[RFC PATCH bpf-next v2 3/3] selftests/bpf: add a selftest with __user tag

[Yonghong Song <yhs@xxxxxx>]

Added a selftest where the argument is a pointer with __user tag.
Directly accessing its field without helper will result
verification failure.
  $ ./test_progs -v -n 21/3
  ...
  Successfully loaded bpf_testmod.ko.
  test_btf_type_tag_user:PASS:btf_type_tag_user 0 nsec
  libbpf: load bpf program failed: Permission denied
  libbpf: -- BEGIN DUMP LOG ---
  libbpf:
  R1 type=ctx expected=fp
  ; int BPF_PROG(sub, struct bpf_testmod_btf_type_tag *arg)
  0: (79) r1 = *(u64 *)(r1 +0)
  func 'bpf_testmod_test_btf_type_tag_user' arg0 accesses user memory
  invalid bpf_context access off=0 size=8
  processed 1 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
  ...
  test_btf_type_tag_user:PASS:btf_type_tag_user 0 nsec
  #21/3 btf_tag/btf_type_tag_user:OK
  #21 btf_tag:OK
  Summary: 1/1 PASSED, 0 SKIPPED, 0 FAILED
  Successfully unloaded bpf_testmod.ko.

Signed-off-by: Yonghong Song <yhs@xxxxxx>
---
 kernel/bpf/verifier.c                         |  1 +
 .../selftests/bpf/bpf_testmod/bpf_testmod.c   |  9 +++++++
 .../selftests/bpf/prog_tests/btf_tag.c        | 23 ++++++++++++++++++
 .../selftests/bpf/progs/btf_type_tag_user.c   | 24 +++++++++++++++++++
 4 files changed, 57 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/btf_type_tag_user.c

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 07ba7c8f6aa3..71fd66b53026 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4372,6 +4372,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
 		enum bpf_reg_type reg_type = SCALAR_VALUE;
 		struct btf *btf = NULL;
 		u32 btf_id = 0;
+		bool is_user;
 
 		if (t == BPF_WRITE && value_regno >= 0 &&
 		    is_pointer_value(env, value_regno)) {
diff --git a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
index 5d52ea2768df..5377dd2959bb 100644
--- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
+++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
@@ -21,6 +21,15 @@ bpf_testmod_test_mod_kfunc(int i)
 	*(int *)this_cpu_ptr(&bpf_testmod_ksym_percpu) = i;
 }
 
+struct bpf_testmod_btf_type_tag {
+	int a;
+};
+
+noinline int
+bpf_testmod_test_btf_type_tag_user(struct bpf_testmod_btf_type_tag __user *arg) {
+	return arg->a;
+}
+
 noinline int bpf_testmod_loop_test(int n)
 {
 	int i, sum = 0;
diff --git a/tools/testing/selftests/bpf/prog_tests/btf_tag.c b/tools/testing/selftests/bpf/prog_tests/btf_tag.c
index 88d63e23e35f..1e4003fa7cd0 100644
--- a/tools/testing/selftests/bpf/prog_tests/btf_tag.c
+++ b/tools/testing/selftests/bpf/prog_tests/btf_tag.c
@@ -8,6 +8,7 @@ struct btf_type_tag_test {
         int **p;
 };
 #include "btf_type_tag.skel.h"
+#include "btf_type_tag_user.skel.h"
 
 static void test_btf_decl_tag(void)
 {
@@ -41,10 +42,32 @@ static void test_btf_type_tag(void)
 	btf_type_tag__destroy(skel);
 }
 
+static void test_btf_type_tag_user(void)
+{
+	struct btf_type_tag_user *skel;
+	int err;
+
+	skel = btf_type_tag_user__open();
+	if (!ASSERT_OK_PTR(skel, "btf_type_tag_user"))
+		return;
+
+	if (skel->rodata->skip_tests) {
+		printf("%s:SKIP: btf_type_tag attribute not supported", __func__);
+		test__skip();
+	} else {
+		err = btf_type_tag_user__load(skel);
+		ASSERT_ERR(err, "btf_type_tag_user");
+	}
+
+	btf_type_tag_user__destroy(skel);
+}
+
 void test_btf_tag(void)
 {
 	if (test__start_subtest("btf_decl_tag"))
 		test_btf_decl_tag();
 	if (test__start_subtest("btf_type_tag"))
 		test_btf_type_tag();
+	if (test__start_subtest("btf_type_tag_user"))
+		test_btf_type_tag_user();
 }
diff --git a/tools/testing/selftests/bpf/progs/btf_type_tag_user.c b/tools/testing/selftests/bpf/progs/btf_type_tag_user.c
new file mode 100644
index 000000000000..41ab1ddd2cf5
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/btf_type_tag_user.c
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Facebook */
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+#if __has_attribute(btf_type_tag)
+volatile const bool skip_tests = false;
+#else
+volatile const bool skip_tests = true;
+#endif
+
+struct bpf_testmod_btf_type_tag {
+	int a;
+};
+
+int g;
+
+SEC("fentry/bpf_testmod_test_btf_type_tag_user")
+int BPF_PROG(sub, struct bpf_testmod_btf_type_tag *arg)
+{
+  g = arg->a;
+  return 0;
+}
-- 
2.30.2