[PATCH bpf-next] selftests/bpf: Fix map_kptr test.

[Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>]

From: Alexei Starovoitov <ast@xxxxxxxxxx>

The compiler is optimizing out majority of unref_ptr read/writes, so the test
wasn't testing much. For example, one could delete '__kptr' tag from
'struct prog_test_ref_kfunc __kptr *unref_ptr;' and the test would still "pass".

Convert it to volatile stores. Confirmed by comparing bpf asm before/after.

Fixes: 2cbc469a6fc3 ("selftests/bpf: Add C tests for kptr")
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
---
 tools/testing/selftests/bpf/progs/map_kptr.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/bpf/progs/map_kptr.c b/tools/testing/selftests/bpf/progs/map_kptr.c
index eb8217803493..228ec45365a8 100644
--- a/tools/testing/selftests/bpf/progs/map_kptr.c
+++ b/tools/testing/selftests/bpf/progs/map_kptr.c
@@ -62,21 +62,23 @@ extern struct prog_test_ref_kfunc *
 bpf_kfunc_call_test_kptr_get(struct prog_test_ref_kfunc **p, int a, int b) __ksym;
 extern void bpf_kfunc_call_test_release(struct prog_test_ref_kfunc *p) __ksym;
 
+#define WRITE_ONCE(x, val) ((*(volatile typeof(x) *) &(x)) = (val))
+
 static void test_kptr_unref(struct map_value *v)
 {
 	struct prog_test_ref_kfunc *p;
 
 	p = v->unref_ptr;
 	/* store untrusted_ptr_or_null_ */
-	v->unref_ptr = p;
+	WRITE_ONCE(v->unref_ptr, p);
 	if (!p)
 		return;
 	if (p->a + p->b > 100)
 		return;
 	/* store untrusted_ptr_ */
-	v->unref_ptr = p;
+	WRITE_ONCE(v->unref_ptr, p);
 	/* store NULL */
-	v->unref_ptr = NULL;
+	WRITE_ONCE(v->unref_ptr, NULL);
 }
 
 static void test_kptr_ref(struct map_value *v)
@@ -85,7 +87,7 @@ static void test_kptr_ref(struct map_value *v)
 
 	p = v->ref_ptr;
 	/* store ptr_or_null_ */
-	v->unref_ptr = p;
+	WRITE_ONCE(v->unref_ptr, p);
 	if (!p)
 		return;
 	if (p->a + p->b > 100)
@@ -99,7 +101,7 @@ static void test_kptr_ref(struct map_value *v)
 		return;
 	}
 	/* store ptr_ */
-	v->unref_ptr = p;
+	WRITE_ONCE(v->unref_ptr, p);
 	bpf_kfunc_call_test_release(p);
 
 	p = bpf_kfunc_call_test_acquire(&(unsigned long){0});
-- 
2.30.2