1  // Copyright (c) 2009-2010 Satoshi Nakamoto
 2  // Copyright (c) 2009-2019 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 <support/cleanse.h>
 7  
 8  #include <cstring>
 9  
10  #if defined(_MSC_VER)
11  #include <Windows.h> // For SecureZeroMemory.
12  #endif
13  
14  void memory_cleanse(void *ptr, size_t len)
15  {
16  #if defined(_MSC_VER)
17      /* SecureZeroMemory is guaranteed not to be optimized out by MSVC. */
18      SecureZeroMemory(ptr, len);
19  #else
20      std::memset(ptr, 0, len);
21  
22      /* Memory barrier that scares the compiler away from optimizing out the memset.
23       *
24       * Quoting Adam Langley <agl@google.com> in commit ad1907fe73334d6c696c8539646c21b11178f20f
25       * in BoringSSL (ISC License):
26       *    As best as we can tell, this is sufficient to break any optimisations that
27       *    might try to eliminate "superfluous" memsets.
28       * This method is used in memzero_explicit() the Linux kernel, too. Its advantage is that it
29       * is pretty efficient because the compiler can still implement the memset() efficiently,
30       * just not remove it entirely. See "Dead Store Elimination (Still) Considered Harmful" by
31       * Yang et al. (USENIX Security 2017) for more background.
32       */
33      __asm__ __volatile__("" : : "r"(ptr) : "memory");
34  #endif
35  }