selftests/kernfs: test xattr retrieval

Make sure that listxattr() returns zero and that getxattr() returns
ENODATA when no extended attributs are set. Use /sys/kernel/warn_count
as that always exists and is a read-only file.

Link: https://lore.kernel.org/20250702-hochmoderne-abklatsch-af9c605b57b2@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>

tools/testing/selftests/filesystems/.gitignore

@@ -3,3 +3,4 @@ dnotify_test
 devpts_pts
 file_stressor
 anon_inode_test
+kernfs_test

tools/testing/selftests/filesystems/Makefile

@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 
 CFLAGS += $(KHDR_INCLUDES)
-TEST_GEN_PROGS := devpts_pts file_stressor anon_inode_test
+TEST_GEN_PROGS := devpts_pts file_stressor anon_inode_test kernfs_test
 TEST_GEN_PROGS_EXTENDED := dnotify_test
 
 include ../lib.mk

tools/testing/selftests/filesystems/kernfs_test.c

@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#define __SANE_USERSPACE_TYPES__
+
+#include <fcntl.h>
+#include <stdio.h>
+#include <sys/stat.h>
+#include <sys/xattr.h>
+
+#include "../kselftest_harness.h"
+#include "wrappers.h"
+
+TEST(kernfs_listxattr)
+{
+	int fd;
+
+	/* Read-only file that can never have any extended attributes set. */
+	fd = open("/sys/kernel/warn_count", O_RDONLY | O_CLOEXEC);
+	ASSERT_GE(fd, 0);
+	ASSERT_EQ(flistxattr(fd, NULL, 0), 0);
+	EXPECT_EQ(close(fd), 0);
+}
+
+TEST(kernfs_getxattr)
+{
+	int fd;
+	char buf[1];
+
+	/* Read-only file that can never have any extended attributes set. */
+	fd = open("/sys/kernel/warn_count", O_RDONLY | O_CLOEXEC);
+	ASSERT_GE(fd, 0);
+	ASSERT_LT(fgetxattr(fd, "user.foo", buf, sizeof(buf)), 0);
+	ASSERT_EQ(errno, ENODATA);
+	EXPECT_EQ(close(fd), 0);
+}
+
+TEST_HARNESS_MAIN
+