bpf/selftests: Test bpf_d_path on rdonly_mem.

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 <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

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

···
 #define MAX_FILES		7
 
 #include "test_d_path.skel.h"
+#include "test_d_path_check_rdonly_mem.skel.h"
 
 static int duration;
 
···
 	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;
···
 
 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_overwriting_rdonly_mem");
+
+	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();
 }

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

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2022 Google */
+
+#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)
+{
+	void *active;
+	__u32 cpu;
+
+	cpu = bpf_get_smp_processor_id();
+	active = (void *)bpf_per_cpu_ptr(&bpf_prog_active, cpu);
+	if (active) {
+		/* FAIL here! 'active' points to readonly memory. 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";