cmake: Check for HAVE_GCC_VOLATILE_MEMORY_PROTECTION.

This ensures that the memset call is not optimized out by the compiler
(works works with gcc and clang).

ConfigureChecks.cmake

@@ -181,6 +181,18 @@ int main(void) {
     return 0;
 }" HAVE_MSC_THREAD_LOCAL_STORAGE)
 
+check_c_source_compiles("
+#include <string.h>
+
+int main(void)
+{
+    char buf[] = \"This is some content\";
+
+    memset(buf, '\\\\0', sizeof(buf)); __asm__ volatile(\"\" : : \"r\"(&buf) : \"memory\");
+
+    return 0;
+}" HAVE_GCC_VOLATILE_MEMORY_PROTECTION)
+
 if (WITH_DEBUG_CRYPTO)
   set(DEBUG_CRYPTO 1)
 endif (WITH_DEBUG_CRYPTO)

config.h.cmake

@@ -126,6 +126,8 @@
 #cmakedefine HAVE_GCC_THREAD_LOCAL_STORAGE 1
 #cmakedefine HAVE_MSC_THREAD_LOCAL_STORAGE 1
 
+#cmakedefine HAVE_GCC_VOLATILE_MEMORY_PROTECTION 1
+
 /* Define to 1 if you want to enable GSSAPI */
 #cmakedefine WITH_GSSAPI 1
 

include/libssh/priv.h

@@ -137,6 +137,17 @@ int gettimeofday(struct timeval *__p, void *__t);
 # define LIBSSH_THREAD
 #endif
 
+/*
+ * This makes sure that the compiler doesn't optimize out the code
+ *
+ * Use it in a macro where the provided variable is 'x'.
+ */
+#if defined(HAVE_GCC_VOLATILE_MEMORY_PROTECTION)
+# define LIBSSH_MEM_PROTECTION __asm__ volatile("" : : "r"(&(x)) : "memory")
+#else
+# define LIBSSH_MEM_PROTECTION
+#endif
+
 #ifdef HAVE_SYS_TIME_H
 #include <sys/time.h>
 #endif
@@ -226,11 +237,33 @@ int match_hostname(const char *host, const char *pattern, unsigned int len);
 /** Get the size of an array */
 #define ARRAY_SIZE(a) (sizeof(a)/sizeof(a[0]))
 
+/*
+ * See http://llvm.org/bugs/show_bug.cgi?id=15495
+ */
+#if defined(HAVE_GCC_VOLATILE_MEMORY_PROTECTION)
 /** Overwrite a string with '\0' */
-#define BURN_STRING(x) do { if ((x) != NULL) memset((x), '\0', strlen((x))); __asm__ volatile ("" : : : "memory"); } while(0)
+# define BURN_STRING(x) do { \
+    if ((x) != NULL) \
+        memset((x), '\0', strlen((x))); __asm__ volatile("" : : "r"(&(x)) : "memory"); \
+  } while(0)
 
 /** Overwrite the buffer with '\0' */
-#define BURN_BUFFER(x, size) do { if ((x) != NULL) memset((x), '\0', (size))); __asm__ volatile ("") : : : "memory"; } while(0)
+# define BURN_BUFFER(x, size) do { \
+    if ((x) != NULL) \
+        memset((x), '\0', (size))); __asm__ volatile("" : : "r"(&(x)) : "memory"); \
+  } while(0)
+#else /* HAVE_GCC_VOLATILE_MEMORY_PROTECTION */
+/** Overwrite a string with '\0' */
+# define BURN_STRING(x) do { \
+    if ((x) != NULL) memset((x), '\0', strlen((x))); \
+  } while(0)
+
+/** Overwrite the buffer with '\0' */
+# define BURN_BUFFER(x, size) do { \
+    if ((x) != NULL) \
+        memset((x), '\0', (size))); __asm__ volatile("" : : "r"(&(x)) : "memory"); \
+  } while(0)
+#endif /* HAVE_GCC_VOLATILE_MEMORY_PROTECTION */
 
 /**
  * This is a hack to fix warnings. The idea is to use this everywhere that we