checkpatch.py: Fix a catastrophic backtracking issue.

The regex_if_macros accidentally implements a catastrophic backtracking
case during matching because it forces a greedy match and evaluation that
causes an explosion of combinations depending on how many blocks appear
in a parenthesized expression.

Consider an expression like:

  if (a b c d e f g h i j k) {

The regex engine will have to create an exponential number of matches that
can overload memory and cpu requirements.

Fix this by switching the inner-content match to use a non-greedy approach.

Fixes: aacad86 ("checkpatch: Fix regexp for if, while, etc inside macros.")
Signed-off-by: Aaron Conole <aconole@redhat.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>

Author: apconole

utilities/checkpatch.py

@@ -158,7 +158,7 @@ def reset_counters():
 __regex_added_doc_rst = re.compile(
                     r'\ndiff .*Documentation/.*rst\nnew file mode')
 __regex_empty_return = re.compile(r'\s*return;')
-__regex_if_macros = re.compile(r'^ +(%s) \([\S]([\s\S]+[\S])*\) { +\\' %
+__regex_if_macros = re.compile(r'^ +(%s) \([\S](?:[\s\S]*?[\S])?\) { +\\' %
                                __parenthesized_constructs)
 __regex_nonascii_characters = re.compile("[^\u0000-\u007f]")
 __regex_efgrep = re.compile(r'.*[ef]grep.*$')