selftests/bpf: Test linking with duplicate extern functions

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 <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/progs/linked_funcs1.c

···
 /* here we'll force set_output_ctx2() to be __hidden in the final obj file */
 __hidden extern void set_output_ctx2(__u64 *ctx);
 
+void *bpf_cast_to_kern_ctx(void *obj) __ksym;
+
 SEC("?raw_tp/sys_enter")
 int BPF_PROG(handler1, struct pt_regs *regs, long id)
 {
···
 	set_output_weak(42);
 
 	return 0;
+}
+
+/* Generate BTF FUNC record and test linking with duplicate extern functions */
+void kfunc_gen1(void)
+{
+	bpf_cast_to_kern_ctx(0);
 }
 
 char LICENSE[] SEC("license") = "GPL";

tools/testing/selftests/bpf/progs/linked_funcs2.c

···
 /* here we'll force set_output_ctx1() to be __hidden in the final obj file */
 __hidden extern void set_output_ctx1(__u64 *ctx);
 
+void *bpf_cast_to_kern_ctx(void *obj) __ksym;
+
 SEC("?raw_tp/sys_enter")
 int BPF_PROG(handler2, struct pt_regs *regs, long id)
 {
···
 	set_output_weak(42);
 
 	return 0;
+}
+
+/* Generate BTF FUNC record and test linking with duplicate extern functions */
+void kfunc_gen2(void)
+{
+	bpf_cast_to_kern_ctx(0);
 }
 
 char LICENSE[] SEC("license") = "GPL";