wg: genkey: account for short reads of /dev/urandom

Apparently Haiku has a misbehaving /dev/urandom.

While we're at it, simplify the function signature to completely succeed
or completely fail and make sure the caller checks the result.

Reported-by: Alexander von Gluck IV <kallisti5@unixzen.com>
Nitpicked-by: Aaron Jones <aaronmdjones@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

contrib/embeddable-wg-library/wireguard.c

@@ -1751,16 +1751,21 @@ void wg_generate_private_key(wg_key private_key)
 void wg_generate_preshared_key(wg_key preshared_key)
 {
 	ssize_t ret;
+	size_t i;
 	int fd;
-
+#if defined(__OpenBSD__) || (defined(__APPLE__) && MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_X_VERSION_10_12) || (defined(__GLIBC__) && (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 25)))
-#if defined(__NR_getrandom)
+	if (!getentropy(preshared_key, sizeof(wg_key)))
-	ret = syscall(__NR_getrandom, preshared_key, sizeof(wg_key), 0);
+		return;
-	if (ret == sizeof(wg_key))
+#endif
+#if defined(__NR_getrandom) && defined(__linux__)
+	if (syscall(__NR_getrandom, preshared_key, sizeof(wg_key), 0) == sizeof(wg_key))
 		return;
 #endif
 	fd = open("/dev/urandom", O_RDONLY);
 	assert(fd >= 0);
-	ret = read(fd, preshared_key, sizeof(wg_key));
+	for (i = 0; i < sizeof(wg_key); i += ret) {
+		ret = read(fd, preshared_key + i, sizeof(wg_key) - i);
+		assert(ret > 0);
+	}
 	close(fd);
-	assert(ret == sizeof(wg_key));
 }

src/genkey.c

@@ -5,6 +5,7 @@
 
 #include <errno.h>
 #include <stdio.h>
+#include <stdbool.h>
 #include <unistd.h>
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -27,29 +28,40 @@
 #include "encoding.h"
 #include "subcommands.h"
 
-static inline ssize_t get_random_bytes(uint8_t *out, size_t len)
+static inline bool __attribute__((__warn_unused_result__)) get_random_bytes(uint8_t *out, size_t len)
 {
-	ssize_t ret;
+	ssize_t ret = 0;
+	size_t i;
 	int fd;
 
+	if (len > 256) {
+		errno = EOVERFLOW;
+		return false;
+	}
+
 #if defined(__OpenBSD__) || (defined(__APPLE__) && MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_X_VERSION_10_12) || (defined(__GLIBC__) && (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 25)))
-	ret = getentropy(out, len);
+	if (!getentropy(out, len))
-	if (!ret)
+		return true;
-		return len;
 #endif
 
 #if defined(__NR_getrandom) && defined(__linux__)
-	ret = syscall(__NR_getrandom, out, len, 0);
+	if (syscall(__NR_getrandom, out, len, 0) == (ssize_t)len)
-	if (ret >= 0)
+		return true;
-		return ret;
 #endif
 
 	fd = open("/dev/urandom", O_RDONLY);
 	if (fd < 0)
-		return fd;
+		return false;
-	ret = read(fd, out, len);
+	for (errno = 0, i = 0; i < len; i += ret, ret = 0) {
+		ret = read(fd, out + i, len - i);
+		if (ret <= 0) {
+			ret = errno ? -errno : -EIO;
+			break;
+		}
+	}
 	close(fd);
-	return ret;
+	errno = -ret;
+	return i == len;
 }
 
 int genkey_main(int argc, char *argv[])
@@ -66,7 +78,7 @@ int genkey_main(int argc, char *argv[])
 	if (!fstat(STDOUT_FILENO, &stat) && S_ISREG(stat.st_mode) && stat.st_mode & S_IRWXO)
 		fputs("Warning: writing to world accessible file.\nConsider setting the umask to 077 and trying again.\n", stderr);
 
-	if (get_random_bytes(key, WG_KEY_LEN) != WG_KEY_LEN) {
+	if (!get_random_bytes(key, WG_KEY_LEN)) {
 		perror("getrandom");
 		return 1;
 	}