[PATCH 09/10] selftests/filesystems/overlayfs: test idmapped overlay mounts

Add a selftest for idmapping the merged overlay mount itself.  It
applies an idmapping to a freshly created (still detached) overlay
mount with mount_setattr(MOUNT_ATTR_IDMAP) and checks that:

  - getattr reports ownership mapped through the mount idmap;
  - a file created through the idmapped mount is stored on the upper
    layer with the corresponding overlay-final id;
  - chown through the idmapped mount round-trips;
  - an nfs_export overlay can be idmapped and decodable file handles
    round-trip through it with correctly mapped ownership.

The layers live on a private tmpfs and are owned by the host id so that
they map to id 0 through the test idmapping, allowing the root caller
to operate on them.

Signed-off-by: Christian Brauner (Amutable) <[email protected]>
---
 .../selftests/filesystems/overlayfs/.gitignore     |   1 +
 .../selftests/filesystems/overlayfs/Makefile       |   2 +
 .../filesystems/overlayfs/idmapped_mounts.c        | 501 +++++++++++++++++++++
 3 files changed, 504 insertions(+)

diff --git a/tools/testing/selftests/filesystems/overlayfs/.gitignore b/tools/testing/selftests/filesystems/overlayfs/.gitignore
index e23a18c8b37f..077f7a128168 100644
--- a/tools/testing/selftests/filesystems/overlayfs/.gitignore
+++ b/tools/testing/selftests/filesystems/overlayfs/.gitignore
@@ -1,3 +1,4 @@
 # SPDX-License-Identifier: GPL-2.0-only
 dev_in_maps
 set_layers_via_fds
+idmapped_mounts
diff --git a/tools/testing/selftests/filesystems/overlayfs/Makefile b/tools/testing/selftests/filesystems/overlayfs/Makefile
index d3ad4a77db9b..b3185f684add 100644
--- a/tools/testing/selftests/filesystems/overlayfs/Makefile
+++ b/tools/testing/selftests/filesystems/overlayfs/Makefile
@@ -8,7 +8,9 @@ LOCAL_HDRS += ../wrappers.h log.h
 
 TEST_GEN_PROGS := dev_in_maps
 TEST_GEN_PROGS += set_layers_via_fds
+TEST_GEN_PROGS += idmapped_mounts
 
 include ../../lib.mk
 
 $(OUTPUT)/set_layers_via_fds: ../utils.c
+$(OUTPUT)/idmapped_mounts: ../utils.c
diff --git a/tools/testing/selftests/filesystems/overlayfs/idmapped_mounts.c b/tools/testing/selftests/filesystems/overlayfs/idmapped_mounts.c
new file mode 100644
index 000000000000..44a75839f4ed
--- /dev/null
+++ b/tools/testing/selftests/filesystems/overlayfs/idmapped_mounts.c
@@ -0,0 +1,501 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+
+#include <linux/mount.h>
+#include <linux/types.h>
+
+#include "kselftest_harness.h"
+#include "../wrappers.h"
+#include "../utils.h"
+
+/*
+ * An idmapping that maps the mount-visible id range [0, ID_RANGE) onto the
+ * host/overlay-final id range [ID_HOST, ID_HOST + ID_RANGE).  Through such an
+ * idmapped overlay mount, an overlay-final id of ID_HOST + n is reported as n,
+ * and an id of n requested through the mount is stored as ID_HOST + n.
+ */
+#define ID_NS	 0
+#define ID_HOST	 10000
+#define ID_RANGE 10000
+
+/*
+ * For the composition test the lower layer's on-disk ids live in a
+ * separate range and are mapped by an idmapped lower layer onto the
+ * overlay-final range [ID_HOST, ID_HOST + ID_RANGE).
