Merge tag 'landlock-6.2-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/mic/linux

Pull landlock updates from Mickaël Salaün:
 "This adds file truncation support to Landlock, contributed by Günther
  Noack. As described by Günther [1], the goal of these patches is to
  work towards a more complete coverage of file system operations that
  are restrictable with Landlock.

  The known set of currently unsupported file system operations in
  Landlock is described at [2]. Out of the operations listed there,
  truncate is the only one that modifies file contents, so these patches
  should make it possible to prevent the direct modification of file
  contents with Landlock.

  The new LANDLOCK_ACCESS_FS_TRUNCATE access right covers both the
  truncate(2) and ftruncate(2) families of syscalls, as well as open(2)
  with the O_TRUNC flag. This includes usages of creat() in the case
  where existing regular files are overwritten.

  Additionally, this introduces a new Landlock security blob associated
  with opened files, to track the available Landlock access rights at
  the time of opening the file. This is in line with Unix's general
  approach of checking the read and write permissions during open(), and
  associating this previously checked authorization with the opened
  file. An ongoing patch documents this use case [3].

  In order to treat truncate(2) and ftruncate(2) calls differently in an
  LSM hook, we split apart the existing security_path_truncate hook into
  security_path_truncate (for truncation by path) and
  security_file_truncate (for truncation of previously opened files)"

Link: https://lore.kernel.org/r/20221018182216.301684-1-gnoack3000@gmail.com [1]
Link: https://www.kernel.org/doc/html/v6.1/userspace-api/landlock.html#filesystem-flags [2]
Link: https://lore.kernel.org/r/20221209193813.972012-1-mic@digikod.net [3]

* tag 'landlock-6.2-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/mic/linux:
  samples/landlock: Document best-effort approach for LANDLOCK_ACCESS_FS_REFER
  landlock: Document Landlock's file truncation support
  samples/landlock: Extend sample tool to support LANDLOCK_ACCESS_FS_TRUNCATE
  selftests/landlock: Test ftruncate on FDs created by memfd_create(2)
  selftests/landlock: Test FD passing from restricted to unrestricted processes
  selftests/landlock: Locally define __maybe_unused
  selftests/landlock: Test open() and ftruncate() in multiple scenarios
  selftests/landlock: Test file truncation support
  landlock: Support file truncation
  landlock: Document init_layer_masks() helper
  landlock: Refactor check_access_path_dual() into is_access_to_paths_allowed()
  security: Create file_truncate hook from path_truncate hook

3 files changed, 546 insertions, 45 deletions

diff --git a/tools/testing/selftests/landlock/base_test.c b/tools/testing/selftests/landlock/base_test.c
index da9290817866..792c3f0a59b4 100644
--- a/tools/testing/selftests/landlock/base_test.c
+++ b/tools/testing/selftests/landlock/base_test.c
@@ -75,7 +75,7 @@ TEST(abi_version)
 	const struct landlock_ruleset_attr ruleset_attr = {
 		.handled_access_fs = LANDLOCK_ACCESS_FS_READ_FILE,
 	};
-	ASSERT_EQ(2, landlock_create_ruleset(NULL, 0,
+	ASSERT_EQ(3, landlock_create_ruleset(NULL, 0,
 					     LANDLOCK_CREATE_RULESET_VERSION));
 
 	ASSERT_EQ(-1, landlock_create_ruleset(&ruleset_attr, 0,
@@ -263,23 +263,6 @@ TEST(ruleset_fd_transfer)
 		.allowed_access = LANDLOCK_ACCESS_FS_READ_DIR,
 	};
 	int ruleset_fd_tx, dir_fd;
-	union {
-		/* Aligned ancillary data buffer. */
-		char buf[CMSG_SPACE(sizeof(ruleset_fd_tx))];
-		struct cmsghdr _align;
-	} cmsg_tx = {};
-	char data_tx = '.';
-	struct iovec io = {
-		.iov_base = &data_tx,
-		.iov_len = sizeof(data_tx),
-	};
-	struct msghdr msg = {
-		.msg_iov = &io,
-		.msg_iovlen = 1,
-		.msg_control = &cmsg_tx.buf,
-		.msg_controllen = sizeof(cmsg_tx.buf),
-	};
-	struct cmsghdr *cmsg;
 	int socket_fds[2];
 	pid_t child;
 	int status;
@@ -298,33 +281,20 @@ TEST(ruleset_fd_transfer)
 				    &path_beneath_attr, 0));
 	ASSERT_EQ(0, close(path_beneath_attr.parent_fd));
 
