[PATCH] [RFC] initial getrandom wrapper to provide getentropy for LibreSSL

[Brent Cook <busterb@gmail.com>]

From: Brent Cook <bcook@openbsd.org>

This is not a kernel patch, but rather an initial test of the API to see
how it might mesh LibreSSL's expectations for how getentropy works.

It is a bit more code to carefully handle the extra return values, as
not reading enough bytes, because there is an unhandled EINTR, might
lead to an unseeded CSPRNG.

The syscall may return EAGAIN depending on the version of getrandom(2)
(this will go away later), but this should give a good example of what
its use would look like in practice.
---
 src/lib/libcrypto/crypto/getentropy_linux.c | 42 ++++++++++++++++++++++++++++-
 1 file changed, 41 insertions(+), 1 deletion(-)

diff --git a/src/lib/libcrypto/crypto/getentropy_linux.c b/src/lib/libcrypto/crypto/getentropy_linux.c
index c16b289..b717d91 100644
--- a/src/lib/libcrypto/crypto/getentropy_linux.c
+++ b/src/lib/libcrypto/crypto/getentropy_linux.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: getentropy_linux.c,v 1.24 2014/07/13 13:37:38 deraadt Exp $	*/
+/*	$OpenBSD: getentropy_linux.c,v 1.25 2014/07/16 14:26:47 kettenis Exp $	*/
 
 /*
  * Copyright (c) 2014 Theo de Raadt <deraadt@openbsd.org>
@@ -73,10 +73,21 @@
 
 int	getentropy(void *buf, size_t len);
 
+#ifndef SYS__getrandom
+#ifdef __LP64__
+#define SYS__getrandom 317
+#else
+#define SYS__getrandom 354
+#endif
+#endif
+
 #if 0
 extern int main(int, char *argv[]);
 #endif
 static int gotdata(char *buf, size_t len);
+#ifdef SYS__getrandom
+static int getentropy_getrandom(void *buf, size_t len);
+#endif
 static int getentropy_urandom(void *buf, size_t len);
 #ifdef CTL_MAXNAME
 static int getentropy_sysctl(void *buf, size_t len);
@@ -95,6 +106,13 @@ getentropy(void *buf, size_t len)
 	}
 
 	/*
+	 * Brand new system call in Linux.  Interface not yet settled.
+	 */
+	ret = getentropy_getrandom(buf, len);
+	if (ret != -1)
+		return (ret);
+
+	/*
 	 * Try to get entropy with /dev/urandom
 	 *
 	 * This can fail if the process is inside a chroot or if file
@@ -180,6 +198,28 @@ gotdata(char *buf, size_t len)
 }
 
 static int
+getentropy_getrandom(void *buf, size_t len)
+{
+	size_t i = 0;
+
+#ifdef SYS__getrandom
+	ssize_t ret;
+
+	for (i = 0; i < len; ) {
+		size_t wanted = len - i;
+		ret = syscall(SYS__getrandom, (char *)buf + i, wanted, 0);
+		if (ret == -1) {
+			if (errno == EAGAIN || errno == EINTR)
+				continue;
+			return (-1);
+		}
+		i += ret;
+	}
+#endif
+	return (i == len ? 0 : -1);
+}
+
+static int
 getentropy_urandom(void *buf, size_t len)
 {
 	struct stat st;
-- 
2.0.1