+ */
+#define LAYER_HOST 20000
+
+#ifndef MOUNT_ATTR_IDMAP
+#define MOUNT_ATTR_IDMAP 0x00100000
+#endif
+
+#ifndef __NR_mount_setattr
+#define __NR_mount_setattr 442
+#endif
+
+static inline int sys_mount_setattr(int dfd, const char *path,
+				    unsigned int flags,
+				    struct mount_attr *attr, size_t size)
+{
+	return syscall(__NR_mount_setattr, dfd, path, flags, attr, size);
+}
+
+static bool ovl_supported(void)
+{
+	int fd = sys_fsopen("overlay", 0);
+
+	if (fd < 0)
+		return false;
+	close(fd);
+	return true;
+}
+
+/* base/{l,u,w} owned by ID_HOST so they map to ID_NS through the idmap. */
+static int setup_layers(const char *base)
+{
+	static const char *sub[] = { "", "/l", "/u", "/w" };
+	char path[PATH_MAX];
+
+	for (size_t i = 0; i < ARRAY_SIZE(sub); i++) {
+		snprintf(path, sizeof(path), "%s%s", base, sub[i]);
+		if (mkdir(path, 0755) && errno != EEXIST)
+			return -1;
+		if (i && chown(path, ID_HOST, ID_HOST))
+			return -1;
+	}
+	return 0;
+}
+
+static int ovl_mount(const char *base, bool nfs_export)
+{
+	char lower[PATH_MAX], upper[PATH_MAX], work[PATH_MAX];
+	int fsfd, ovl;
+
+	snprintf(lower, sizeof(lower), "%s/l", base);
+	snprintf(upper, sizeof(upper), "%s/u", base);
+	snprintf(work, sizeof(work), "%s/w", base);
+
+	fsfd = sys_fsopen("overlay", 0);
+	if (fsfd < 0)
+		return -1;
+
+	if (sys_fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "test", 0) ||
+	    sys_fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir", lower, 0) ||
+	    sys_fsconfig(fsfd, FSCONFIG_SET_STRING, "upperdir", upper, 0) ||
+	    sys_fsconfig(fsfd, FSCONFIG_SET_STRING, "workdir", work, 0))
+		goto err;
+	if (nfs_export &&
+	    (sys_fsconfig(fsfd, FSCONFIG_SET_STRING, "index", "on", 0) ||
+	     sys_fsconfig(fsfd, FSCONFIG_SET_STRING, "nfs_export", "on", 0)))
+		goto err;
+	if (sys_fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0))
+		goto err;
+
+	ovl = sys_fsmount(fsfd, 0, 0);
+	close(fsfd);
+	return ovl;
+err:
+	close(fsfd);
+	return -1;
+}
+
+/* Idmap the (still detached, not yet visible) overlay mount @mfd. */
+static int ovl_idmap(int mfd)
+{
+	struct mount_attr attr = {
+		.attr_set = MOUNT_ATTR_IDMAP,
+	};
+	int ret, userns_fd;
+
+	/*
+	 * get_userns_fd(fs_id, mount_id, range): a file whose filesystem id
+	 * is fs_id + n is shown through the idmapped mount as mount_id + n.
+	 * Here the overlay-final (fs side) range is [ID_HOST, ..) and the
+	 * caller-visible (mount side) range is [ID_NS, ..).