-	cmsg = CMSG_FIRSTHDR(&msg);
-	ASSERT_NE(NULL, cmsg);
-	cmsg->cmsg_len = CMSG_LEN(sizeof(ruleset_fd_tx));
-	cmsg->cmsg_level = SOL_SOCKET;
-	cmsg->cmsg_type = SCM_RIGHTS;
-	memcpy(CMSG_DATA(cmsg), &ruleset_fd_tx, sizeof(ruleset_fd_tx));
-
 	/* Sends the ruleset FD over a socketpair and then close it. */
 	ASSERT_EQ(0, socketpair(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0,
 				socket_fds));
-	ASSERT_EQ(sizeof(data_tx), sendmsg(socket_fds[0], &msg, 0));
+	ASSERT_EQ(0, send_fd(socket_fds[0], ruleset_fd_tx));
 	ASSERT_EQ(0, close(socket_fds[0]));
 	ASSERT_EQ(0, close(ruleset_fd_tx));
 
 	child = fork();
 	ASSERT_LE(0, child);
 	if (child == 0) {
-		int ruleset_fd_rx;
+		const int ruleset_fd_rx = recv_fd(socket_fds[1]);
 
-		*(char *)msg.msg_iov->iov_base = '\0';
-		ASSERT_EQ(sizeof(data_tx),
-			  recvmsg(socket_fds[1], &msg, MSG_CMSG_CLOEXEC));
-		ASSERT_EQ('.', *(char *)msg.msg_iov->iov_base);
+		ASSERT_LE(0, ruleset_fd_rx);
 		ASSERT_EQ(0, close(socket_fds[1]));
-		cmsg = CMSG_FIRSTHDR(&msg);
-		ASSERT_EQ(cmsg->cmsg_len, CMSG_LEN(sizeof(ruleset_fd_tx)));
-		memcpy(&ruleset_fd_rx, CMSG_DATA(cmsg), sizeof(ruleset_fd_tx));
 
 		/* Enforces the received ruleset on the child. */
 		ASSERT_EQ(0, prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0));
diff --git a/tools/testing/selftests/landlock/common.h b/tools/testing/selftests/landlock/common.h
index 7ba18eb23783..d7987ae8d7fc 100644
--- a/tools/testing/selftests/landlock/common.h
+++ b/tools/testing/selftests/landlock/common.h
@@ -10,6 +10,7 @@
 #include <errno.h>
 #include <linux/landlock.h>
 #include <sys/capability.h>
+#include <sys/socket.h>
 #include <sys/syscall.h>
 #include <sys/types.h>
 #include <sys/wait.h>
@@ -17,6 +18,10 @@
 
 #include "../kselftest_harness.h"
 
+#ifndef __maybe_unused
+#define __maybe_unused __attribute__((__unused__))
+#endif
+
 /*
  * TEST_F_FORK() is useful when a test drop privileges but the corresponding
  * FIXTURE_TEARDOWN() requires them (e.g. to remove files from a directory
@@ -140,14 +145,12 @@ static void _init_caps(struct __test_metadata *const _metadata, bool drop_all)
 }
 
 /* We cannot put such helpers in a library because of kselftest_harness.h . */
-__attribute__((__unused__)) static void
-disable_caps(struct __test_metadata *const _metadata)
+static void __maybe_unused disable_caps(struct __test_metadata *const _metadata)
 {
 	_init_caps(_metadata, false);
 }
 
-__attribute__((__unused__)) static void
-drop_caps(struct __test_metadata *const _metadata)
+static void __maybe_unused drop_caps(struct __test_metadata *const _metadata)
 {
 	_init_caps(_metadata, true);
 }
@@ -176,14 +179,80 @@ static void _effective_cap(struct __test_metadata *const _metadata,
 	}
 }
 
