lib/randutils: use getrandom(2) when it is available

System call getrandom(2) is relatively new, available since kernel 3.17 but
not supported by glibc 2.24.  That in mind autotools is made to check
availability of this function and keep old code as fallback.  It is
reasonable assume it will take years before the syscall(2) and fallback are
unproblematic to remove.

One might ask why bother using getrandom(2).  Main reason is to avoid
unnecessary system calls to achieve exactly same end result.  That
demonstrated with 'strace -c ./mcookie' showing 36 calls before, and 32
after this change.  Secondly the getrandom(2) function got to kernel with
promise it can be used to avoid file descriptor run down, and since uuidd
uses random_get_bytes() it should fulfill it's promise here.

Reference: http://man7.org/linux/man-pages/man2/getrandom.2.html
Reference: https://lwn.net/Articles/606141/
Reviewed-by: Karel Zak <kzak@redhat.com>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>

2 files changed, 54 insertions, 16 deletions

diff --git a/configure.ac b/configure.ac
index e0240b322..d301b3da2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -385,6 +385,7 @@ AC_CHECK_FUNCS([ \
 	getdtablesize \
 	getexecname \
 	getmntinfo \
+	getrandom \
 	getrlimit \
 	getsgnam \
 	inotify_init \
diff --git a/lib/randutils.c b/lib/randutils.c
index b93b9ba47..8defe744c 100644
--- a/lib/randutils.c
+++ b/lib/randutils.c
@@ -25,6 +25,25 @@
 #define THREAD_LOCAL static
 #endif
 
+#ifdef HAVE_GETRANDOM
+# include <linux/random.h>
+#elif defined (__linux__)
+# include <sys/syscall.h>
+# if !defined(SYS_getrandom) && defined(__NR_getrandom)
+   /* usable kernel-headers, but old glibc-headers */
+#  define SYS_getrandom __NR_getrandom
+# endif
+#endif
+
+#if !defined(HAVE_GETRANDOM) && defined(SYS_getrandom)
+/* libc without function, but we have syscal */
+static int getrandom(void *buf, size_t buflen, unsigned int flags)
+{
+	return (syscall(SYS_getrandom, buf, buflen, flags));
+}
+# define HAVE_GETRANDOM
+#endif
+
 #if defined(__linux__) && defined(__NR_gettid) && defined(HAVE_JRAND48)
 #define DO_JRAND_MIX
 THREAD_LOCAL unsigned short ul_jrand_seed[3];
@@ -35,20 +54,11 @@ int rand_get_number(int low_n, int high_n)
 	return rand() % (high_n - low_n + 1) + low_n;
 }
 
-int random_get_fd(void)
+static void crank_random(void)
 {
-	int i, fd;
+	int i;
 	struct timeval tv;
 
-	fd = open("/dev/urandom", O_RDONLY | O_CLOEXEC);
-	if (fd == -1)
-		fd = open("/dev/random", O_RDONLY | O_NONBLOCK | O_CLOEXEC);
-	if (fd >= 0) {
-		i = fcntl(fd, F_GETFD);
-		if (i >= 0)
-			fcntl(fd, F_SETFD, i | FD_CLOEXEC);
-	}
-
 	gettimeofday(&tv, 0);
 	srand((getpid() << 16) ^ getuid() ^ tv.tv_sec ^ tv.tv_usec);
 
@@ -61,9 +71,23 @@ int random_get_fd(void)
 	gettimeofday(&tv, 0);
 	for (i = (tv.tv_sec ^ tv.tv_usec) & 0x1F; i > 0; i--)
 		rand();
-	return fd;
 }
 
+int random_get_fd(void)
+{
+	int i, fd;
+
+	fd = open("/dev/urandom", O_RDONLY | O_CLOEXEC);
+	if (fd == -1)
+		fd = open("/dev/random", O_RDONLY | O_NONBLOCK | O_CLOEXEC);
+	if (fd >= 0) {
+		i = fcntl(fd, F_GETFD);
+		if (i >= 0)
+			fcntl(fd, F_SETFD, i | FD_CLOEXEC);
+	}
+	crank_random();
+	return fd;
+}
 
 /*
  * Generate a stream of random nbytes into buf.
@@ -72,10 +96,19 @@ int random_get_fd(void)
  */
 void random_get_bytes(void *buf, size_t nbytes)
 {
-	size_t i, n = nbytes;
+	size_t i;
+	unsigned char *cp = (unsigned char *)buf;
+
+#ifdef HAVE_GETRANDOM
+	while (getrandom(buf, nbytes, 0) < 0) {
+		if (errno == EINTR)
+			continue;
+		break;
+	}
+#else
+	size_t n = nbytes;
 	int fd = random_get_fd();
 	int lose_counter = 0;
-	unsigned char *cp = (unsigned char *) buf;
 
 	if (fd >= 0) {
 		while (n > 0) {
@@ -92,11 +125,12 @@ void random_get_bytes(void *buf, size_t nbytes)
 
 		close(fd);
 	}
-
+#endif
 	/*
 	 * We do this all the time, but this is the only source of
 	 * randomness if /dev/random/urandom is out to lunch.
 	 */
+	crank_random();
 	for (cp = buf, i = 0; i < nbytes; i++)
 		*cp++ ^= (rand() >> 7) & 0xFF;
 
@@ -122,6 +156,9 @@ void random_get_bytes(void *buf, size_t nbytes)
  */
 const char *random_tell_source(void)
 {
+#ifdef HAVE_GETRANDOM
+	return _("getrandom() function");
+#else
 	size_t i;
 	static const char *random_sources[] = {
 		"/dev/urandom",
@@ -132,7 +169,7 @@ const char *random_tell_source(void)
 		if (!access(random_sources[i], R_OK))
 			return random_sources[i];
 	}
-
+#endif
 	return _("libc pseudo-random functions");
 }