+	 */
+	userns_fd = get_userns_fd(ID_HOST, ID_NS, ID_RANGE);
+	if (userns_fd < 0)
+		return -1;
+
+	attr.userns_fd = userns_fd;
+	ret = sys_mount_setattr(mfd, "", AT_EMPTY_PATH, &attr, sizeof(attr));
+	close(userns_fd);
+	return ret;
+}
+
+/* Clone @path into a detached, idmapped mount usable as an overlay layer. */
+static int idmapped_layer_fd(const char *path, int nsid, int hostid, int range)
+{
+	struct mount_attr attr = {
+		.attr_set = MOUNT_ATTR_IDMAP,
+	};
+	int fd_tree, userns_fd;
+
+	fd_tree = sys_open_tree(AT_FDCWD, path,
+			       OPEN_TREE_CLONE | OPEN_TREE_CLOEXEC);
+	if (fd_tree < 0)
+		return -1;
+	userns_fd = get_userns_fd(nsid, hostid, range);
+	if (userns_fd < 0) {
+		close(fd_tree);
+		return -1;
+	}
+	attr.userns_fd = userns_fd;
+	if (sys_mount_setattr(fd_tree, "", AT_EMPTY_PATH, &attr,
+			      sizeof(attr))) {
+		close(userns_fd);
+		close(fd_tree);
+		return -1;
+	}
+	close(userns_fd);
+	return fd_tree;
+}
+
+/* Overlay with a layer passed by fd (idmapped) plus a plain upper/work. */
+static int ovl_mount_lower_fd(const char *upper, const char *work, int fd_lower)
+{
+	int fsfd, ovl;
+
+	fsfd = sys_fsopen("overlay", 0);
+	if (fsfd < 0)
+		return -1;
+
+	if (sys_fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "test", 0) ||
+	    sys_fsconfig(fsfd, FSCONFIG_SET_STRING, "upperdir", upper, 0) ||
+	    sys_fsconfig(fsfd, FSCONFIG_SET_STRING, "workdir", work, 0) ||
+	    sys_fsconfig(fsfd, FSCONFIG_SET_FD, "lowerdir+", NULL, fd_lower) ||
+	    sys_fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0))
+		goto err;
+
+	ovl = sys_fsmount(fsfd, 0, 0);
+	close(fsfd);
+	return ovl;
+err:
+	close(fsfd);
+	return -1;
+}
+
+/*
+ * Mount an overlay inside user namespace @u1 (so the overlay sb's s_user_ns is
+ * not the initial namespace) and idmap that overlay mount with @u2.  Runs in a
+ * child that joins @u1; returns 0 on success.
+ */
+static int userns_overlay_child(int u1)
+{
+	struct mount_attr attr = {
+		.attr_set = MOUNT_ATTR_IDMAP,
+	};
+	struct stat st;
+	int ovl, u2;
+
+	/* Become root in the overlay sb's user namespace u1. */
+	if (!switch_userns(u1, 0, 0, false))
+		return fprintf(stderr, "userns: switch_userns: %m\n"), -1;
+	if (unshare(CLONE_NEWNS) ||
+	    sys_mount(NULL, "/", NULL, MS_SLAVE | MS_REC, NULL))
+		return fprintf(stderr, "userns: unshare/slave: %m\n"), -1;
+	if (sys_mount("tmpfs", "/tmp", "tmpfs", 0, NULL))
+		return fprintf(stderr, "userns: mount tmpfs: %m\n"), -1;
+	if (setup_layers("/tmp/ovl"))
+		return fprintf(stderr, "userns: setup_layers: %m\n"), -1;
+	if (mknod("/tmp/ovl/l/file", S_IFREG | 0644, 0) ||
+	    chown("/tmp/ovl/l/file", ID_HOST + 5, ID_HOST + 5))
+		return fprintf(stderr, "userns: lower file: %m\n"), -1;
+
+	ovl = ovl_mount("/tmp/ovl", false);
+	if (ovl < 0)
+		return fprintf(stderr, "userns: ovl_mount: %m\n"), -1;
+
+	/*
+	 * mount_setattr() requires CAP_SYS_ADMIN over the idmap user
+	 * namespace, so it must be a child of u1.  Create it now, from
+	 * inside u1.