-__attribute__((__unused__)) static void
-set_cap(struct __test_metadata *const _metadata, const cap_value_t caps)
+static void __maybe_unused set_cap(struct __test_metadata *const _metadata,
+				   const cap_value_t caps)
 {
 	_effective_cap(_metadata, caps, CAP_SET);
 }
 
-__attribute__((__unused__)) static void
-clear_cap(struct __test_metadata *const _metadata, const cap_value_t caps)
+static void __maybe_unused clear_cap(struct __test_metadata *const _metadata,
+				     const cap_value_t caps)
 {
 	_effective_cap(_metadata, caps, CAP_CLEAR);
 }
+
+/* Receives an FD from a UNIX socket. Returns the received FD, or -errno. */
+static int __maybe_unused recv_fd(int usock)
+{
+	int fd_rx;
+	union {
+		/* Aligned ancillary data buffer. */
+		char buf[CMSG_SPACE(sizeof(fd_rx))];
+		struct cmsghdr _align;
+	} cmsg_rx = {};
+	char data = '\0';
+	struct iovec io = {
+		.iov_base = &data,
+		.iov_len = sizeof(data),
+	};
+	struct msghdr msg = {
+		.msg_iov = &io,
+		.msg_iovlen = 1,
+		.msg_control = &cmsg_rx.buf,
+		.msg_controllen = sizeof(cmsg_rx.buf),
+	};
+	struct cmsghdr *cmsg;
+	int res;
+
+	res = recvmsg(usock, &msg, MSG_CMSG_CLOEXEC);
+	if (res < 0)
+		return -errno;
+
+	cmsg = CMSG_FIRSTHDR(&msg);
+	if (cmsg->cmsg_len != CMSG_LEN(sizeof(fd_rx)))
+		return -EIO;
+
+	memcpy(&fd_rx, CMSG_DATA(cmsg), sizeof(fd_rx));
+	return fd_rx;
+}
+
+/* Sends an FD on a UNIX socket. Returns 0 on success or -errno. */
+static int __maybe_unused send_fd(int usock, int fd_tx)
+{
+	union {
+		/* Aligned ancillary data buffer. */
+		char buf[CMSG_SPACE(sizeof(fd_tx))];
+		struct cmsghdr _align;
+	} cmsg_tx = {};
+	char data_tx = '.';
+	struct iovec io = {
+		.iov_base = &data_tx,
+		.iov_len = sizeof(data_tx),
+	};
+	struct msghdr msg = {
+		.msg_iov = &io,
+		.msg_iovlen = 1,
+		.msg_control = &cmsg_tx.buf,
+		.msg_controllen = sizeof(cmsg_tx.buf),
+	};
+	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+
+	cmsg->cmsg_len = CMSG_LEN(sizeof(fd_tx));
+	cmsg->cmsg_level = SOL_SOCKET;
+	cmsg->cmsg_type = SCM_RIGHTS;
+	memcpy(CMSG_DATA(cmsg), &fd_tx, sizeof(fd_tx));
+
+	if (sendmsg(usock, &msg, 0) < 0)
+		return -errno;
+	return 0;
+}
diff --git a/tools/testing/selftests/landlock/fs_test.c b/tools/testing/selftests/landlock/fs_test.c
index 45de42a027c5..d5dab986f612 100644
--- a/tools/testing/selftests/landlock/fs_test.c
+++ b/tools/testing/selftests/landlock/fs_test.c
@@ -58,6 +58,7 @@ static const char file1_s2d3[] = TMP_DIR "/s2d1/s2d2/s2d3/f1";
 static const char file2_s2d3[] = TMP_DIR "/s2d1/s2d2/s2d3/f2";
 
 static const char dir_s3d1[] = TMP_DIR "/s3d1";
+static const char file1_s3d1[] = TMP_DIR "/s3d1/f1";
 /* dir_s3d2 is a mount point. */
 static const char dir_s3d2[] = TMP_DIR "/s3d1/s3d2";
 static const char dir_s3d3[] = TMP_DIR "/s3d1/s3d2/s3d3";
@@ -83,6 +84,7 @@ static const char dir_s3d3[] = TMP_DIR "/s3d1/s3d2/s3d3";
  * │           ├── f1
  * │           └── f2
  * └── s3d1
+ *     ├── f1
  *     └── s3d2
  *         └── s3d3
  */
@@ -208,6 +210,7 @@ static void create_layout1(struct __test_metadata *const _metadata)
 	create_file(_metadata, file1_s2d3);
 	create_file(_metadata, file2_s2d3);
 
