lib/xalloc-oversized.h

   1 /* xalloc-oversized.h -- memory allocation size checking
   2
   3    Copyright (C) 1990-2000, 2003-2004, 2006-2016 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  17
  18 #ifndef XALLOC_OVERSIZED_H_
  19 #define XALLOC_OVERSIZED_H_
  20
  21 #include <stddef.h>
  22
  23 /* Default for (non-Clang) compilers that lack __has_builtin.  */
  24 #ifndef __has_builtin
  25 # define __has_builtin(x) 0
  26 #endif
  27
  28 /* True if N * S would overflow in a size calculation.
  29    This expands to a constant expression if N and S are both constants.
  30    By gnulib convention, SIZE_MAX represents overflow in size
  31    calculations, so the conservative dividend to use here is
  32    SIZE_MAX - 1, since SIZE_MAX might represent an overflowed value.
  33    However, malloc (SIZE_MAX) fails on all known hosts where
  34    sizeof (ptrdiff_t) <= sizeof (size_t), so do not bother to test for
  35    exactly-SIZE_MAX allocations on such hosts; this avoids a test and
  36    branch when S is known to be 1.  */
  37 #define __xalloc_oversized(n, s) \
  38     ((size_t) (sizeof (ptrdiff_t) <= sizeof (size_t) ? -1 : -2) / (s) < (n))
  39
  40
  41 /* Return 1 if an array of N objects, each of size S, cannot exist due
  42    to size arithmetic overflow.  S must be positive and N must be
  43    nonnegative.  This is a macro, not a function, so that it
  44    works correctly even when SIZE_MAX < N.  */
  45
  46 /* GCC 7 __builtin_mul_overflow should easily compute this.  See:
  47    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68120  */
  48 #if 7 <= __GNUC__
  49 # define xalloc_oversized(n, s) __builtin_mul_overflow (n, s, (size_t *) NULL)
  50
  51 /* GCC 5 and Clang __builtin_mul_overflow needs a temporary, and
  52    should be used only for non-constant operands, so that
  53    xalloc_oversized is a constant expression if both arguments are.
  54    Do not use this if pedantic, since pedantic GCC issues a diagnostic
  55    for ({ ... }).  */
  56 #elif ((5 <= __GNUC__ \
  57         || (__has_builtin (__builtin_mul_overflow) \
  58             && __has_builtin (__builtin_constant_p))) \
  59        && !__STRICT_ANSI__)
  60 # define xalloc_oversized(n, s) \
  61    (__builtin_constant_p (n) && __builtin_constant_p (s) \
  62     ? __xalloc_oversized (n, s) \
  63     : ({ size_t __xalloc_size; __builtin_mul_overflow (n, s, &__xalloc_size); }))
  64
  65 /* Other compilers use integer division; this may be slower but is
  66    more portable.  */
  67 #else
  68 # define xalloc_oversized(n, s) __xalloc_oversized (n, s)
  69 #endif
  70
  71 #endif /* !XALLOC_OVERSIZED_H_ */