posix: Introduce getentropy() function

The function writes random data to the provided buffer. Maximum
permitted buffer size is 256 bytes. On success the function returns 0,
otherwise it returns -1 and sets errno to appropriate value.

Signed-off-by: Patryk Duda <patrykd@google.com>

include/zephyr/posix/unistd.h

@@ -263,6 +263,7 @@ extern char *optarg;
 extern int opterr, optind, optopt;
 #endif
 
+int getentropy(void *buffer, size_t length);
 pid_t getpid(void);
 unsigned sleep(unsigned int seconds);
 int usleep(useconds_t useconds);

lib/posix/options/CMakeLists.txt

@@ -36,6 +36,7 @@ zephyr_library()
 add_subdirectory_ifdef(CONFIG_GETOPT getopt)
 zephyr_library_sources_ifdef(CONFIG_EVENTFD eventfd.c)
 zephyr_library_sources_ifdef(CONFIG_FNMATCH fnmatch.c)
+zephyr_library_sources_ifdef(CONFIG_GETENTROPY getentropy.c)
 zephyr_library_sources_ifdef(CONFIG_POSIX_API perror.c)
 zephyr_library_sources_ifdef(CONFIG_POSIX_ASYNCHRONOUS_IO aio.c)
 zephyr_library_sources_ifdef(CONFIG_POSIX_CLOCK clock.c)

lib/posix/options/Kconfig

@@ -35,6 +35,7 @@ rsource "Kconfig.eventfd"
 rsource "Kconfig.fdtable"
 rsource "Kconfig.fnmatch"
 rsource "Kconfig.fs"
+rsource "Kconfig.getentropy"
 rsource "Kconfig.getopt"
 rsource "Kconfig.key"
 rsource "Kconfig.mqueue"

lib/posix/options/Kconfig.getentropy

@@ -0,0 +1,14 @@
+# Copyright (c) 2024 Google LLC
+#
+# SPDX-License-Identifier: Apache-2.0
+
+DT_CHOSEN_ZEPHYR_ENTROPY := zephyr,entropy
+
+config GETENTROPY
+	bool "Support for getentropy"
+	depends on !NATIVE_APPLICATION
+	select NATIVE_LIBC_INCOMPATIBLE
+	depends on ENTROPY_HAS_DRIVER
+	depends on $(dt_chosen_enabled,$(DT_CHOSEN_ZEPHYR_ENTROPY))
+	help
+	  Enable support for getentropy() function.

lib/posix/options/getentropy.c

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2024 Google LLC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <errno.h>
+
+#include <zephyr/drivers/entropy.h>
+#include <zephyr/kernel.h>
+#include <zephyr/posix/unistd.h>
+
+#define ENTROPY_NODE DT_CHOSEN(zephyr_entropy)
+
+int getentropy(void *buffer, size_t length)
+{
+	const struct device * const entropy = DEVICE_DT_GET(ENTROPY_NODE);
+
+	if (!buffer) {
+		errno = EFAULT;
+		return -1;
+	}
+
+	if (length > 256) {
+		errno = EIO;
+		return -1;
+	}
+
+	if (!device_is_ready(entropy)) {
+		errno = EIO;
+		return -1;
+	}
+
+	/*
+	 * getentropy() uses size_t to represent buffer size, but Zephyr uses
+	 * uint16_t. The length check above allows us to safely cast without
+	 * overflow.
+	 */
+	if (entropy_get_entropy(entropy, buffer, (uint16_t)length)) {
+		errno = EIO;
+		return -1;
+	}
+
+	return 0;
+}

tests/posix/getentropy/CMakeLists.txt

@@ -0,0 +1,10 @@
+# Copyright (c) 2024 Google LLC
+#
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(getentropy)
+
+FILE(GLOB app_sources src/*.c)
+target_sources(app PRIVATE ${app_sources})

tests/posix/getentropy/prj.conf

@@ -0,0 +1,4 @@
+CONFIG_ENTROPY_GENERATOR=y
+CONFIG_GETENTROPY=y
+CONFIG_POSIX_API=y
+CONFIG_ZTEST=y

tests/posix/getentropy/src/main.c

@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2024 Google LLC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/ztest.h>
+
+#include <unistd.h>
+
+ZTEST(getentropy_test_suite, test_getentropy_too_large)
+{
+	uint8_t buf[256 + 1] = { 0 };
+	int ret;
+
+	ret = getentropy(buf, sizeof(buf));
+	zassert_equal(ret, -1);
+	zassert_equal(errno, EIO);
+}
+
+ZTEST(getentropy_test_suite, test_getentropy_null_buffer)
+{
+	int ret;
+
+	ret = getentropy(NULL, 0);
+	zassert_equal(ret, -1);
+	zassert_equal(errno, EFAULT);
+}
+
+ZTEST(getentropy_test_suite, test_getentropy_max_size)
+{
+	uint8_t buf[256] = { 0 };
+	int ret;
+
+	ret = getentropy(buf, sizeof(buf));
+	zassert_equal(ret, 0);
+}
+
+ZTEST(getentropy_test_suite, test_getentropy)
+{
+	uint8_t zero[16] = { 0 };
+	uint8_t buf1[16];
+	uint8_t buf2[16];
+	int ret;
+
+	ret = getentropy(buf1, sizeof(buf1));
+	zassert_equal(ret, 0);
+
+	ret = getentropy(buf2, sizeof(buf2));
+	zassert_equal(ret, 0);
+
+	zassert_true(memcmp(buf1, zero, sizeof(zero)) != 0);
+	zassert_true(memcmp(buf2, zero, sizeof(zero)) != 0);
+	zassert_true(memcmp(buf1, buf2, sizeof(buf1)) != 0);
+}
+
+ZTEST_SUITE(getentropy_test_suite, NULL, NULL, NULL, NULL, NULL);

tests/posix/getentropy/testcase.yaml

@@ -0,0 +1,20 @@
+common:
+  filter: dt_chosen_enabled("zephyr,entropy") and CONFIG_ENTROPY_HAS_DRIVER and
+    not CONFIG_NATIVE_LIBC
+  integration_platforms:
+    - native_sim
+  tags:
+    - posix
+    - getentropy
+tests:
+  portability.posix.getentropy: {}
+  portability.posix.getentropy.newlib:
+    filter: CONFIG_NEWLIB_LIBC_SUPPORTED
+    extra_configs:
+      - CONFIG_NEWLIB_LIBC=y
+      - CONFIG_NEWLIB_LIBC_MIN_REQUIRED_HEAP_SIZE=4096
+  portability.posix.getentropy.picolibc:
+    tags: picolibc
+    filter: CONFIG_PICOLIBC_SUPPORTED
+    extra_configs:
+      - CONFIG_PICOLIBC=y