src/support/cleanse.cpp

   1 // Copyright (c) 2009-2010 Satoshi Nakamoto
   2 // Copyright (c) 2009-2015 The Bitcoin Core developers
   3 // Distributed under the MIT software license, see the accompanying
   4 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
   5
   6 #include "cleanse.h"
   7
   8 #include <cstring>
   9
  10 /* Compilers have a bad habit of removing "superfluous" memset calls that
  11  * are trying to zero memory. For example, when memset()ing a buffer and
  12  * then free()ing it, the compiler might decide that the memset is
  13  * unobservable and thus can be removed.
  14  *
  15  * Previously we used OpenSSL which tried to stop this by a) implementing
  16  * memset in assembly on x86 and b) putting the function in its own file
  17  * for other platforms.
  18  *
  19  * This change removes those tricks in favour of using asm directives to
  20  * scare the compiler away. As best as our compiler folks can tell, this is
  21  * sufficient and will continue to be so.
  22  *
  23  * Adam Langley <agl@google.com>
  24  * Commit: ad1907fe73334d6c696c8539646c21b11178f20f
  25  * BoringSSL (LICENSE: ISC)
  26  */
  27 void memory_cleanse(void *ptr, size_t len)
  28 {
  29     std::memset(ptr, 0, len);
  30
  31     /* As best as we can tell, this is sufficient to break any optimisations that
  32        might try to eliminate "superfluous" memsets. If there's an easy way to
  33        detect memset_s, it would be better to use that. */
  34 #if defined(_MSC_VER)
  35     __asm;
  36 #else
  37     __asm__ __volatile__("" : : "r"(ptr) : "memory");
  38 #endif
  39 }