selftests/landlock: Add filesystem access benchmark

fs_bench benchmarks the performance of Landlock's path walk
by exercising it in a scenario that amplifies Landlock's overhead:

* Create a large number of nested directories
* Enforce a Landlock policy in which a rule is associated with each of
  these subdirectories
* Benchmark openat() applied to the deepest directory,
  forcing Landlock to walk the entire path.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20260206151154.97915-3-gnoack3000@gmail.com
[mic: Fix missing mode with O_CREAT, improve text consistency, sort
includes]
Signed-off-by: Mickaël Salaün <mic@digikod.net>

tools/testing/selftests/landlock/.gitignore

@@ -1,4 +1,5 @@
 /*_test
+/fs_bench
 /sandbox-and-launch
 /true
 /wait-pipe

tools/testing/selftests/landlock/Makefile

@@ -9,6 +9,7 @@ LOCAL_HDRS += $(wildcard *.h)
 src_test := $(wildcard *_test.c)
 
 TEST_GEN_PROGS := $(src_test:.c=)
+TEST_GEN_PROGS += fs_bench
 
 TEST_GEN_PROGS_EXTENDED := \
 	true \

tools/testing/selftests/landlock/fs_bench.c

@@ -0,0 +1,214 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Landlock filesystem benchmark
+ *
+ * This program benchmarks the time required for file access checks.  We use a
+ * large number (-d flag) of nested directories where each directory inode has
+ * an associated Landlock rule, and we repeatedly (-n flag) exercise a file
+ * access for which Landlock has to walk the path all the way up to the root.
+ *
+ * With an increasing number of nested subdirectories, Landlock's portion of the
+ * overall system call time increases, which makes the effects of Landlock
+ * refactorings more measurable.
+ *
+ * This benchmark does *not* measure the building of the Landlock ruleset.  The
+ * time required to add all these rules is not large enough to be easily
+ * measurable.  A separate benchmark tool would be better to test that, and that
+ * tool could then also use a simpler file system layout.
+ *
+ * Copyright © 2026 Google LLC
+ */
+
+#define _GNU_SOURCE
+#include <err.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/landlock.h>
+#include <linux/prctl.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/prctl.h>
+#include <sys/stat.h>
+#include <sys/times.h>
+#include <time.h>
+#include <unistd.h>
+
+#include "wrappers.h"
+
+static void usage(const char *const argv0)
+{
+	printf("Usage:\n");
+	printf("  %s [OPTIONS]\n", argv0);
+	printf("\n");
+	printf("  Benchmark expensive Landlock checks for D nested dirs\n");
+	printf("\n");
+	printf("Options:\n");
+	printf("  -h	help\n");
+	printf("  -L	disable Landlock (as a baseline)\n");
+	printf("  -d D	set directory depth to D\n");
+	printf("  -n N	set number of benchmark iterations to N\n");
+}
+
+/*
+ * Build a deep directory, enforce Landlock and return the FD to the
+ * deepest dir.  On any failure, exit the process with an error.
+ */
+static int build_directory(size_t depth, const bool use_landlock)
+{
+	const char *path = "d"; /* directory name */
+	int abi, ruleset_fd, curr, prev;
+
+	if (use_landlock) {
+		abi = landlock_create_ruleset(NULL, 0,
+					      LANDLOCK_CREATE_RULESET_VERSION);
+		if (abi < 7)
+			err(1, "Landlock ABI too low: got %d, wanted 7+", abi);
+	}
+
+	ruleset_fd = -1;
+	if (use_landlock) {
