Add evutil_secure_rng_set_urandom_device_file

This experimental function is needed for some seccomp2 hackery to
work, and should have no effect for systems that don't use it.

arc4random.c

@@ -293,6 +293,27 @@ arc4_seed_proc_sys_kernel_random_uuid(void)
 
 #ifndef WIN32
 #define TRY_SEED_URANDOM
+static char *arc4random_urandom_filename = NULL;
+
+static int arc4_seed_urandom_helper_(const char *fname)
+{
+	unsigned char buf[ADD_ENTROPY];
+	int fd;
+	size_t n;
+
+	fd = evutil_open_closeonexec(fname, O_RDONLY, 0);
+	if (fd<0)
+		return -1;
+	n = read_all(fd, buf, sizeof(buf));
+	close(fd);
+	if (n != sizeof(buf))
+		return -1;
+	arc4_addrandom(buf, sizeof(buf));
+	memset(buf, 0, sizeof(buf));
+	arc4_seeded_ok = 1;
+	return 0;
+}
+
 static int
 arc4_seed_urandom(void)
 {
@@ -300,22 +321,13 @@ arc4_seed_urandom(void)
 	static const char *filenames[] = {
 		"/dev/srandom", "/dev/urandom", "/dev/random", NULL
 	};
-	unsigned char buf[ADD_ENTROPY];
-	int fd, i;
-	size_t n;
+	int i;
+	if (arc4random_urandom_filename)
+		return arc4_seed_urandom_helper_(arc4random_urandom_filename);
 
 	for (i = 0; filenames[i]; ++i) {
-		fd = evutil_open_closeonexec(filenames[i], O_RDONLY, 0);
-		if (fd<0)
-			continue;
-		n = read_all(fd, buf, sizeof(buf));
-		close(fd);
-		if (n != sizeof(buf))
-			return -1;
-		arc4_addrandom(buf, sizeof(buf));
-		memset(buf, 0, sizeof(buf));
-		arc4_seeded_ok = 1;
-		return 0;
+		if (arc4_seed_urandom_helper_(filenames[i]) == 0)
+			return 0;
 	}
 
 	return -1;

evutil_rand.c

@@ -43,6 +43,12 @@
 #include <stdlib.h>
 #include <string.h>
 int
+evutil_secure_rng_set_urandom_device_file(char *fname)
+{
+	(void) fname;
+	return -1;
+}
+int
 evutil_secure_rng_init(void)
 {
 	/* call arc4random() now to force it to self-initialize */
@@ -123,6 +129,17 @@ evutil_secure_rng_global_setup_locks_(const int enable_locks)
 }
 #endif
 
+int
+evutil_secure_rng_set_urandom_device_file(char *fname)
+{
+#ifdef TRY_SEED_URANDOM
+	_ARC4_LOCK();
+	arc4random_urandom_filename = fname;
+	_ARC4_UNLOCK();
+#endif
+	return 0;
+}
+
 int
 evutil_secure_rng_init(void)
 {

include/event2/util.h

@@ -675,6 +675,20 @@ void evutil_secure_rng_get_bytes(void *buf, size_t n);
  */
 int evutil_secure_rng_init(void);
 
+/**
+ * Set a filename to use in place of /dev/urandom for seeding the secure
+ * PRNG. Return 0 on success, -1 on failure.
+ *
+ * Call this function BEFORE calling any other initialization or .
+ *
+ * (This string will _NOT_ be copied internally. Do not free it while any
+ * user of the secure RNG might be running. Don't pass anything other than a
+ * real /dev/...random device file here, or you might lose security.)
+ *
+ * This API is unstable, and might change in a future libevent version.
+ */
+int evutil_secure_rng_set_urandom_device_file(char *fname);
+
 /** Seed the random number generator with extra random bytes.
 
     You should almost never need to call this function; it should be