[RFC v2 3/3] selftests/bpf: verify bpftool emits preserve_static_offset

[Eduard Zingerman <eddyz87@gmail.com>]

Extend test_bpftool.py with following test cases:
- Load a small program that has some context types in it's BTF,
  verify that "bpftool btf dump file ... format c" emits
  preserve_static_offset attribute.
- Load a small program that has no context types in it's BTF,
  verify that "bpftool btf dump file ... format c" does not emit
  preserve_static_offset attribute.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
 .../bpf/progs/dummy_no_context_btf.c          | 12 ++++
 .../selftests/bpf/progs/dummy_sk_buff_user.c  | 14 +++++
 tools/testing/selftests/bpf/test_bpftool.py   | 61 +++++++++++++++++++
 3 files changed, 87 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/dummy_no_context_btf.c
 create mode 100644 tools/testing/selftests/bpf/progs/dummy_sk_buff_user.c

diff --git a/tools/testing/selftests/bpf/progs/dummy_no_context_btf.c b/tools/testing/selftests/bpf/progs/dummy_no_context_btf.c
new file mode 100644
index 000000000000..5a1df4984dce
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/dummy_no_context_btf.c
@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+/* A dummy program that does not reference context types in it's BTF */
+SEC("tc")
+__u32 dummy_prog(void *ctx)
+{
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
diff --git a/tools/testing/selftests/bpf/progs/dummy_sk_buff_user.c b/tools/testing/selftests/bpf/progs/dummy_sk_buff_user.c
new file mode 100644
index 000000000000..f271881bcbd0
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/dummy_sk_buff_user.c
@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+/* A dummy program that references __sk_buff type in it's BTF,
+ * used by test_bpftool.py.
+ */
+SEC("tc")
+int sk_buff_user(struct __sk_buff *skb)
+{
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
diff --git a/tools/testing/selftests/bpf/test_bpftool.py b/tools/testing/selftests/bpf/test_bpftool.py
index 1c2408ee1f5d..3117f431dd92 100644
--- a/tools/testing/selftests/bpf/test_bpftool.py
+++ b/tools/testing/selftests/bpf/test_bpftool.py
@@ -8,6 +8,7 @@ import os
 import socket
 import subprocess
 import unittest
+import io
 
 
 # Add the source tree of bpftool and /usr/local/sbin to PATH
@@ -25,6 +26,10 @@ class UnprivilegedUserError(Exception):
     pass
 
 
+class MissingDependencyError(Exception):
+    pass
+
+
 def _bpftool(args, json=True):
     _args = ["bpftool"]
     if json:
@@ -63,12 +68,22 @@ DMESG_EMITTING_HELPERS = [
         "bpf_trace_vprintk",
     ]
 
+DUMMY_SK_BUFF_USER_OBJ = cur_dir + "/dummy_sk_buff_user.bpf.o"
+DUMMY_NO_CONTEXT_BTF_OBJ = cur_dir + "/dummy_no_context_btf.bpf.o"
+
 class TestBpftool(unittest.TestCase):
     @classmethod
     def setUpClass(cls):
         if os.getuid() != 0:
             raise UnprivilegedUserError(
                 "This test suite needs root privileges")
+        objs = [DUMMY_SK_BUFF_USER_OBJ,
+                DUMMY_NO_CONTEXT_BTF_OBJ]
+        for obj in objs:
+            if os.path.exists(obj):
+                continue
+            raise MissingDependencyError(
+                "File " + obj + " does not exist, make sure progs/*.c are compiled")
 
     @default_iface
     def test_feature_dev_json(self, iface):
@@ -172,3 +187,49 @@ class TestBpftool(unittest.TestCase):
         res = bpftool(["feature", "probe", "macros"])
         for pattern in expected_patterns:
             self.assertRegex(res, pattern)
+
+    def assertStringsPresent(self, text, patterns):
+        pos = 0
+        for i, pat in enumerate(patterns):
+            m = text.find(pat, pos)
+            if m == -1:
+                with io.StringIO() as msg:
+                    print("Can't find expected string:", file=msg)
+                    for s in patterns[0:i]:
+                        print("    MATCHED: " + s, file=msg)
+                    print("NOT MATCHED: " + pat, file=msg)
+                    print("", file=msg)
+                    print("Searching in:", file=msg)
+                    print(text, file=msg)
+                    self.fail(msg.getvalue())
+            pos += len(pat)
+
+    # Load a small program that has some context types in it's BTF,
+    # verify that "bpftool btf dump file ... format c" emits
+    # preserve_static_offset attribute.
+    def test_c_dump_preserve_static_offset_present(self):
+        res = bpftool(["btf", "dump", "file", DUMMY_SK_BUFF_USER_OBJ, "format", "c"])
+        self.assertStringsPresent(res, [
+            "#if !defined(BPF_NO_PRESERVE_STATIC_OFFSET) && " +
+              "__has_attribute(preserve_static_offset)",
+            "#pragma clang attribute push " +
+              "(__attribute__((preserve_static_offset)), apply_to = record)",
+            "struct __sk_buff;",
+            "struct bpf_sock;",
+            "#pragma clang attribute pop",
+            "#endif /* BPF_NO_PRESERVE_STATIC_OFFSET */",
+            "#pragma clang attribute push " +
+              "(__attribute__((preserve_access_index)), apply_to = record)",
+            "struct __sk_buff {",
+        ])
+
+    # Load a small program that has no context types in it's BTF,
+    # verify that "bpftool btf dump file ... format c" does not emit
+    # preserve_static_offset attribute.
+    def test_c_dump_no_preserve_static_offset(self):
+        res = bpftool(["btf", "dump", "file", DUMMY_NO_CONTEXT_BTF_OBJ, "format", "c"])
+        self.assertNotRegex(res, "preserve_static_offset")
+        self.assertStringsPresent(res, [
+            "preserve_access_index",
+            "typedef unsigned int __u32;"
+        ])
-- 
2.42.1