selftests/bpf: Add selftest for calling global functions from freplace

Add a selftest that calls a global function with a context object parameter
from an freplace function to check that the program context type is
correctly converted to the freplace target when fetching the context type
from the kernel BTF.

v2:
- Trim includes
- Get rid of global function
- Use __noinline

Signed-off-by: Toke Høiland-Jørgensen <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c

···
 				     "./test_attach_probe.o");
 }
 
+static void test_func_replace_global_func(void)
+{
+	const char *prog_name[] = {
+		"freplace/test_pkt_access",
+	};
+
+	test_fexit_bpf2bpf_common("./freplace_global_func.o",
+				  "./test_pkt_access.o",
+				  ARRAY_SIZE(prog_name),
+				  prog_name, false, NULL);
+}
+
 /* NOTE: affect other tests, must run in serial mode */
 void serial_test_fexit_bpf2bpf(void)
 {
···
 		test_func_replace_multi();
 	if (test__start_subtest("fmod_ret_freplace"))
 		test_fmod_ret_freplace();
+	if (test__start_subtest("func_replace_global_func"))
+		test_func_replace_global_func();
 }

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

···
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+__noinline
+int test_ctx_global_func(struct __sk_buff *skb)
+{
+	volatile int retval = 1;
+	return retval;
+}
+
+SEC("freplace/test_pkt_access")
+int new_test_pkt_access(struct __sk_buff *skb)
+{
+	return test_ctx_global_func(skb);
+}
+
+char _license[] SEC("license") = "GPL";