tools: 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>

1 files changed, 24 insertions, 12 deletions

diff --git a/src/tools/genkey.c b/src/tools/genkey.c
index 645f614..21d2f7a 100644
--- a/src/tools/genkey.c
+++ b/src/tools/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 (!ret)
-		return len;
+	if (!getentropy(out, len))
+		return true;
 #endif
 
 #if defined(__NR_getrandom) && defined(__linux__)
-	ret = syscall(__NR_getrandom, out, len, 0);
-	if (ret >= 0)
-		return ret;
+	if (syscall(__NR_getrandom, out, len, 0) == (ssize_t)len)
+		return true;
 #endif
 
 	fd = open("/dev/urandom", O_RDONLY);
 	if (fd < 0)
-		return fd;
-	ret = read(fd, out, len);
+		return false;
+	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;
 	}