feat(newlib): Implement getentropy() function

Closes https://github.com/espressif/esp-idf/issues/11963

components/cxx/CMakeLists.txt

@@ -53,15 +53,18 @@ target_link_libraries(${COMPONENT_LIB} INTERFACE "-u __cxa_guard_dummy")
 # Furthermore, force libcxx to appear later than libgcc because some libgcc unwind code is wrapped, if C++
 # exceptions are disabled. libcxx (this component) provides the unwind code wrappers.
 # This is to prevent linking of libgcc's unwind code which considerably increases the binary size.
+# Also force libnewlib to appear later than libstdc++ in link line since libstdc++ depends on
+# some functions in libnewlib, e.g. getentropy().
 idf_component_get_property(pthread pthread COMPONENT_LIB)
+idf_component_get_property(newlib newlib COMPONENT_LIB)
 idf_component_get_property(cxx cxx COMPONENT_LIB)
-add_library(stdcpp_pthread INTERFACE)
+add_library(stdcpp_deps INTERFACE)
 if(CMAKE_C_COMPILER_ID MATCHES "Clang")
-    target_link_libraries(stdcpp_pthread INTERFACE stdc++ c $<TARGET_FILE:${pthread}>)
+    target_link_libraries(stdcpp_deps INTERFACE stdc++ c $<TARGET_FILE:${pthread}> $<TARGET_FILE:${newlib}>)
 else()
-    target_link_libraries(stdcpp_pthread INTERFACE stdc++ $<TARGET_FILE:${pthread}>)
+    target_link_libraries(stdcpp_deps INTERFACE stdc++ $<TARGET_FILE:${pthread}> $<TARGET_FILE:${newlib}>)
 endif()
-target_link_libraries(${COMPONENT_LIB} PUBLIC stdcpp_pthread)
+target_link_libraries(${COMPONENT_LIB} PUBLIC stdcpp_deps)
 add_library(libgcc_cxx INTERFACE)
 target_link_libraries(libgcc_cxx INTERFACE ${CONFIG_COMPILER_RT_LIB_NAME} $<TARGET_FILE:${cxx}>)
 target_link_libraries(${COMPONENT_LIB} PUBLIC libgcc_cxx)

components/newlib/CMakeLists.txt

@@ -12,6 +12,7 @@ set(srcs
     "poll.c"
     "pthread.c"
     "random.c"
+    "getentropy.c"
     "reent_init.c"
     "newlib_init.c"
     "syscalls.c"

components/newlib/getentropy.c

@@ -0,0 +1,30 @@
+/*
+ * SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <sys/random.h>
+#include <errno.h>
+
+int getentropy(void *buffer, size_t length)
+{
+    ssize_t ret;
+
+    if (buffer == NULL) {
+        errno = EFAULT;
+        return -1;
+    }
+
+    if (length > 256) {
+        errno = EIO;
+        return -1;
+    }
+
+    ret = getrandom(buffer, length, 0);
+    if (ret == -1) {
+        return -1;
+    }
+
+    return 0;
+}

components/newlib/platform_include/sys/unistd.h

@@ -16,6 +16,7 @@ extern "C" {
 
 int truncate(const char *, off_t __length);
 int gethostname(char *__name, size_t __len);
+int getentropy(void *buffer, size_t length);
 
 #ifdef __cplusplus
 }

docs/en/api-reference/system/random.rst

@@ -62,5 +62,24 @@ The ``flags`` argument is ignored, this function is always non-blocking but the
 
 Return value is -1 (with ``errno`` set to ``EFAULT``) if the ``buf`` argument is NULL, and equal to ``buflen`` otherwise.
 
+getentropy
+----------
+
+A compatible version of the Linux ``getentropy()`` function is also provided for ease of porting:
+
+.. code-block:: c
+
+   #include <unistd.h>
+
+   int getentropy(void *buffer, size_t length);
+
+This function is implemented by calling :cpp:func:`getrandom` internally.
+
+Strength of any random numbers is dependent on the same conditions described above.
+
+Return value is 0 on success and -1 otherwise with ``errno`` set to:
+   - ``EFAULT`` if the ``buffer`` argument is NULL.
+   - ``EIO`` if the ``length`` is more then 256.
+
 .. _Dieharder: https://webhome.phy.duke.edu/~rgb/General/dieharder.php