Code to get random bytes in C, cross-platform

kitty/cross-platform-random.h

@@ -0,0 +1,35 @@
+/*
+ * Copyright (C) 2020 Kovid Goyal <kovid at kovidgoyal.net>
+ *
+ * Distributed under terms of the GPL3 license.
+ */
+
+#pragma once
+#include <stdlib.h>
+#include <stdbool.h>
+
+#if __linux__
+#include <sys/random.h>
+#include <errno.h>
+
+static inline bool
+secure_random_bytes(void *buf, size_t nbytes) {
+    unsigned char* p = buf;
+    ssize_t left = nbytes;
+    while(1) {
+        ssize_t n = getrandom(p, left, 0);
+        if (n >= left) return true;
+        if (n < 0) {
+            if (errno != EINTR) return false;  // should never happen but if it does, we fail without any feedback
+            continue;
+        }
+        left -= n; p += n;
+    }
+}
+#else
+static inline bool
+secure_random_bytes(void *buf, size_t nbytes) {
+    arc4random_buf(buf, nbytes);
+    return true;
+}
+#endif

kitty/disk-cache.c

@@ -10,6 +10,7 @@
 #include "disk-cache.h"
 #include "uthash.h"
 #include "loop-utils.h"
+#include "cross-platform-random.h"
 #include <stdlib.h>
 #include <sys/stat.h>
 #include <fcntl.h>
@@ -186,6 +187,7 @@ add_to_disk_cache(PyObject *self_, const void *key, size_t key_sz, const uint8_t
     if (s == NULL) {
         s = calloc(1, sizeof(CacheEntry));
         if (!s) { PyErr_NoMemory(); goto end; }
+        if (!secure_random_bytes(s->encryption_key, sizeof(s->encryption_key))) { free(s); PyErr_SetFromErrno(PyExc_OSError); goto end; }
         s->hash_key = malloc(key_sz);
         if (!s->hash_key) { free(s); PyErr_NoMemory(); goto end; }
         s->hash_keylen = key_sz;