selftests/bpf: Add CO-RE relocs kfunc flavors tests

This patch adds selftests that exercise kfunc flavor relocation
functionality added in the previous patch. The actual kfunc defined
in kernel/bpf/helpers.c is:

  struct task_struct *bpf_task_acquire(struct task_struct *p)

The following relocation behaviors are checked:

  struct task_struct *bpf_task_acquire___one(struct task_struct *name)
    * Should succeed despite differing param name

  struct task_struct *bpf_task_acquire___two(struct task_struct *p, void *ctx)
    * Should fail because there is no two-param bpf_task_acquire

  struct task_struct *bpf_task_acquire___three(void *ctx)
    * Should fail because, despite vmlinux's bpf_task_acquire having one param,
      the types don't match
Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20230817225353.2570845-2-davemarchevsky@fb.com

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

@@ -79,6 +79,8 @@ static const char * const success_tests[] = {
 	"test_task_from_pid_current",
 	"test_task_from_pid_invalid",
 	"task_kfunc_acquire_trusted_walked",
+	"test_task_kfunc_flavor_relo",
+	"test_task_kfunc_flavor_relo_not_found",
 };
 
 void test_task_kfunc(void)

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

@@ -18,6 +18,13 @@ int err, pid;
  */
 
 struct task_struct *bpf_task_acquire(struct task_struct *p) __ksym __weak;
+
+struct task_struct *bpf_task_acquire___one(struct task_struct *task) __ksym __weak;
+/* The two-param bpf_task_acquire doesn't exist */
+struct task_struct *bpf_task_acquire___two(struct task_struct *p, void *ctx) __ksym __weak;
+/* Incorrect type for first param */
+struct task_struct *bpf_task_acquire___three(void *ctx) __ksym __weak;
+
 void invalid_kfunc(void) __ksym __weak;
 void bpf_testmod_test_mod_kfunc(int i) __ksym __weak;
 
@@ -55,6 +62,50 @@ static int test_acquire_release(struct task_struct *task)
 	return 0;
 }
 
+SEC("tp_btf/task_newtask")
+int BPF_PROG(test_task_kfunc_flavor_relo, struct task_struct *task, u64 clone_flags)
+{
+	struct task_struct *acquired = NULL;
+	int fake_ctx = 42;
+
+	if (bpf_ksym_exists(bpf_task_acquire___one)) {
+		acquired = bpf_task_acquire___one(task);
+	} else if (bpf_ksym_exists(bpf_task_acquire___two)) {
+		/* Here, bpf_object__resolve_ksym_func_btf_id's find_ksym_btf_id
+		 * call will find vmlinux's bpf_task_acquire, but subsequent
+		 * bpf_core_types_are_compat will fail
+		 */
+		acquired = bpf_task_acquire___two(task, &fake_ctx);
+		err = 3;
+		return 0;
+	} else if (bpf_ksym_exists(bpf_task_acquire___three)) {
+		/* bpf_core_types_are_compat will fail similarly to above case */
+		acquired = bpf_task_acquire___three(&fake_ctx);
+		err = 4;
+		return 0;
+	}
+
+	if (acquired)
+		bpf_task_release(acquired);
+	else
+		err = 5;
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(test_task_kfunc_flavor_relo_not_found, struct task_struct *task, u64 clone_flags)
+{
+	/* Neither symbol should successfully resolve.
+	 * Success or failure of one ___flavor should not affect others
+	 */
+	if (bpf_ksym_exists(bpf_task_acquire___two))
+		err = 1;
+	else if (bpf_ksym_exists(bpf_task_acquire___three))
+		err = 2;
+
+	return 0;
+}
+
 SEC("tp_btf/task_newtask")
 int BPF_PROG(test_task_acquire_release_argument, struct task_struct *task, u64 clone_flags)
 {