getentropy: Add cpu counter based implementation.

Update #3239.

1 files changed, 54 insertions, 0 deletions

diff --git a/c/src/lib/libbsp/shared/getentropy-cpucounter.c b/c/src/lib/libbsp/shared/getentropy-cpucounter.c
new file mode 100644
index 0000000000..15c8648047
--- /dev/null
+++ b/c/src/lib/libbsp/shared/getentropy-cpucounter.c
@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2017 embedded brains GmbH.  All rights reserved.
+ *
+ *  embedded brains GmbH
+ *  Dornierstr. 4
+ *  82178 Puchheim
+ *  Germany
+ *  <rtems@embedded-brains.de>
+ *
+ * The license and distribution terms for this file may be
+ * found in the file LICENSE in this distribution or at
+ * http://www.rtems.org/license/LICENSE.
+ */
+
+/*
+ * ATTENTION: THIS IS A VERY LIMITED ENTROPY SOURCE.
+ *
+ * This implementation uses a time-based value for it's entropy. The only thing
+ * that makes it random are interrupts from external sources. Don't use it if
+ * you need for example a strong crypto.
+ */
+
+#include <unistd.h>
+#include <string.h>
+#include <rtems/sysinit.h>
+#include <rtems/counter.h>
+
+int getentropy(void *ptr, size_t n)
+{
+	uint8_t *dest = ptr;
+
+	while (n > 0) {
+		rtems_counter_ticks ticks;
+
+		ticks = rtems_counter_read();
+
+		if (n >= sizeof(ticks)) {
+			memcpy(dest, &ticks, sizeof(ticks));
+			n -= sizeof(ticks);
+			dest += sizeof(ticks);
+		} else {
+			/*
+			 * Fill the remaining bytes with only the least
+			 * significant byte of the time. That is the byte with
+			 * the most changes.
+			 */
+			*dest = ticks & 0xFF;
+			--n;
+			++dest;
+		}
+	}
+
+	return 0;
+}