libc: implement getentropy function

specify here:
https://man7.org/linux/man-pages/man3/getentropy.3.html

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

include/unistd.h

@@ -410,6 +410,8 @@ gid_t   getegid(void);
 int     setreuid(uid_t ruid, uid_t euid);
 int     setregid(gid_t rgid, gid_t egid);
 
+int     getentropy(FAR void *buffer, size_t length);
+
 #undef EXTERN
 #if defined(__cplusplus)
 }

libs/libc/unistd/Make.defs

@@ -21,7 +21,7 @@
 # Add the unistd C files to the build
 
 CSRCS += lib_access.c lib_daemon.c lib_swab.c lib_pathconf.c lib_sysconf.c
-CSRCS += lib_getopt_common.c lib_getopt.c lib_getopt_long.c
+CSRCS += lib_getentropy.c lib_getopt_common.c lib_getopt.c lib_getopt_long.c
 CSRCS += lib_getopt_longonly.c lib_getoptvars.c lib_getoptargp.c
 CSRCS += lib_getopterrp.c lib_getoptindp.c lib_getoptoptp.c lib_times.c
 CSRCS += lib_alarm.c lib_fstatvfs.c lib_statvfs.c lib_sleep.c lib_nice.c

libs/libc/unistd/lib_getentropy.c

@@ -0,0 +1,83 @@
+/****************************************************************************
+ * libs/libc/unistd/lib_getentropy.c
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.  The
+ * ASF licenses this file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the
+ * License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ ****************************************************************************/
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <sys/random.h>
+#include <errno.h>
+#include <unistd.h>
+
+/****************************************************************************
+ * Public Functions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Name: getentropy
+ *
+ * Description:
+ *   The getentropy() function writes length bytes of high-quality
+ *   random data to the buffer starting at the location pointed to by
+ *   buffer. The maximum permitted value for the length argument is
+ *   256.
+ *
+ *   A successful call to getentropy() always provides the requested
+ *   number of bytes of entropy.
+ *
+ * Input Parameters:
+ *   buffer - Buffer for returned random bytes
+ *   length - Number of bytes requested.
+ *
+ * Returned Value:
+ *   On success, this function returns zero.  On error, -1 is
+ *   returned, and errno is set to indicate the error.
+ *
+ ****************************************************************************/
+
+int getentropy(FAR void *buffer, size_t length)
+{
+  FAR char *pos = buffer;
+
+  if (length > 256)
+    {
+      errno = EIO;
+      return -1;
+    }
+
+  while (length > 0)
+    {
+      int ret = getrandom(pos, length, 0);
+      if (ret < 0)
+        {
+          if (errno == EINTR)
+            {
+              continue;
+            }
+
+          return ret;
+        }
+
+      pos += ret;
+      length -= ret;
+    }
+
+  return 0;
+}