+	 */
+	u2 = get_userns_fd(ID_HOST, ID_NS, ID_RANGE);
+	if (u2 < 0)
+		return fprintf(stderr, "userns: get_userns_fd: %m\n"), -1;
+	attr.userns_fd = u2;
+	if (sys_mount_setattr(ovl, "", AT_EMPTY_PATH, &attr, sizeof(attr)))
+		return fprintf(stderr, "userns: mount_setattr: %m\n"), -1;
+	close(u2);
+
+	if (fstatat(ovl, "file", &st, 0))
+		return fprintf(stderr, "userns: fstatat: %m\n"), -1;
+	if (st.st_uid != ID_NS + 5 || st.st_gid != ID_NS + 5) {
+		fprintf(stderr, "userns: got %u:%u expected %u:%u\n",
+			st.st_uid, st.st_gid, ID_NS + 5, ID_NS + 5);
+		return -1;
+	}
+	return 0;
+}
+
+FIXTURE(idmapped_overlay) {
+	char base[64];
+};
+
+FIXTURE_SETUP(idmapped_overlay)
+{
+	/* Private mount namespace so test mounts need no cleanup. */
+	ASSERT_EQ(unshare(CLONE_NEWNS), 0);
+	ASSERT_EQ(sys_mount(NULL, "/", NULL, MS_SLAVE | MS_REC, NULL), 0);
+
+	/* tmpfs for the layers so we can chown them to arbitrary ids. */
+	ASSERT_EQ(sys_mount("tmpfs", "/tmp", "tmpfs", 0, NULL), 0);
+
+	snprintf(self->base, sizeof(self->base), "/tmp/ovl");
+	ASSERT_EQ(setup_layers(self->base), 0);
+}
+
+FIXTURE_TEARDOWN(idmapped_overlay)
+{
+}
+
+/* A file owned by ID_HOST + 5 is reported as ID_NS + 5 through the idmap. */
+TEST_F(idmapped_overlay, getattr)
+{
+	char path[PATH_MAX];
+	struct stat st;
+	int ovl;
+
+	if (!ovl_supported())
+		SKIP(return, "overlayfs not supported");
+
+	snprintf(path, sizeof(path), "%s/l/file", self->base);
+	ASSERT_EQ(mknod(path, S_IFREG | 0644, 0), 0);
+	ASSERT_EQ(chown(path, ID_HOST + 5, ID_HOST + 5), 0);
+
+	ovl = ovl_mount(self->base, false);
+	ASSERT_GE(ovl, 0);
+	ASSERT_EQ(ovl_idmap(ovl), 0);
+
+	ASSERT_EQ(fstatat(ovl, "file", &st, 0), 0);
+	EXPECT_EQ(st.st_uid, ID_NS + 5);
+	EXPECT_EQ(st.st_gid, ID_NS + 5);
+
+	EXPECT_EQ(close(ovl), 0);
+}
+
+/*
+ * Every creation path initializes the new owner through the mount idmap:
+ * created as caller id ID_NS, stored on the upper layer as overlay-final
+ * ID_HOST.  Covers ovl_create() (regular file), ovl_mkdir(), ovl_mknod()
+ * and ovl_symlink() (which share ovl_create_object()), plus the separate
+ * ovl_tmpfile() path.
