char *ksym_name,
 				break;
 		}
 	}
-	if (id <= 0) {
-		pr_warn("extern (%s ksym) '%s': failed to find BTF ID in kernel BTF(s).\n",
-			__btf_kind_str(kind), ksym_name);
+	if (id <= 0)
 		return -ESRCH;
-	}
 
 	*res_btf = btf;
 	*res_btf_fd = btf_fd;
@@ -6629,8 +6626,13 @@ static int bpf_object__resolve_ksym_var_btf_id(struct bpf_object *obj,
 	struct btf *btf = NULL;
 
 	id = find_ksym_btf_id(obj, ext->name, BTF_KIND_VAR, &btf, &btf_fd);
-	if (id < 0)
+	if (id == -ESRCH && ext->is_weak) {
+		return 0;
+	} else if (id < 0) {
+		pr_warn("extern (var ksym) '%s': not found in kernel BTF\n",
+			ext->name);
 		return id;
+	}
 
 	/* find local type_id */
 	local_type_id = ext->ksym.type_id;
diff --git a/tools/testing/selftests/bpf/prog_tests/ksyms_btf.c b/tools/testing/selftests/bpf/prog_tests/ksyms_btf.c
index 67bebd324147..cf3acfa5a91d 100644
--- a/tools/testing/selftests/bpf/prog_tests/ksyms_btf.c
+++ b/tools/testing/selftests/bpf/prog_tests/ksyms_btf.c
@@ -6,6 +6,7 @@
 #include <bpf/btf.h>
 #include "test_ksyms_btf.skel.h"
 #include "test_ksyms_btf_null_check.skel.h"
+#include "test_ksyms_weak.skel.h"
 
 static int duration;
 
@@ -81,6 +82,33 @@ static void test_null_check(void)
 	test_ksyms_btf_null_check__destroy(skel);
 }
 
+static void test_weak_syms(void)
+{
+	struct test_ksyms_weak *skel;
+	struct test_ksyms_weak__data *data;
+	int err;
+
+	skel = test_ksyms_weak__open_and_load();
+	if (CHECK(!skel, "test_ksyms_weak__open_and_load", "failed\n"))
+		return;
+
+	err = test_ksyms_weak__attach(skel);
+	if (CHECK(err, "test_ksyms_weak__attach", "skeleton attach failed: %d\n", err))
+		goto cleanup;
+
+	/* trigger tracepoint */
+	usleep(1);
+
+	data = skel->data;
+	ASSERT_EQ(data->out__existing_typed, 0, "existing typed ksym");
+	ASSERT_NEQ(data->out__existing_typeless, -1, "existing typeless ksym");
+	ASSERT_EQ(data->out__non_existent_typeless, 0, "nonexistent typeless ksym");
+	ASSERT_EQ(data->out__non_existent_typed, 0, "nonexistent typed ksym");
+
+cleanup:
+	test_ksyms_weak__destroy(skel);
+}
+
 void test_ksyms_btf(void)
 {
 	int percpu_datasec;
@@ -105,4 +133,7 @@ void test_ksyms_btf(void)
 
 	if (test__start_subtest("null_check"))
 		test_null_check();
+
+	if (test__start_subtest("weak_ksyms"))
+		test_weak_syms();
 }
diff --git a/tools/testing/selftests/bpf/progs/test_ksyms_weak.c b/tools/testing/selftests/bpf/progs/test_ksyms_weak.c
new file mode 100644
index 000000000000..5f8379aadb29
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_ksyms_weak.c
@@ -0,0 +1,56 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Test weak ksyms.
+ *
+ * Copyright (c) 2021 Google
+ */
+
+#include "vmlinux.h"
+
+#include <bpf/bpf_helpers.h>
+
+int out__existing_typed = -1;
+__u64 out__existing_typeless = -1;
+
+__u64 out__non_existent_typeless = -1;
+__u64 out__non_existent_typed = -1;
+
+/* existing weak symbols */
+
+/* test existing weak symbols can be resolved. */
+extern const struct rq runqueues __ksym __weak; /* typed */
+extern const void bpf_prog_active __ksym __weak; /* typeless */
+
+
+/* non-existent weak symbols. */
+
+/* typeless symbols, default to zero. */
+extern const void bpf_link_fops1 __ksym __weak;
+
+/* typed symbols, default to zero. */
+extern const int bpf_link_fops2 __ksym __weak;
+
+SEC("raw_tp/sys_enter")
+int pass_handler(const void *ctx)
+{
+	struct rq *rq;
+
+	/* tests existing symbols. */
+	rq = (struct rq *)bpf_per_cpu_ptr(&runqueues, 0);
+	if (rq)
+		out__existing_typed = rq->cpu;
+	out__existing_typeless = (__u64)&bpf_prog_active;
+
+	/* tests non-existent symbols. */
+	out__non_existent_typeless = (__u64)&bpf_link_fops1;
+
+	/* tests non-existent symbols. */
+	out__non_existent_typed = (__u64)&bpf_link_fops2;
+
+	if (&bpf_link_fops2) /* can't happen */
+		out__non_existent_typed = (__u64)bpf_per_cpu_ptr(&bpf_link_fops2, 0);
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";