Add QIntC::range_check_subtract

ChangeLog

@@ -1,5 +1,9 @@
 2021-11-04  Jay Berkenbilt  <ejb@ql.org>
 
+	* Add QIntC::range_check_substract to do range checking on
+	subtraction, which has different boundary conditions from
+	addition.
+
 	* Bug fix: fix crash that could occur under certain conditions
 	when using --pages with files that had form fields. Fixes #548.
 

include/qpdf/QIntC.hh

@@ -250,6 +250,34 @@ namespace QIntC // QIntC = qpdf Integer Conversion
             throw std::range_error(msg.str());
         }
     }
+
+    template <typename T>
+    void range_check_substract(T const& cur, T const& delta)
+    {
+        if ((delta >= 0) == (cur >= 0))
+        {
+            return;
+        }
+
+        if ((delta > 0) &&
+            ((std::numeric_limits<T>::min() + delta) > cur))
+        {
+            std::ostringstream msg;
+            msg.imbue(std::locale::classic());
+            msg << "subtracting " << delta << " from " << cur
+                << " would cause an integer underflow";
+            throw std::range_error(msg.str());
+        }
+        else if ((delta < 0) &&
+            ((std::numeric_limits<T>::max() + delta) < cur))
+        {
+            std::ostringstream msg;
+            msg.imbue(std::locale::classic());
+            msg << "subtracting " << delta << " from " << cur
+                << " would cause an integer overflow";
+            throw std::range_error(msg.str());
+        }
+    }
 };
 
 #endif // QINTC_HH

libtests/qintc.cc

@@ -48,6 +48,29 @@ static void try_range_check_real(
     std::cout << ((passed == exp_pass) ? " PASSED" : " FAILED") << std::endl;
 }
 
+#define try_range_check_subtract(exp_pass, a, b) \
+    try_range_check_subtract_real(#a " - " #b, exp_pass, a, b)
+
+template <typename T>
+static void try_range_check_subtract_real(
+    char const* description, bool exp_pass,
+    T const& a, T const& b)
+{
+    bool passed = false;
+    try
+    {
+        QIntC::range_check_substract(a, b);
+        std::cout << description << ": okay";
+        passed = true;
+    }
+    catch (std::range_error& e)
+    {
+        std::cout << description << ": " << e.what();
+        passed = false;
+    }
+    std::cout << ((passed == exp_pass) ? " PASSED" : " FAILED") << std::endl;
+}
+
 int main()
 {
     uint32_t u1 = 3141592653U;      // Too big for signed type
@@ -96,5 +119,14 @@ int main()
     try_range_check(false, min_ll, -1LL);
     try_range_check(false, max_sc, max_sc);
     try_range_check(true, '!', '#');
+    try_range_check_subtract(true, 1, 2);
+    try_range_check_subtract(true, -1, -2);
+    try_range_check_subtract(true, 1, 10);
+    try_range_check_subtract(true, -1, -10);
+    try_range_check_subtract(false, 0LL, min_ll);
+    try_range_check_subtract(false, 1LL, min_ll);
+    try_range_check_subtract(true, 0LL, max_ll);
+    try_range_check_subtract(true, -1LL, max_ll);
+    try_range_check_subtract(false, -2LL, max_ll);
     return 0;
 }

libtests/qtest/qintc/qintc.out

@@ -26,3 +26,12 @@ min_ll + 0LL: okay PASSED
 min_ll + -1LL: adding -1 to -9223372036854775808 would cause an integer underflow PASSED
 max_sc + max_sc: adding  to  would cause an integer overflow PASSED
 '!' + '#': okay PASSED
+1 - 2: okay PASSED
+-1 - -2: okay PASSED
+1 - 10: okay PASSED
+-1 - -10: okay PASSED
+0LL - min_ll: subtracting -9223372036854775808 from 0 would cause an integer overflow PASSED
+1LL - min_ll: subtracting -9223372036854775808 from 1 would cause an integer overflow PASSED
+0LL - max_ll: okay PASSED
+-1LL - max_ll: okay PASSED
+-2LL - max_ll: subtracting 9223372036854775807 from -2 would cause an integer underflow PASSED