sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h

   1 /* Optional code to distinguish library flavours.  x86-64 version.
   2    Copyright (C) 2015-2016 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library 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 GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 #ifndef _DL_LIBRECON_H
  20
  21 #include <sysdeps/unix/sysv/linux/dl-librecon.h>
  22
  23 /* Recognizing extra environment variables.  For 64-bit applications,
  24    branch prediction performance may be negatively impacted when the
  25    target of a branch is more than 4GB away from the branch.  Add the
  26    Prefer_MAP_32BIT_EXEC bit so that mmap will try to map executable
  27    pages with MAP_32BIT first.  NB: MAP_32BIT will map to lower 2GB,
  28    not lower 4GB, address.  Prefer_MAP_32BIT_EXEC reduces bits available
  29    for address space layout randomization (ASLR).  Prefer_MAP_32BIT_EXEC
  30    is always disabled for SUID programs and can be enabled by setting
  31    environment variable, LD_PREFER_MAP_32BIT_EXEC.  */
  32 #define EXTRA_LD_ENVVARS \
  33   case 21:                                                                \
  34     if (memcmp (envline, "PREFER_MAP_32BIT_EXEC", 21) == 0)               \
  35       GLRO(dl_x86_cpu_features).feature[index_arch_Prefer_MAP_32BIT_EXEC] \
  36         |= bit_arch_Prefer_MAP_32BIT_EXEC;                                \
  37     break;
  38
  39 /* Extra unsecure variables.  The names are all stuffed in a single
  40    string which means they have to be terminated with a '\0' explicitly.  */
  41 #define EXTRA_UNSECURE_ENVVARS \
  42   "LD_PREFER_MAP_32BIT_EXEC\0"
  43
  44 #endif /* dl-librecon.h */