+ */
+TEST_F(idmapped_overlay, create)
+{
+	static const char *names[] = { "reg", "dir", "fifo", "lnk" };
+	char path[PATH_MAX];
+	struct stat st;
+	int ovl, fd;
+
+	if (!ovl_supported())
+		SKIP(return, "overlayfs not supported");
+
+	ovl = ovl_mount(self->base, false);
+	ASSERT_GE(ovl, 0);
+	ASSERT_EQ(ovl_idmap(ovl), 0);
+
+	/* One object per creation operation, all as caller id ID_NS. */
+	fd = openat(ovl, "reg", O_CREAT | O_WRONLY | O_EXCL, 0644);
+	ASSERT_GE(fd, 0);
+	EXPECT_EQ(close(fd), 0);
+	ASSERT_EQ(mkdirat(ovl, "dir", 0755), 0);
+	ASSERT_EQ(mknodat(ovl, "fifo", S_IFIFO | 0644, 0), 0);
+	ASSERT_EQ(symlinkat("target", ovl, "lnk"), 0);
+
+	for (size_t i = 0; i < ARRAY_SIZE(names); i++) {
+		/* Reported as ID_NS through the idmapped mount ... */
+		ASSERT_EQ(fstatat(ovl, names[i], &st, AT_SYMLINK_NOFOLLOW), 0);
+		EXPECT_EQ(st.st_uid, ID_NS);
+		EXPECT_EQ(st.st_gid, ID_NS);
+		/* ... and stored as ID_HOST on the upper layer. */
+		snprintf(path, sizeof(path), "%s/u/%s", self->base, names[i]);
+		ASSERT_EQ(lstat(path, &st), 0);
+		EXPECT_EQ(st.st_uid, ID_HOST);
+		EXPECT_EQ(st.st_gid, ID_HOST);
+	}
+
+	/* O_TMPFILE goes through the separate ovl_tmpfile() path. */
+	fd = openat(ovl, ".", O_TMPFILE | O_WRONLY, 0644);
+	ASSERT_GE(fd, 0);
+	/* Inside the mount: caller id ID_NS. */
+	ASSERT_EQ(fstat(fd, &st), 0);
+	EXPECT_EQ(st.st_uid, ID_NS);
+	EXPECT_EQ(st.st_gid, ID_NS);
+	/* Link it in so the upper backing file can be inspected too. */
+	ASSERT_EQ(linkat(fd, "", ovl, "tmp", AT_EMPTY_PATH), 0);
+	EXPECT_EQ(close(fd), 0);
+	snprintf(path, sizeof(path), "%s/u/tmp", self->base);
+	ASSERT_EQ(lstat(path, &st), 0);
+	EXPECT_EQ(st.st_uid, ID_HOST);
+	EXPECT_EQ(st.st_gid, ID_HOST);
+
+	EXPECT_EQ(close(ovl), 0);
+}
+
+/* chown through the idmapped mount round-trips: ID_NS + 5 <-> ID_HOST + 5. */
+TEST_F(idmapped_overlay, chown)
+{
+	char path[PATH_MAX];
+	struct stat st;
+	int ovl, fd;
+
+	if (!ovl_supported())
+		SKIP(return, "overlayfs not supported");
+
+	ovl = ovl_mount(self->base, false);
+	ASSERT_GE(ovl, 0);
+	ASSERT_EQ(ovl_idmap(ovl), 0);
+
+	fd = openat(ovl, "f", O_CREAT | O_WRONLY | O_EXCL, 0644);
+	ASSERT_GE(fd, 0);
+	EXPECT_EQ(close(fd), 0);
+
+	ASSERT_EQ(fchownat(ovl, "f", ID_NS + 5, ID_NS + 5, 0), 0);
+
+	ASSERT_EQ(fstatat(ovl, "f", &st, 0), 0);
+	EXPECT_EQ(st.st_uid, ID_NS + 5);
+	EXPECT_EQ(st.st_gid, ID_NS + 5);
+
+	snprintf(path, sizeof(path), "%s/u/f", self->base);
+	ASSERT_EQ(stat(path, &st), 0);
+	EXPECT_EQ(st.st_uid, ID_HOST + 5);
+	EXPECT_EQ(st.st_gid, ID_HOST + 5);
+
+	EXPECT_EQ(close(ovl), 0);
+}
+
+/*
+ * Composition: an idmapped lower layer underneath an idmapped overlay mount.
