Check for exact function in unwanted function check

check-unwanted-cpp-functions.py has false positives. The script reports
`asprintf` as usage of `sprintf`.

To prevent issues like this, modify the regex for checking functions so
that we only report exact matches.

BUG=b:320231979
TEST=`devtools/check-unwanted-cpp-functions.py --check_all_files` and
check that there are no `sprintf` reports.

Change-Id: Ieed8121b525e3e84ab3cc4414acd3db1535aad9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/adhd/+/5248447
Commit-Queue: Baili Deng <bailideng@google.com>
Tested-by: chromeos-cop-builder@chromeos-cop.iam.gserviceaccount.com <chromeos-cop-builder@chromeos-cop.iam.gserviceaccount.com>
Reviewed-by: Li-Yu Yu <aaronyu@google.com>

devtools/blocked_functions.json

@@ -1,43 +1,43 @@
 {
   "blocked_functions": [
     {
-      "regex": "atoi",
+      "function": "atoi",
       "replacement": "strtol"
     },
     {
-      "regex": "atof",
+      "function": "atof",
       "replacement": "strtof"
     },
     {
-      "regex": "sprintf",
+      "function": "sprintf",
       "replacement": "snprintf or scnprintf if using the return value"
     },
     {
-      "regex": "strcpy",
+      "function": "strcpy",
       "replacement": "strlcpy"
     },
     {
-      "regex": "strncpy",
+      "function": "strncpy",
       "replacement": "strlcpy"
     },
     {
-      "regex": "strlen",
+      "function": "strlen",
       "replacement": "strnlen"
     },
     {
-      "regex": "strdup",
+      "function": "strdup",
       "replacement": "strndup"
     },
     {
-      "regex": "strstr",
+      "function": "strstr",
       "replacement": "strnstr"
     },
     {
-      "regex": "strchr",
+      "function": "strchr",
       "replacement": "strnchr"
     },
     {
-      "regex": "strdupa",
+      "function": "strdupa",
       "replacement": "strndupa"
     }
   ]

devtools/check-unwanted-cpp-functions.py

@@ -100,9 +100,10 @@ def _check_line(line):
             return None
 
         for function in blocked_functions:
-            match = re.search(function["regex"], line)
+            regex = r"(?<![a-zA-Z])" + function["function"] + r"\("
+            match = re.search(regex, line)
             if match:
-                return f'Found usage of "{function["regex"]}", please replace it with "{function["replacement"]}"'
+                return f'Found usage of "{function["function"]}", please replace it with "{function["replacement"]}"'
         return None
 
     error_list = []