1 files changed, 14 insertions, 22 deletions
diff --git a/src/support/cleanse.cpp b/src/support/cleanse.cpp
index 17a4a4c2b2..ecb00510f7 100644
--- a/src/support/cleanse.cpp
+++ b/src/support/cleanse.cpp
@@ -11,33 +11,25 @@
 #include <Windows.h> // For SecureZeroMemory.
 #endif
 
-/* Compilers have a bad habit of removing "superfluous" memset calls that
- * are trying to zero memory. For example, when memset()ing a buffer and
- * then free()ing it, the compiler might decide that the memset is
- * unobservable and thus can be removed.
- *
- * Previously we used OpenSSL which tried to stop this by a) implementing
- * memset in assembly on x86 and b) putting the function in its own file
- * for other platforms.
- *
- * This change removes those tricks in favour of using asm directives to
- * scare the compiler away. As best as our compiler folks can tell, this is
- * sufficient and will continue to be so.
- *
- * Adam Langley <agl@google.com>
- * Commit: ad1907fe73334d6c696c8539646c21b11178f20f
- * BoringSSL (LICENSE: ISC)
- */
 void memory_cleanse(void *ptr, size_t len)
 {
-    std::memset(ptr, 0, len);
-
-    /* As best as we can tell, this is sufficient to break any optimisations that
-       might try to eliminate "superfluous" memsets. If there's an easy way to
-       detect memset_s, it would be better to use that. */
 #if defined(_MSC_VER)
+    /* SecureZeroMemory is guaranteed not to be optimized out by MSVC. */
     SecureZeroMemory(ptr, len);
 #else
+    std::memset(ptr, 0, len);
+
+    /* Memory barrier that scares the compiler away from optimizing out the memset.
+     *
+     * Quoting Adam Langley <agl@google.com> in commit ad1907fe73334d6c696c8539646c21b11178f20f
+     * in BoringSSL (ISC License):
+     *    As best as we can tell, this is sufficient to break any optimisations that
+     *    might try to eliminate "superfluous" memsets.
+     * This method is used in memzero_explicit() the Linux kernel, too. Its advantage is that it
+     * is pretty efficient because the compiler can still implement the memset() efficiently,
+     * just not remove it entirely. See "Dead Store Elimination (Still) Considered Harmful" by
+     * Yang et al. (USENIX Security 2017) for more background.
+     */
     __asm__ __volatile__("" : : "r"(ptr) : "memory");
 #endif
 }