Explicit_bzero and bzero

As stated in the manpage:

The bzero() function writes len zero bytes to the string b. If len is zero, bzero() does nothing. The explicit_bzero() variant behaves the same, but will not be removed by a compiler’s dead store optimization pass, making it useful for clearing sensitive memory such as a password.

This is how it’s implemented:

/*	$OpenBSD: explicit_bzero.c,v 1.4 2015/08/31 02:53:57 guenther Exp $ */
/*
 * Public domain.
 * Written by Matthew Dempsky.
 */

#include <string.h>

__attribute__((weak)) void
__explicit_bzero_hook(void *buf, size_t len)
{
}

void
explicit_bzero(void *buf, size_t len)
{
	memset(buf, 0, len);
	__explicit_bzero_hook(buf, len);
}
DEF_WEAK(explicit_bzero);

Unfortunately, it might get optimized away when using static linking with LTO.

A better implementation would be:

void *
explicit_memset(void *buf, size_t len)
{
	memset(buf, 0, len);
	__asm__ __volatile__("" :: "r"(buf) : "memory");
	return s;
}

But it’s sill a neat way of improving forward secrecy, by trying to remove cryptographic materials from memory as soon as possible.