safemath: Add some arithmetic primitives that check for overflow

This adds a new header, include/grub/safemath.h, that includes easy to
use wrappers for __builtin_{add,sub,mul}_overflow() declared like:

  bool OP(a, b, res)

where OP is grub_add, grub_sub or grub_mul. OP() returns true in the
case where the operation would overflow and res is not modified.
Otherwise, false is returned and the operation is executed.

These arithmetic primitives require newer compiler versions. So, bump
these requirements in the INSTALL file too.

Signed-off-by: Peter Jones <pjones@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

INSTALL

@@ -11,27 +11,9 @@ GRUB depends on some software packages installed into your system. If
 you don't have any of them, please obtain and install them before
 configuring the GRUB.
 
-* GCC 4.1.3 or later
+* GCC 5.1.0 or later
-  Note: older versions may work but support is limited
+  Experimental support for clang 3.8.0 or later (results in much bigger binaries)
-
-  Experimental support for clang 3.3 or later (results in much bigger binaries)
   for i386, x86_64, arm (including thumb), arm64, mips(el), powerpc, sparc64
-  Note: clang 3.2 or later works for i386 and x86_64 targets but results in
-        much bigger binaries.
-	earlier versions not tested
-  Note: clang 3.2 or later works for arm
-	earlier versions not tested
-  Note: clang on arm64 is not supported due to
-	https://llvm.org/bugs/show_bug.cgi?id=26030
-  Note: clang 3.3 or later works for mips(el)
-	earlier versions fail to generate .reginfo and hence gprel relocations
-	fail.
-  Note: clang 3.2 or later works for powerpc
-	earlier versions not tested
-  Note: clang 3.5 or later works for sparc64
-        earlier versions return "error: unable to interface with target machine"
-  Note: clang has no support for ia64 and hence you can't compile GRUB
-	for ia64 with clang
 * GNU Make
 * GNU Bison 2.3 or later
 * GNU gettext 0.17 or later

include/grub/compiler.h

@@ -48,4 +48,12 @@
 #  define WARN_UNUSED_RESULT
 #endif
 
+#if defined(__clang__) && defined(__clang_major__) && defined(__clang_minor__)
+#  define CLANG_PREREQ(maj,min) \
+          ((__clang_major__ > (maj)) || \
+	   (__clang_major__ == (maj) && __clang_minor__ >= (min)))
+#else
+#  define CLANG_PREREQ(maj,min) 0
+#endif
+
 #endif /* ! GRUB_COMPILER_HEADER */

include/grub/safemath.h

@@ -0,0 +1,37 @@
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2020  Free Software Foundation, Inc.
+ *
+ *  GRUB is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  GRUB is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ *  Arithmetic operations that protect against overflow.
+ */
+
+#ifndef GRUB_SAFEMATH_H
+#define GRUB_SAFEMATH_H 1
+
+#include <grub/compiler.h>
+
+/* These appear in gcc 5.1 and clang 3.8. */
+#if GNUC_PREREQ(5, 1) || CLANG_PREREQ(3, 8)
+
+#define grub_add(a, b, res)	__builtin_add_overflow(a, b, res)
+#define grub_sub(a, b, res)	__builtin_sub_overflow(a, b, res)
+#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
+#endif
+
+#endif /* GRUB_SAFEMATH_H */