+	create_file(_metadata, file1_s3d1);
 	create_directory(_metadata, dir_s3d2);
 	set_cap(_metadata, CAP_SYS_ADMIN);
 	ASSERT_EQ(0, mount("tmp", dir_s3d2, "tmpfs", 0, "size=4m,mode=700"));
@@ -230,6 +233,7 @@ static void remove_layout1(struct __test_metadata *const _metadata)
 	EXPECT_EQ(0, remove_path(file1_s2d2));
 	EXPECT_EQ(0, remove_path(file1_s2d1));
 
+	EXPECT_EQ(0, remove_path(file1_s3d1));
 	EXPECT_EQ(0, remove_path(dir_s3d3));
 	set_cap(_metadata, CAP_SYS_ADMIN);
 	umount(dir_s3d2);
@@ -406,9 +410,10 @@ TEST_F_FORK(layout1, inval)
 #define ACCESS_FILE ( \
 	LANDLOCK_ACCESS_FS_EXECUTE | \
 	LANDLOCK_ACCESS_FS_WRITE_FILE | \
-	LANDLOCK_ACCESS_FS_READ_FILE)
+	LANDLOCK_ACCESS_FS_READ_FILE | \
+	LANDLOCK_ACCESS_FS_TRUNCATE)
 
-#define ACCESS_LAST LANDLOCK_ACCESS_FS_REFER
+#define ACCESS_LAST LANDLOCK_ACCESS_FS_TRUNCATE
 
 #define ACCESS_ALL ( \
 	ACCESS_FILE | \
@@ -422,7 +427,7 @@ TEST_F_FORK(layout1, inval)
 	LANDLOCK_ACCESS_FS_MAKE_FIFO | \
 	LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
 	LANDLOCK_ACCESS_FS_MAKE_SYM | \
-	ACCESS_LAST)
+	LANDLOCK_ACCESS_FS_REFER)
 
 /* clang-format on */
 
@@ -3157,6 +3162,463 @@ TEST_F_FORK(layout1, proc_pipe)
 	ASSERT_EQ(0, close(pipe_fds[1]));
 }
 
