util/nvmutil: rand: use getrandom on newer linux

we still fall back to the old /dev/urandom read
on older linux, via runtime detection (ENOSYS).

getrandom is better, because it guarantees entropy
via blocking, and works even when /dev/urandom
is unavailable.

it has the same practical benefit as arc4random,
which i use on bsd. linux can have arc4random,
but not every linux libc has it, so it's better
to use getrandom on linux.

older linux will fall back to /dev/urandom

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/nvmutil/include/common.h

@@ -9,6 +9,25 @@
 #include <sys/stat.h>
 #include <limits.h>
 
+/* for linux getrandom
+ */
+#if defined(__linux__)
+#include <errno.h>
+#if defined(__has_include)
+#if __has_include(<sys/random.h>)
+#include <sys/random.h>
+#define HAVE_GETRANDOM 1
+#endif
+#endif
+#if !defined(HAVE_GETRANDOM)
+#include <sys/syscall.h>
+#if defined(SYS_getrandom)
+#define HAVE_GETRANDOM_SYSCALL 1
+#endif
+#endif
+
+#endif
+
 #define items(x) (sizeof((x)) / sizeof((x)[0]))
 
 /* system prototypes
@@ -318,6 +337,12 @@ void set_mac_nib(unsigned long mac_str_pos,
     unsigned long mac_byte_pos, unsigned long mac_nib_pos);
 unsigned short hextonum(char ch_s);
 unsigned long rlong(void);
+#if defined(__linux__)
+#if defined(HAVE_GETRANDOM) || \
+    defined(HAVE_GETRANDOM_SYSCALL)
+int fallback_rand_getrandom(void *buf, size_t len);
+#endif
+#endif
 void write_mac_part(unsigned long partnum);
 
 /* Helper functions for command: dump

util/nvmutil/lib/num.c

@@ -75,6 +75,37 @@ rlong(void)
 	unsigned long rval;
 	long new_nr;
 
+	int retries = 0;
+	int max_retries = 100;
+
+#if defined(__linux__)
+#if defined(HAVE_GETRANDOM) || \
+    defined(HAVE_GETRANDOM_SYSCALL)
+
+	/* linux getrandom()
+	 *
+	 * we *can* use arc4random on
+	 * modern linux, but not on
+	 * every libc. better use the
+	 * official linux function
+	 *
+	 * similar benefits to arc4random
+	 * e.g. works in chroot, blocks
+	 * until it has enough entropy,
+	 * and works even when /dev/urandom
+	 * is available (doesn't use it);
+	 * it's generally more reliable
+	 */
+
+	if (fallback_rand_getrandom(&rval, sizeof(rval)) == 0)
+		return rval;
+
+	/*
+	 * now fall back to urandom if getrandom failed:
+	 */
+#endif
+#endif
+
 	/* reading from urandom is inherently
 	 * unreliable on old systems, even if
 	 * newer systems make it more reliable
@@ -87,9 +118,6 @@ rlong(void)
 	 * tricks to mitigate possible os bugs
 	 */
 
-	int retries = 0;
-	int max_retries = 100;
-
 retry_urandom_read:
 
 	if (++retries > max_retries)
@@ -104,6 +132,10 @@ retry_urandom_read:
 			    O_CLOEXEC);
 
 #ifdef __OpenBSD__
+/* TODO: dead code
+	openbsd 2.1 had arc4random.
+	arandam was introduced **later**
+ */
 			if (fd < 0) /* old openbsd */
 				fd = open("/dev/arandom",
 				    O_RDONLY | O_BINARY | O_NOFOLLOW |
@@ -178,6 +210,51 @@ rlong_next:
 #endif
 }
 
+#if defined(__linux__)
+#if defined(HAVE_GETRANDOM) || \
+    defined(HAVE_GETRANDOM_SYSCALL)
+int
+fallback_rand_getrandom(void *buf, size_t len)
+{
+	ssize_t rval = -1;
+
+	/* keep strict compiler
+	 * happy if unused
+	 */
+	(void)rval;
+	(void)buf;
+	(void)len;
+
+#if defined(HAVE_GETRANDOM)
+	do {
+		rval = getrandom(buf, len, 0);
+	} while (rval < 0 && errno == EINTR);
+
+#elif defined(HAVE_GETRANDOM_SYSCALL)
+	do {
+		rval = syscall(SYS_getrandom, buf, len, 0);
+	} while (ret < 0 && errno == EINTR);
+
+#else
+	return -1;
+#endif
+
+#if defined(HAVE_GETRANDOM) || \
+    defined(HAVE_GETRANDOM_SYSCALL)
+
+	if (rval == (ssize_t)len) {
+		return 0;
+	}
+
+	if (rval < 0 && errno == ENOSYS)
+		return -1; /* not supported by kernel */
+
+	return -1;
+#endif
+}
+#endif
+#endif
+
 void
 check_bin(unsigned long a, const char *a_name)
 {