Why a normal memset is not always enough

1// Copyright (c) 2009-2010 Satoshi Nakamoto
2// Copyright (c) 2009-present 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(WIN32)
11#include <windows.h>
12#endif
13
14void memory_cleanse(void *ptr, size_t len)
15{
16#if defined(WIN32)
17    /* SecureZeroMemory is guaranteed not to be optimized out. */
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 <[email protected]> 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}
36