+/* Invokes truncate(2) and returns its errno or 0. */
+static int test_truncate(const char *const path)
+{
+	if (truncate(path, 10) < 0)
+		return errno;
+	return 0;
+}
+
+/*
+ * Invokes creat(2) and returns its errno or 0.
+ * Closes the opened file descriptor on success.
+ */
+static int test_creat(const char *const path)
+{
+	int fd = creat(path, 0600);
+
+	if (fd < 0)
+		return errno;
+
+	/*
+	 * Mixing error codes from close(2) and creat(2) should not lead to any
+	 * (access type) confusion for this test.
+	 */
+	if (close(fd) < 0)
+		return errno;
+	return 0;
+}
+
+/*
+ * Exercises file truncation when it's not restricted,
+ * as it was the case before LANDLOCK_ACCESS_FS_TRUNCATE existed.
+ */
+TEST_F_FORK(layout1, truncate_unhandled)
+{
+	const char *const file_r = file1_s1d1;
+	const char *const file_w = file2_s1d1;
+	const char *const file_none = file1_s1d2;
+	const struct rule rules[] = {
+		{
+			.path = file_r,
+			.access = LANDLOCK_ACCESS_FS_READ_FILE,
+		},
+		{
+			.path = file_w,
+			.access = LANDLOCK_ACCESS_FS_WRITE_FILE,
+		},
+		/* Implicitly: No rights for file_none. */
+		{},
+	};
+
+	const __u64 handled = LANDLOCK_ACCESS_FS_READ_FILE |
+			      LANDLOCK_ACCESS_FS_WRITE_FILE;
+	int ruleset_fd;
+
+	/* Enable Landlock. */
+	ruleset_fd = create_ruleset(_metadata, handled, rules);
+
+	ASSERT_LE(0, ruleset_fd);
+	enforce_ruleset(_metadata, ruleset_fd);
+	ASSERT_EQ(0, close(ruleset_fd));
+
+	/*
+	 * Checks read right: truncate and open with O_TRUNC work, unless the
+	 * file is attempted to be opened for writing.
+	 */
+	EXPECT_EQ(0, test_truncate(file_r));
+	EXPECT_EQ(0, test_open(file_r, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_open(file_r, O_WRONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_creat(file_r));
+
+	/*
+	 * Checks write right: truncate and open with O_TRUNC work, unless the
+	 * file is attempted to be opened for reading.
+	 */
+	EXPECT_EQ(0, test_truncate(file_w));
+	EXPECT_EQ(EACCES, test_open(file_w, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(0, test_open(file_w, O_WRONLY | O_TRUNC));
+	EXPECT_EQ(0, test_creat(file_w));
+
+	/*
+	 * Checks "no rights" case: truncate works but all open attempts fail,
+	 * including creat.
+	 */
+	EXPECT_EQ(0, test_truncate(file_none));
+	EXPECT_EQ(EACCES, test_open(file_none, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_open(file_none, O_WRONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_creat(file_none));
+}
+
+TEST_F_FORK(layout1, truncate)
+{
+	const char *const file_rwt = file1_s1d1;
+	const char *const file_rw = file2_s1d1;
+	const char *const file_rt = file1_s1d2;
+	const char *const file_t = file2_s1d2;
+	const char *const file_none = file1_s1d3;
+	const char *const dir_t = dir_s2d1;
+	const char *const file_in_dir_t = file1_s2d1;
+	const char *const dir_w = dir_s3d1;
+	const char *const file_in_dir_w = file1_s3d1;
+	const struct rule rules[] = {
+		{
+			.path = file_rwt,
+			.access = LANDLOCK_ACCESS_FS_READ_FILE |
+				  LANDLOCK_ACCESS_FS_WRITE_FILE |
+				  LANDLOCK_ACCESS_FS_TRUNCATE,
+		},
+		{
+			.path = file_rw,
+			.access = LANDLOCK_ACCESS_FS_READ_FILE |
+				  LANDLOCK_ACCESS_FS_WRITE_FILE,
+		},
