common/mischelp: use platform memory zeroing function for wipememory

* common/mischelp.h (wipememory): Replace macro with function
prototype.
(wipememory2): Remove.
* common/mischelp.c (wipememory): New.
* configure.ac (AC_CHECK_FUNCS): Check for 'explicit_bzero'.
--

In new wipememory function, memory is cleared through platform
provided secure memory zeroing function, SecureZeroMemory
or explicit_bzero.

If none of these is available, memset is called through
volatile function pointer to so that compiler won't optimize
away the call.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>

common/mischelp.c

@@ -49,6 +49,22 @@
 #include "mischelp.h"
 
 
+void
+wipememory (void *ptr, size_t len)
+{
+#if defined(HAVE_W32_SYSTEM) && defined(SecureZeroMemory)
+  SecureZeroMemory (ptr, len);
+#elif defined(HAVE_EXPLICIT_BZERO)
+  explicit_bzero (ptr, len);
+#else
+  /* Prevent compiler from optimizing away the call to memset by accessing
+     memset through volatile pointer. */
+  static void *(*volatile memset_ptr)(void *, int, size_t) = (void *)memset;
+  memset_ptr (ptr, 0, len);
+#endif
+}
+
+
 /* Check whether the files NAME1 and NAME2 are identical.  This is for
    example achieved by comparing the inode numbers of the files.  */
 int

common/mischelp.h

@@ -47,15 +47,9 @@ time_t timegm (struct tm *tm);
 #define DIM(v)		     (sizeof(v)/sizeof((v)[0]))
 #define DIMof(type,member)   DIM(((type *)0)->member)
 
-/* To avoid that a compiler optimizes certain memset calls away, these
-   macros may be used instead. */
-#define wipememory2(_ptr,_set,_len) do { \
-              volatile char *_vptr=(volatile char *)(_ptr); \
-              size_t _vlen=(_len); \
-              while(_vlen) { *_vptr=(_set); _vptr++; _vlen--; } \
-                  } while(0)
-#define wipememory(_ptr,_len) wipememory2(_ptr,0,_len)
-
+/* To avoid that a compiler optimizes certain memset calls away,
+   wipememory function may be used instead.  */
+void wipememory(void *ptr, size_t len);
 
 /* Include hacks which are mainly required for Slowaris.  */
 #ifdef GNUPG_COMMON_NEED_AFLOCAL