libstdc++: Check for overflow in regex back-reference [PR106607]

Currently we fail to notice integer overflow when parsing a
back-reference expression, or when converting the parsed result from
long to int. This changes the result to be int, so no conversion is
needed, and uses the overflow-checking built-ins to detect an
out-of-range back-reference.

libstdc++-v3/ChangeLog:

	PR libstdc++/106607
	* include/bits/regex_compiler.tcc (_Compiler::_M_cur_int_value):
	Use built-ins to check for integer overflow in back-reference
	number.
	* testsuite/28_regex/basic_regex/106607.cc: New test.

libstdc++-v3/include/bits/regex_compiler.tcc

@@ -583,10 +583,12 @@ namespace __detail
     _Compiler<_TraitsT>::
     _M_cur_int_value(int __radix)
     {
-      long __v = 0;
-      for (typename _StringT::size_type __i = 0;
-	   __i < _M_value.length(); ++__i)
-	__v =__v * __radix + _M_traits.value(_M_value[__i], __radix);
+      int __v = 0;
+      for (_CharT __c : _M_value)
+	if (__builtin_mul_overflow(__v, __radix, &__v)
+	    || __builtin_add_overflow(__v, _M_traits.value(__c, __radix), &__v))
+	    std::__throw_regex_error(regex_constants::error_backref,
+				     "invalid back reference");
       return __v;
     }
 

libstdc++-v3/testsuite/28_regex/basic_regex/106607.cc

@@ -0,0 +1,25 @@
+// { dg-do run { target c++11 } }
+
+#include <regex>
+#include <string>
+#include <climits>
+#include <testsuite_hooks.h>
+
+// PR libstdc++/106607 - Regex integer overflow on large backreference value
+
+int main()
+{
+  std::regex r("(.)\\1"); // OK
+
+  try
+  {
+    long long n = (unsigned)-1 + 2LL;          // 4294967297 for 32-bit int
+    VERIFY( (int)n == 1 );                     // 4294967297 % 2^32 == 1
+    std::regex r("(.)\\" + std::to_string(n)); // Invalid back reference.
+    VERIFY(false);
+  }
+  catch (const std::regex_error& e)
+  {
+    VERIFY( e.code() == std::regex_constants::error_backref );
+  }
+}