#define grub_mul(a, b, res) __builtin_mul_overflow(a, b, res)
#else
-#error gcc 5.1 or newer or clang 3.8 or newer is required
+/*
+ * Copyright 2020 Rasmus Villemoes
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to
+ * deal in the Software without restriction, including without limitation the
+ * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+ * sell copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+/*
+ * The code used in this header was taken from linux kernel commit
+ * f0907827a8a9152aedac2833ed1b674a7b2a44f2
+ * Rasmus Villemoes <linux@rasmusvillemoes.dk>, the original author of the
+ * patch, was contacted directly, confirmed his authorship of the code, and
+ * gave his permission on treating that dual license as MIT and including into
+ * GRUB2 sources
+ */
+
+#include <grub/types.h>
+#define is_signed_type(type) (((type)(-1)) < (type)1)
+#define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type)))
+#define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
+#define type_min(T) ((T)((T)-type_max(T)-(T)1))
+
+#define __unsigned_add_overflow(a, b, d) ({ \
+ typeof(+(a)) __a = (a); \
+ typeof(+(b)) __b = (b); \
+ typeof(d) __d = (d); \
+ (void) (&__a == &__b); \
+ (void) (&__a == __d); \
+ *__d = __a + __b; \
+ *__d < __a; \
+})
+#define __unsigned_sub_overflow(a, b, d) ({ \
+ typeof(+(a)) __a = (a); \
+ typeof(+(b)) __b = (b); \
+ typeof(d) __d = (d); \
+ (void) (&__a == &__b); \
+ (void) (&__a == __d); \
+ *__d = __a - __b; \
+ __a < __b; \
+})
+#define __unsigned_mul_overflow(a, b, d) ({ \
+ typeof(+(a)) __a = (a); \
+ typeof(+(b)) __b = (b); \
+ typeof(d) __d = (d); \
+ (void) (&__a == &__b); \
+ (void) (&__a == __d); \
+ *__d = __a * __b; \
+ __builtin_constant_p(__b) ? \
+ __b > 0 && __a > type_max(typeof(__a)) / __b :\
+ __a > 0 && __b > type_max(typeof(__b)) / __a; \
+})
+
+#define __signed_add_overflow(a, b, d) ({ \
+ typeof(+(a)) __a = (a); \
+ typeof(+(b)) __b = (b); \
+ typeof(d) __d = (d); \
+ (void) (&__a == &__b); \
+ (void) (&__a == __d); \
+ *__d = (grub_uint64_t)__a + (grub_uint64_t)__b; \
+ (((~(__a ^ __b)) & (*__d ^ __a)) \
+ & type_min(typeof(__a))) != 0; \
+})
+
+#define __signed_sub_overflow(a, b, d) ({ \
+ typeof(+(a)) __a = (a); \
+ typeof(+(b)) __b = (b); \
+ typeof(d) __d = (d); \
+ (void) (&__a == &__b); \
+ (void) (&__a == __d); \
+ *__d = (grub_uint64_t)__a - (grub_uint64_t)__b; \
+ ((((__a ^ __b)) & (*__d ^ __a)) \
+ & type_min(typeof(__a))) != 0; \
+})
+
+#define __signed_mul_overflow(a, b, d) ({ \
+ typeof(+(a)) __a = (a); \
+ typeof(+(b)) __b = (b); \
+ typeof(d) __d = (d); \
+ typeof(+(a)) __tmax = type_max(typeof(+(a))); \
+ typeof(+(a)) __tmin = type_min(typeof(+(a))); \
+ (void) (&__a == &__b); \
+ (void) (&__a == __d); \
+ *__d = (grub_uint64_t)__a * (grub_uint64_t)__b; \
+ (__b > 0 && (__a > __tmax/__b || __a < __tmin/__b)) ||\
+ (__b < (typeof(__b))-1 && \
+ (__a > __tmin/__b || __a < __tmax/__b)) || \
+ (__b == (typeof(__b))-1 && __a == __tmin); \
+})
+
+#define grub_add(a, b, d) \
+ __builtin_choose_expr(is_signed_type(typeof(+(a))), \
+ __signed_add_overflow(a, b, d), \
+ __unsigned_add_overflow(a, b, d))
+
+#define grub_sub(a, b, d) \
+ __builtin_choose_expr(is_signed_type(typeof(+(a))), \
+ __signed_sub_overflow(a, b, d), \
+ __unsigned_sub_overflow(a, b, d))
+
+#define grub_mul(a, b, d) \
+ __builtin_choose_expr(is_signed_type(typeof(+(a))), \
+ __signed_mul_overflow(a, b, d), \
+ __unsigned_mul_overflow(a, b, d))
+
#endif
#endif /* GRUB_SAFEMATH_H */