ftrace: Improve parsing for string fields

With this commit:

* The code now accepts string fields with an indentifier, instead of a
  literal, for array size. This is important since commit
  3087c61ed2c4("tools/testing/selftests/bpf: replace open-coded 16 with
  TASK_COMM_LEN"), included in kernel 5.17, which exposes TASK_COMM_LEN
  in the format.
* The code now accepts fields whose name include a digit (because that's
  a valid C identifier.).

Tested: unit tests
Bug: 227677227
Change-Id: I6f0074607f6ce41880a235a00ee7a4fbc53a5bb9

Author: ddiproietto

src/traced/probes/ftrace/proto_translation_table.cc

@@ -256,15 +256,20 @@ bool InferFtraceType(const std::string& type_and_name,
                      size_t size,
                      bool is_signed,
                      FtraceFieldType* out) {
-  // Fixed length strings: e.g. "char foo[16]" we don't care about the number
-  // since we get the size as it's own field. Somewhat awkwardly these fields
-  // are both fixed size and null terminated meaning that we can't just drop
-  // them directly into the protobuf (since if the string is shorter than 15
-  // characters we want only the bit up to the null terminator).
+  // Fixed length strings: e.g. "char foo[16]".
+  //
+  // We don't care about the number, since we get the size as it's own field and
+  // since it can be a string defined elsewhere in a kernel header file.
+  //
+  // Somewhat awkwardly these fields are both fixed size and null terminated
+  // meaning that we can't just drop them directly into the protobuf (since if
+  // the string is shorter than 15 characters we want only the bit up to the
+  // null terminator).
   //
   // In some rare cases (e.g. old kernel bugs) these strings might not be null
   // terminated (b/205763418).
-  if (Match(type_and_name.c_str(), R"(char [a-zA-Z_]+\[[0-9]+\])")) {
+  if (Match(type_and_name.c_str(),
+            R"(char [a-zA-Z_][a-zA-Z_0-9]*\[[a-zA-Z_0-9]+\])")) {
     *out = kFtraceFixedCString;
     return true;
   }

src/traced/probes/ftrace/proto_translation_table_unittest.cc

@@ -319,6 +319,14 @@ TEST(TranslationTableTest, InferFtraceType) {
   ASSERT_TRUE(InferFtraceType("char foo[16]", 16, false, &type));
   EXPECT_EQ(type, kFtraceFixedCString);
 
+  ASSERT_TRUE(InferFtraceType("char comm[TASK_COMM_LEN]", 16, false, &type));
+  EXPECT_EQ(type, kFtraceFixedCString);
+
+  ASSERT_TRUE(InferFtraceType("char identifier22[16]", 16, false, &type));
+  EXPECT_EQ(type, kFtraceFixedCString);
+
+  EXPECT_FALSE(InferFtraceType("char 2invalid[16]", 16, false, &type));
+
   ASSERT_TRUE(InferFtraceType("char[] foo", 8, false, &type));
   EXPECT_EQ(type, kFtraceStringPtr);