Some test files reference functions defined in other test translation
units. When those units are not compiled (e.g. because a BPF skeleton
could not be generated), the link step fails with undefined references.

Replace forward declarations with weak stub definitions for:

 - uprobe_multi_func_{1,2,3}() in bpf_cookie.c (defined in
   uprobe_multi_test.c)
 - stack_mprotect() in bpf_cookie.c and iters.c (defined in test_lsm.c)

The linker will prefer the strong definitions from the original objects
when they are present, and fall back to the stubs otherwise.

The uprobe stubs are marked noinline because uprobe_multi_test_run()
takes their addresses directly to configure the BPF program's uprobe
attachment points. An inlined function has no stable address in the
binary, which would cause the attachment to probe the wrong location.

Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
---
 .../testing/selftests/bpf/prog_tests/bpf_cookie.c  | 24 +++++++++++++++++-----
 tools/testing/selftests/bpf/prog_tests/iters.c     | 10 ++++++++-
 2 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_cookie.c b/tools/testing/selftests/bpf/prog_tests/bpf_cookie.c
index 35adc3f6d443..5fc6b2cd0bb9 100644
--- a/tools/testing/selftests/bpf/prog_tests/bpf_cookie.c
+++ b/tools/testing/selftests/bpf/prog_tests/bpf_cookie.c
@@ -252,10 +252,17 @@ static void kprobe_multi_attach_api_subtest(void)
 	kprobe_multi__destroy(skel);
 }
 
-/* defined in prog_tests/uprobe_multi_test.c */
-void uprobe_multi_func_1(void);
-void uprobe_multi_func_2(void);
-void uprobe_multi_func_3(void);
+/*
+ * Weak uprobe target stubs. noinline is required because
+ * uprobe_multi_test_run() takes their addresses to configure the BPF
+ * program's attachment points; an inlined function has no stable
+ * address in the binary to probe. The strong definitions in
+ * uprobe_multi_test.c take precedence when that translation unit is
+ * linked.
+ */
+noinline __weak void uprobe_multi_func_1(void) {}
+noinline __weak void uprobe_multi_func_2(void) {}
+noinline __weak void uprobe_multi_func_3(void) {}
 
 static void uprobe_multi_test_run(struct uprobe_multi *skel)
 {
@@ -574,7 +581,14 @@ static void tracing_subtest(struct test_bpf_cookie *skel)
 		close(fmod_ret_fd);
 }
 
-int stack_mprotect(void);
+/*
+ * Weak stub for stack_mprotect(); the real definition lives in
+ * test_lsm.c and takes precedence when that object is linked in.
+ */
+__weak int stack_mprotect(void)
+{
+	return 0;
+}
 
 static void lsm_subtest(struct test_bpf_cookie *skel)
 {
diff --git a/tools/testing/selftests/bpf/prog_tests/iters.c b/tools/testing/selftests/bpf/prog_tests/iters.c
index a539980a2fbe..6888bc9173fd 100644
--- a/tools/testing/selftests/bpf/prog_tests/iters.c
+++ b/tools/testing/selftests/bpf/prog_tests/iters.c
@@ -7,6 +7,7 @@
 #include <unistd.h>
 #include <malloc.h>
 #include <stdlib.h>
+#include <linux/compiler.h>
 #include <test_progs.h>
 #include "cgroup_helpers.h"
 
@@ -202,7 +203,14 @@ static void subtest_task_iters(void)
 	iters_task__destroy(skel);
 }
 
-extern int stack_mprotect(void);
+/*
+ * Weak stub for stack_mprotect(); the real definition lives in
+ * test_lsm.c and takes precedence when that object is linked in.
+ */
+__weak int stack_mprotect(void)
+{
+	return 0;
+}
 
 static void subtest_css_task_iters(void)
 {

-- 
2.53.0