+		{
+			.path = file_rt,
+			.access = LANDLOCK_ACCESS_FS_READ_FILE |
+				  LANDLOCK_ACCESS_FS_TRUNCATE,
+		},
+		{
+			.path = file_t,
+			.access = LANDLOCK_ACCESS_FS_TRUNCATE,
+		},
+		/* Implicitly: No access rights for file_none. */
+		{
+			.path = dir_t,
+			.access = LANDLOCK_ACCESS_FS_TRUNCATE,
+		},
+		{
+			.path = dir_w,
+			.access = LANDLOCK_ACCESS_FS_WRITE_FILE,
+		},
+		{},
+	};
+	const __u64 handled = LANDLOCK_ACCESS_FS_READ_FILE |
+			      LANDLOCK_ACCESS_FS_WRITE_FILE |
+			      LANDLOCK_ACCESS_FS_TRUNCATE;
+	int ruleset_fd;
+
+	/* Enable Landlock. */
+	ruleset_fd = create_ruleset(_metadata, handled, rules);
+
+	ASSERT_LE(0, ruleset_fd);
+	enforce_ruleset(_metadata, ruleset_fd);
+	ASSERT_EQ(0, close(ruleset_fd));
+
+	/* Checks read, write and truncate rights: truncation works. */
+	EXPECT_EQ(0, test_truncate(file_rwt));
+	EXPECT_EQ(0, test_open(file_rwt, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(0, test_open(file_rwt, O_WRONLY | O_TRUNC));
+
+	/* Checks read and write rights: no truncate variant works. */
+	EXPECT_EQ(EACCES, test_truncate(file_rw));
+	EXPECT_EQ(EACCES, test_open(file_rw, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_open(file_rw, O_WRONLY | O_TRUNC));
+
+	/*
+	 * Checks read and truncate rights: truncation works.
+	 *
+	 * Note: Files can get truncated using open() even with O_RDONLY.
+	 */
+	EXPECT_EQ(0, test_truncate(file_rt));
+	EXPECT_EQ(0, test_open(file_rt, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_open(file_rt, O_WRONLY | O_TRUNC));
+
+	/* Checks truncate right: truncate works, but can't open file. */
+	EXPECT_EQ(0, test_truncate(file_t));
+	EXPECT_EQ(EACCES, test_open(file_t, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_open(file_t, O_WRONLY | O_TRUNC));
+
+	/* Checks "no rights" case: No form of truncation works. */
+	EXPECT_EQ(EACCES, test_truncate(file_none));
+	EXPECT_EQ(EACCES, test_open(file_none, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_open(file_none, O_WRONLY | O_TRUNC));
+
+	/*
+	 * Checks truncate right on directory: truncate works on contained
+	 * files.
+	 */
+	EXPECT_EQ(0, test_truncate(file_in_dir_t));
+	EXPECT_EQ(EACCES, test_open(file_in_dir_t, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_open(file_in_dir_t, O_WRONLY | O_TRUNC));
+
+	/*
+	 * Checks creat in dir_w: This requires the truncate right when
+	 * overwriting an existing file, but does not require it when the file
+	 * is new.
+	 */
+	EXPECT_EQ(EACCES, test_creat(file_in_dir_w));
+
+	ASSERT_EQ(0, unlink(file_in_dir_w));
+	EXPECT_EQ(0, test_creat(file_in_dir_w));
+}
+
+/* Invokes ftruncate(2) and returns its errno or 0. */
+static int test_ftruncate(int fd)
+{
+	if (ftruncate(fd, 10) < 0)
+		return errno;
+	return 0;
+}
+
+TEST_F_FORK(layout1, ftruncate)
+{
+	/*
+	 * This test opens a new file descriptor at different stages of
+	 * Landlock restriction:
+	 *
+	 * without restriction:                    ftruncate works
+	 * something else but truncate restricted: ftruncate works
+	 * truncate restricted and permitted:      ftruncate works
+	 * truncate restricted and not permitted:  ftruncate fails
+	 *
+	 * Whether this works or not is expected to depend on the time when the
