selftests/landlock: Add hostfs tests

Add tests for the hostfs filesystems to make sure it has a consistent
inode management, which is required for Landlock's file hierarchy
identification. This adds 5 new tests for layout3_fs with the hostfs
variant.

Add hostfs to the new (architecture-specific) config.um file.

The hostfs filesystem, only available for an User-Mode Linux kernel, is
special because we cannot explicitly mount it. The layout3_fs.hostfs
variant tests are skipped if the current test directory is not backed by
this filesystem.

The layout3_fs.hostfs.tag_inode_dir_child and
layout3_fs.hostfs.tag_inode_file tests pass thanks to a previous commit
fixing hostfs inode management. Without this fix, the deny-by-default
policy would apply and all access requests would be denied.

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mickaël Salaün <[e-mail]>

tools/testing/selftests/landlock/config.um

···
+CONFIG_HOSTFS=y

tools/testing/selftests/landlock/fs_test.c

···
 #define _GNU_SOURCE
 #include <fcntl.h>
 #include <linux/landlock.h>
+#include <linux/magic.h>
 #include <sched.h>
 #include <stdio.h>
 #include <string.h>
···
 #include <sys/sendfile.h>
 #include <sys/stat.h>
 #include <sys/sysmacros.h>
+#include <sys/vfs.h>
 #include <unistd.h>
 
 #include "common.h"
···
 	res = fgrep(inf, str);
 	fclose(inf);
 	return res;
+}
+
+static bool cwd_matches_fs(unsigned int fs_magic)
+{
+	struct statfs statfs_buf;
+
+	if (!fs_magic)
+		return true;
+
+	if (statfs(".", &statfs_buf))
+		return true;
+
+	return statfs_buf.f_type == fs_magic;
 }
 
 static void mkdir_parents(struct __test_metadata *const _metadata,
···
 {
 	const struct mnt_opt mnt;
 	const char *const file_path;
+	unsigned int cwd_fs_magic;
 };
 
 /* clang-format off */
···
 	.file_path = TMP_DIR "/kernel/notes",
 };
 
+FIXTURE_VARIANT_ADD(layout3_fs, hostfs) {
+	.mnt = {
+		.source = TMP_DIR,
+		.flags = MS_BIND,
+	},
+	.file_path = TMP_DIR "/dir/file",
+	.cwd_fs_magic = HOSTFS_SUPER_MAGIC,
+};
+
 FIXTURE_SETUP(layout3_fs)
 {
 	struct stat statbuf;
 	const char *slash;
 	size_t dir_len;
 
-	if (!supports_filesystem(variant->mnt.type)) {
+	if (!supports_filesystem(variant->mnt.type) ||
+	    !cwd_matches_fs(variant->cwd_fs_magic)) {
 		self->skip_test = true;
 		SKIP(return, "this filesystem is not supported (setup)");
 	}