[PATCH bpf-next] bpf/selftests: Test bpf_d_path on rdonly_mem.

[Hao Luo <haoluo@xxxxxxxxxx>]

The second parameter of bpf_d_path() can only accept writable
memories. rdonly_mem obtained from bpf_per_cpu_ptr() can not
be passed into bpf_d_path for modification. This patch adds
a selftest to verify this behavior.

Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
---
 .../testing/selftests/bpf/prog_tests/d_path.c | 22 +++++++++++++-
 .../bpf/progs/test_d_path_check_rdonly_mem.c  | 30 +++++++++++++++++++
 2 files changed, 51 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/bpf/progs/test_d_path_check_rdonly_mem.c

diff --git a/tools/testing/selftests/bpf/prog_tests/d_path.c b/tools/testing/selftests/bpf/prog_tests/d_path.c
index 0a577a248d34..f8d8c5a5dfba 100644
--- a/tools/testing/selftests/bpf/prog_tests/d_path.c
+++ b/tools/testing/selftests/bpf/prog_tests/d_path.c
@@ -9,6 +9,7 @@
 #define MAX_FILES		7
 
 #include "test_d_path.skel.h"
+#include "test_d_path_check_rdonly_mem.skel.h"
 
 static int duration;
 
@@ -99,7 +100,7 @@ static int trigger_fstat_events(pid_t pid)
 	return ret;
 }
 
-void test_d_path(void)
+static void test_d_path_basic(void)
 {
 	struct test_d_path__bss *bss;
 	struct test_d_path *skel;
@@ -155,3 +156,22 @@ void test_d_path(void)
 cleanup:
 	test_d_path__destroy(skel);
 }
+
+static void test_d_path_check_rdonly_mem(void)
+{
+	struct test_d_path_check_rdonly_mem *skel;
+
+	skel = test_d_path_check_rdonly_mem__open_and_load();
+	ASSERT_ERR_PTR(skel, "unexpected load of a prog using d_path to write rdonly_mem\n");
+
+	test_d_path_check_rdonly_mem__destroy(skel);
+}
+
+void test_d_path(void)
+{
+	if (test__start_subtest("basic"))
+		test_d_path_basic();
+
+	if (test__start_subtest("check_rdonly_mem"))
+		test_d_path_check_rdonly_mem();
+}
diff --git a/tools/testing/selftests/bpf/progs/test_d_path_check_rdonly_mem.c b/tools/testing/selftests/bpf/progs/test_d_path_check_rdonly_mem.c
new file mode 100644
index 000000000000..c7a9655d5850
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_d_path_check_rdonly_mem.c
@@ -0,0 +1,30 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Google */
+
+#include "vmlinux.h"
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+extern const int bpf_prog_active __ksym;
+
+SEC("fentry/security_inode_getattr")
+int BPF_PROG(d_path_check_rdonly_mem, struct path *path, struct kstat *stat,
+	     __u32 request_mask, unsigned int query_flags)
+{
+	char *active;
+	__u32 cpu;
+
+	cpu = bpf_get_smp_processor_id();
+	active = (char *)bpf_per_cpu_ptr(&bpf_prog_active, cpu);
+	if (active) {
+		/* FAIL here! 'active' is a rdonly_mem. bpf helpers that
+		 * update its arguments can not write into it.
+		 */
+		bpf_d_path(path, active, sizeof(int));
+	}
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.34.1.307.g9b7440fafd-goog