+	 * FD was opened, not to depend on the time when ftruncate() was
+	 * called.
+	 */
+	const char *const path = file1_s1d1;
+	const __u64 handled1 = LANDLOCK_ACCESS_FS_READ_FILE |
+			       LANDLOCK_ACCESS_FS_WRITE_FILE;
+	const struct rule layer1[] = {
+		{
+			.path = path,
+			.access = LANDLOCK_ACCESS_FS_WRITE_FILE,
+		},
+		{},
+	};
+	const __u64 handled2 = LANDLOCK_ACCESS_FS_TRUNCATE;
+	const struct rule layer2[] = {
+		{
+			.path = path,
+			.access = LANDLOCK_ACCESS_FS_TRUNCATE,
+		},
+		{},
+	};
+	const __u64 handled3 = LANDLOCK_ACCESS_FS_TRUNCATE |
+			       LANDLOCK_ACCESS_FS_WRITE_FILE;
+	const struct rule layer3[] = {
+		{
+			.path = path,
+			.access = LANDLOCK_ACCESS_FS_WRITE_FILE,
+		},
+		{},
+	};
+	int fd_layer0, fd_layer1, fd_layer2, fd_layer3, ruleset_fd;
+
+	fd_layer0 = open(path, O_WRONLY);
+	EXPECT_EQ(0, test_ftruncate(fd_layer0));
+
+	ruleset_fd = create_ruleset(_metadata, handled1, layer1);
+	ASSERT_LE(0, ruleset_fd);
+	enforce_ruleset(_metadata, ruleset_fd);
+	ASSERT_EQ(0, close(ruleset_fd));
+
+	fd_layer1 = open(path, O_WRONLY);
+	EXPECT_EQ(0, test_ftruncate(fd_layer0));
+	EXPECT_EQ(0, test_ftruncate(fd_layer1));
+
+	ruleset_fd = create_ruleset(_metadata, handled2, layer2);
+	ASSERT_LE(0, ruleset_fd);
+	enforce_ruleset(_metadata, ruleset_fd);
+	ASSERT_EQ(0, close(ruleset_fd));
+
+	fd_layer2 = open(path, O_WRONLY);
+	EXPECT_EQ(0, test_ftruncate(fd_layer0));
+	EXPECT_EQ(0, test_ftruncate(fd_layer1));
+	EXPECT_EQ(0, test_ftruncate(fd_layer2));
+
+	ruleset_fd = create_ruleset(_metadata, handled3, layer3);
+	ASSERT_LE(0, ruleset_fd);
+	enforce_ruleset(_metadata, ruleset_fd);
+	ASSERT_EQ(0, close(ruleset_fd));
+
+	fd_layer3 = open(path, O_WRONLY);
+	EXPECT_EQ(0, test_ftruncate(fd_layer0));
+	EXPECT_EQ(0, test_ftruncate(fd_layer1));
+	EXPECT_EQ(0, test_ftruncate(fd_layer2));
+	EXPECT_EQ(EACCES, test_ftruncate(fd_layer3));
+
+	ASSERT_EQ(0, close(fd_layer0));
+	ASSERT_EQ(0, close(fd_layer1));
+	ASSERT_EQ(0, close(fd_layer2));
+	ASSERT_EQ(0, close(fd_layer3));
+}
+
+/* clang-format off */
+FIXTURE(ftruncate) {};
+/* clang-format on */
+
+FIXTURE_SETUP(ftruncate)
+{
+	prepare_layout(_metadata);
+	create_file(_metadata, file1_s1d1);
+}
+
+FIXTURE_TEARDOWN(ftruncate)
+{
+	EXPECT_EQ(0, remove_path(file1_s1d1));
+	cleanup_layout(_metadata);
+}
+
+FIXTURE_VARIANT(ftruncate)
+{
+	const __u64 handled;
+	const __u64 permitted;
+	const int expected_open_result;
+	const int expected_ftruncate_result;
+};
+
+/* clang-format off */
+FIXTURE_VARIANT_ADD(ftruncate, w_w) {
+	/* clang-format on */
+	.handled = LANDLOCK_ACCESS_FS_WRITE_FILE,
+	.permitted = LANDLOCK_ACCESS_FS_WRITE_FILE,
+	.expected_open_result = 0,
+	.expected_ftruncate_result = 0,
+};
+
+/* clang-format off */
+FIXTURE_VARIANT_ADD(ftruncate, t_t) {
+	/* clang-format on */
+	.handled = LANDLOCK_ACCESS_FS_TRUNCATE,
+	.permitted = LANDLOCK_ACCESS_FS_TRUNCATE,
+	.expected_open_result = 0,
+	.expected_ftruncate_result = 0,
+};
+
+/* clang-format off */
+FIXTURE_VARIANT_ADD(ftruncate, wt_w) {
+	/* clang-format on */
+	.handled = LANDLOCK_ACCESS_FS_WRITE_FILE | LANDLOCK_ACCESS_FS_TRUNCATE,
+	.permitted = LANDLOCK_ACCESS_FS_WRITE_FILE,