+		struct landlock_ruleset_attr attr = {
+			.handled_access_fs = LANDLOCK_ACCESS_FS_IOCTL_DEV |
+					     LANDLOCK_ACCESS_FS_WRITE_FILE |
+					     LANDLOCK_ACCESS_FS_MAKE_REG,
+		};
+		ruleset_fd = landlock_create_ruleset(&attr, sizeof(attr), 0U);
+		if (ruleset_fd < 0)
+			err(1, "landlock_create_ruleset");
+	}
+
+	curr = open(".", O_PATH);
+	if (curr < 0)
+		err(1, "open(.)");
+
+	while (depth--) {
+		if (use_landlock) {
+			struct landlock_path_beneath_attr attr = {
+				.allowed_access = LANDLOCK_ACCESS_FS_IOCTL_DEV,
+				.parent_fd = curr,
+			};
+			if (landlock_add_rule(ruleset_fd,
+					      LANDLOCK_RULE_PATH_BENEATH, &attr,
+					      0) < 0)
+				err(1, "landlock_add_rule");
+		}
+
+		if (mkdirat(curr, path, 0700) < 0)
+			err(1, "mkdirat(%s)", path);
+
+		prev = curr;
+		curr = openat(curr, path, O_PATH);
+		if (curr < 0)
+			err(1, "openat(%s)", path);
+
+		close(prev);
+	}
+
+	if (use_landlock) {
+		if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0)
+			err(1, "prctl");
+
+		if (landlock_restrict_self(ruleset_fd, 0) < 0)
+			err(1, "landlock_restrict_self");
+	}
+
+	close(ruleset_fd);
+	return curr;
+}
+
+static void remove_recursively(const size_t depth)
+{
+	const char *path = "d"; /* directory name */
+
+	int fd = openat(AT_FDCWD, ".", O_PATH);
+
+	if (fd < 0)
+		err(1, "openat(.)");
+
+	for (size_t i = 0; i < depth - 1; i++) {
+		int oldfd = fd;
+
+		fd = openat(fd, path, O_PATH);
+		if (fd < 0)
+			err(1, "openat(%s)", path);
+		close(oldfd);
+	}
+
+	for (size_t i = 0; i < depth; i++) {
+		if (unlinkat(fd, path, AT_REMOVEDIR) < 0)
+			err(1, "unlinkat(%s)", path);
+		int newfd = openat(fd, "..", O_PATH);
+
+		close(fd);
+		fd = newfd;
+	}
+	close(fd);
+}
+
+int main(int argc, char *argv[])
+{
+	bool use_landlock = true;
+	size_t num_iterations = 100000;
+	size_t num_subdirs = 10000;
+	int c, curr, fd;
+	struct tms start_time, end_time;
+
+	setbuf(stdout, NULL);
+	while ((c = getopt(argc, argv, "hLd:n:")) != -1) {
+		switch (c) {
+		case 'h':
+			usage(argv[0]);
+			return EXIT_SUCCESS;
+		case 'L':
+			use_landlock = false;
+			break;
+		case 'd':
+			num_subdirs = atoi(optarg);
+			break;
+		case 'n':
+			num_iterations = atoi(optarg);
+			break;
+		default:
+			usage(argv[0]);
+			return EXIT_FAILURE;
+		}
+	}
+
+	printf("*** Benchmark ***\n");
+	printf("%zu dirs, %zu iterations, %s Landlock\n", num_subdirs,
+	       num_iterations, use_landlock ? "with" : "without");
+
+	if (times(&start_time) == -1)
+		err(1, "times");
+
+	curr = build_directory(num_subdirs, use_landlock);
+
+	for (int i = 0; i < num_iterations; i++) {
+		fd = openat(curr, "file.txt", O_CREAT | O_TRUNC | O_WRONLY,
+			    0600);
+		if (use_landlock) {
+			if (fd == 0)
+				errx(1, "openat succeeded, expected EACCES");
+			if (errno != EACCES)
+				err(1, "openat expected EACCES, but got");
+		}
+		if (fd != -1)
+			close(fd);
+	}
+
+	if (times(&end_time) == -1)
+		err(1, "times");
+
+	printf("*** Benchmark concluded ***\n");
+	printf("System: %ld clocks\n",
+	       end_time.tms_stime - start_time.tms_stime);
+	printf("User  : %ld clocks\n",
+	       end_time.tms_utime - start_time.tms_utime);
+	printf("Clocks per second: %ld\n", CLOCKS_PER_SEC);
+
+	close(curr);
+
+	remove_recursively(num_subdirs);
+}