selftests/bpf: make sure linking objects with duplicate extern functi…

…ons doesn't fail

Previously when multiple BPF object files referencing the same extern
function (usually kfunc) are statically linked using `bpftool gen
object`, libbpf tries to get the nonexistent size of BTF_KIND_FUNC_PROTO
and fails. This test ensures it is fixed.

Signed-off-by: Eric Long <i@hack3r.moe>

tools/testing/selftests/bpf/Makefile

@@ -496,7 +496,7 @@ SKEL_BLACKLIST := btf__% test_pinning_invalid.c test_sk_assign.c
 LINKED_SKELS := test_static_linked.skel.h linked_funcs.skel.h		\
 		linked_vars.skel.h linked_maps.skel.h 			\
 		test_subskeleton.skel.h test_subskeleton_lib.skel.h	\
-		test_usdt.skel.h
+		test_usdt.skel.h dup_extern_funcs.skel.h
 
 LSKELS := fentry_test.c fexit_test.c fexit_sleep.c atomics.c 		\
 	trace_printk.c trace_vprintk.c map_ptr_kern.c 			\
@@ -520,6 +520,7 @@ test_usdt.skel.h-deps := test_usdt.bpf.o test_usdt_multispec.bpf.o
 xsk_xdp_progs.skel.h-deps := xsk_xdp_progs.bpf.o
 xdp_hw_metadata.skel.h-deps := xdp_hw_metadata.bpf.o
 xdp_features.skel.h-deps := xdp_features.bpf.o
+dup_extern_funcs.skel.h-deps := dup_extern_funcs1.bpf.o dup_extern_funcs2.bpf.o
 
 LINKED_BPF_OBJS := $(foreach skel,$(LINKED_SKELS),$($(skel)-deps))
 LINKED_BPF_SRCS := $(patsubst %.bpf.o,%.c,$(LINKED_BPF_OBJS))

tools/testing/selftests/bpf/prog_tests/dup_extern_funcs.c

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <test_progs.h>
+#include "dup_extern_funcs.skel.h"
+
+void test_dup_extern_funcs(void)
+{
+	RUN_TESTS(dup_extern_funcs);
+}

tools/testing/selftests/bpf/progs/dup_extern_funcs1.c

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+void *bpf_cast_to_kern_ctx(void *obj) __ksym;
+
+SEC("tc")
+int handler1(struct __sk_buff *skb) {
+	struct sk_buff *skb_kern = bpf_cast_to_kern_ctx(skb);
+	if (!skb_kern)
+		return -1;
+	return 0;
+}

tools/testing/selftests/bpf/progs/dup_extern_funcs2.c

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+void *bpf_cast_to_kern_ctx(void *obj) __ksym;
+
+SEC("xdp")
+int handler2(struct xdp_md *xdp) {
+	struct xdp_buff *xdp_kern = bpf_cast_to_kern_ctx(xdp);
+	if (!xdp_kern)
+		return -1;
+	return 0;
+}