+	.expected_open_result = 0,
+	.expected_ftruncate_result = EACCES,
+};
+
+/* clang-format off */
+FIXTURE_VARIANT_ADD(ftruncate, wt_wt) {
+	/* clang-format on */
+	.handled = LANDLOCK_ACCESS_FS_WRITE_FILE | LANDLOCK_ACCESS_FS_TRUNCATE,
+	.permitted = LANDLOCK_ACCESS_FS_WRITE_FILE |
+		     LANDLOCK_ACCESS_FS_TRUNCATE,
+	.expected_open_result = 0,
+	.expected_ftruncate_result = 0,
+};
+
+/* clang-format off */
+FIXTURE_VARIANT_ADD(ftruncate, wt_t) {
+	/* clang-format on */
+	.handled = LANDLOCK_ACCESS_FS_WRITE_FILE | LANDLOCK_ACCESS_FS_TRUNCATE,
+	.permitted = LANDLOCK_ACCESS_FS_TRUNCATE,
+	.expected_open_result = EACCES,
+};
+
+TEST_F_FORK(ftruncate, open_and_ftruncate)
+{
+	const char *const path = file1_s1d1;
+	const struct rule rules[] = {
+		{
+			.path = path,
+			.access = variant->permitted,
+		},
+		{},
+	};
+	int fd, ruleset_fd;
+
+	/* Enable Landlock. */
+	ruleset_fd = create_ruleset(_metadata, variant->handled, rules);
+	ASSERT_LE(0, ruleset_fd);
+	enforce_ruleset(_metadata, ruleset_fd);
+	ASSERT_EQ(0, close(ruleset_fd));
+
+	fd = open(path, O_WRONLY);
+	EXPECT_EQ(variant->expected_open_result, (fd < 0 ? errno : 0));
+	if (fd >= 0) {
+		EXPECT_EQ(variant->expected_ftruncate_result,
+			  test_ftruncate(fd));
+		ASSERT_EQ(0, close(fd));
+	}
+}
+
+TEST_F_FORK(ftruncate, open_and_ftruncate_in_different_processes)
+{
+	int child, fd, status;
+	int socket_fds[2];
+
+	ASSERT_EQ(0, socketpair(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0,
+				socket_fds));
+
+	child = fork();
+	ASSERT_LE(0, child);
+	if (child == 0) {
+		/*
+		 * Enables Landlock in the child process, open a file descriptor
+		 * where truncation is forbidden and send it to the
+		 * non-landlocked parent process.
+		 */
+		const char *const path = file1_s1d1;
+		const struct rule rules[] = {
+			{
+				.path = path,
+				.access = variant->permitted,
+			},
+			{},
+		};
+		int fd, ruleset_fd;
+
+		ruleset_fd = create_ruleset(_metadata, variant->handled, rules);
+		ASSERT_LE(0, ruleset_fd);
+		enforce_ruleset(_metadata, ruleset_fd);
+		ASSERT_EQ(0, close(ruleset_fd));
+
+		fd = open(path, O_WRONLY);
+		ASSERT_EQ(variant->expected_open_result, (fd < 0 ? errno : 0));
+
+		if (fd >= 0) {
+			ASSERT_EQ(0, send_fd(socket_fds[0], fd));
+			ASSERT_EQ(0, close(fd));
+		}
+
+		ASSERT_EQ(0, close(socket_fds[0]));
+
+		_exit(_metadata->passed ? EXIT_SUCCESS : EXIT_FAILURE);
+		return;
+	}
+
+	if (variant->expected_open_result == 0) {
+		fd = recv_fd(socket_fds[1]);
+		ASSERT_LE(0, fd);
+
+		EXPECT_EQ(variant->expected_ftruncate_result,
+			  test_ftruncate(fd));
+		ASSERT_EQ(0, close(fd));
+	}
+
+	ASSERT_EQ(child, waitpid(child, &status, 0));
+	ASSERT_EQ(1, WIFEXITED(status));
+	ASSERT_EQ(EXIT_SUCCESS, WEXITSTATUS(status));
+
+	ASSERT_EQ(0, close(socket_fds[0]));
+	ASSERT_EQ(0, close(socket_fds[1]));
+}
+
+TEST(memfd_ftruncate)
+{
+	int fd;
+
+	fd = memfd_create("name", MFD_CLOEXEC);
+	ASSERT_LE(0, fd);
+
+	/*
+	 * Checks that ftruncate is permitted on file descriptors that are
+	 * created in ways other than open(2).
+	 */
+	EXPECT_EQ(0, test_ftruncate(fd));
+
+	ASSERT_EQ(0, close(fd));
+}
+
 /* clang-format off */
 FIXTURE(layout1_bind) {};
 /* clang-format on */