selftests, arm64: add a selftest for passing tagged pointers to kernel

This patch is a part of a series that extends kernel ABI to allow to pass
tagged user pointers (with the top byte set to something else other than
0x00) as syscall arguments.

This patch adds a simple test, that calls the uname syscall with a
tagged user pointer as an argument. Without the kernel accepting tagged
user pointers the test fails with EFAULT.
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Will Deacon <will@kernel.org>

tools/testing/selftests/arm64/.gitignore

+tags_test

tools/testing/selftests/arm64/Makefile

+# SPDX-License-Identifier: GPL-2.0
+
+# ARCH can be overridden by the user for cross compiling
+ARCH ?= $(shell uname -m 2>/dev/null || echo not)
+
+ifneq (,$(filter $(ARCH),aarch64 arm64))
+TEST_GEN_PROGS := tags_test
+TEST_PROGS := run_tags_test.sh
+endif
+
+include ../lib.mk

tools/testing/selftests/arm64/run_tags_test.sh

+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+
+echo "--------------------"
+echo "running tags test"
+echo "--------------------"
+./tags_test
+if [ $? -ne 0 ]; then
+	echo "[FAIL]"
+else
+	echo "[PASS]"
+fi

tools/testing/selftests/arm64/tags_test.c

+// SPDX-License-Identifier: GPL-2.0
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <stdint.h>
+#include <sys/prctl.h>
+#include <sys/utsname.h>
+
+#define SHIFT_TAG(tag)		((uint64_t)(tag) << 56)
+#define SET_TAG(ptr, tag)	(((uint64_t)(ptr) & ~SHIFT_TAG(0xff)) | \
+					SHIFT_TAG(tag))
+
+int main(void)
+{
+	static int tbi_enabled = 0;
+	struct utsname *ptr, *tagged_ptr;
+	int err;
+
+	if (prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0, 0, 0) == 0)
+		tbi_enabled = 1;
+	ptr = (struct utsname *)malloc(sizeof(*ptr));
+	if (tbi_enabled)
+		tagged_ptr = (struct utsname *)SET_TAG(ptr, 0x42);
+	err = uname(tagged_ptr);
+	free(ptr);
+
+	return err;
+}