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       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                 :      22421 : 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                 :      22421 :     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                 :      22421 :     __asm__ __volatile__("" : : "r"(ptr) : "memory");
      34                 :            : #endif
      35                 :      22421 : }