+ * An on-disk id is mapped by the layer idmap into the overlay-final range and
+ * then by the mount idmap into the caller's range:
+ *
+ *   on-disk LAYER_HOST+7  --layer-->  ID_HOST+7  --mount-->  ID_NS+7
+ */
+TEST_F(idmapped_overlay, composition)
+{
+	char lower[PATH_MAX], upper[PATH_MAX], work[PATH_MAX], path[PATH_MAX];
+	struct stat st;
+	int ovl, fd_lower;
+
+	if (!ovl_supported())
+		SKIP(return, "overlayfs not supported");
+
+	snprintf(lower, sizeof(lower), "%s/l", self->base);
+	snprintf(upper, sizeof(upper), "%s/u", self->base);
+	snprintf(work, sizeof(work), "%s/w", self->base);
+
+	/* Put the lower layer's ids in the on-disk [LAYER_HOST, ..) range. */
+	ASSERT_EQ(chown(lower, LAYER_HOST, LAYER_HOST), 0);
+	snprintf(path, sizeof(path), "%s/l/file", self->base);
+	ASSERT_EQ(mknod(path, S_IFREG | 0644, 0), 0);
+	ASSERT_EQ(chown(path, LAYER_HOST + 7, LAYER_HOST + 7), 0);
+
+	/* Idmapped lower: on-disk LAYER_HOST <-> overlay-final ID_HOST. */
+	fd_lower = idmapped_layer_fd(lower, LAYER_HOST, ID_HOST, ID_RANGE);
+	ASSERT_GE(fd_lower, 0);
+
+	ovl = ovl_mount_lower_fd(upper, work, fd_lower);
+	ASSERT_GE(ovl, 0);
+	EXPECT_EQ(close(fd_lower), 0);
+
+	/* Idmap the overlay mount: overlay-final ID_HOST <-> caller ID_NS. */
+	ASSERT_EQ(ovl_idmap(ovl), 0);
+
+	ASSERT_EQ(fstatat(ovl, "file", &st, 0), 0);
+	EXPECT_EQ(st.st_uid, ID_NS + 7);
+	EXPECT_EQ(st.st_gid, ID_NS + 7);
+
+	EXPECT_EQ(close(ovl), 0);
+}
+
+/* An idmapped overlay mount whose sb lives inside a user namespace. */
+TEST_F(idmapped_overlay, userns)
+{
+	int u1;
+	pid_t pid;
+
+	if (!ovl_supported())
+		SKIP(return, "overlayfs not supported");
+
+	/* u1 backs the overlay sb: identity-mapped, but not the init ns. */
+	u1 = get_userns_fd(0, 0, 65536);
+	if (u1 < 0)
+		SKIP(return, "user namespaces not available");
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+	if (pid == 0) {
+		int ret = userns_overlay_child(u1);
+
+		_exit(ret ? EXIT_FAILURE : EXIT_SUCCESS);
+	}
+	EXPECT_EQ(wait_for_pid(pid), 0);
+
+	EXPECT_EQ(close(u1), 0);
+}
+
+/*
+ * An nfs_export overlay can be idmapped, and decodable file handles round-trip
+ * through the idmapped mount with correctly mapped ownership.  Overlay file
+ * handles encode object identity, not ownership, so the mount idmap does not
+ * affect them; it only maps the owner reported once a handle is reopened.
+ */
+TEST_F(idmapped_overlay, nfs_export_handles)
+{
+	char path[PATH_MAX], mnt[128];
+	union {
+		struct file_handle fh;
+		char buf[sizeof(struct file_handle) + MAX_HANDLE_SZ];
+	} fhu;
+	struct file_handle *fh = &fhu.fh;
+	struct stat st;
+	int ovl, mfd, fd, mount_id;
+
+	if (!ovl_supported())
+		SKIP(return, "overlayfs not supported");
+
+	snprintf(path, sizeof(path), "%s/l/file", self->base);
+	ASSERT_EQ(mknod(path, S_IFREG | 0644, 0), 0);
+	ASSERT_EQ(chown(path, ID_HOST + 7, ID_HOST + 7), 0);
+
+	/* nfs_export=on gives decodable overlay file handles. */
+	ovl = ovl_mount(self->base, true);
+	if (ovl < 0)
+		SKIP(return, "overlayfs nfs_export not supported");
+	ASSERT_EQ(ovl_idmap(ovl), 0);
+
+	/* Attach the idmapped mount so handles can be resolved against it. */
+	snprintf(mnt, sizeof(mnt), "%s/mnt", self->base);
+	ASSERT_EQ(mkdir(mnt, 0755), 0);
+	ASSERT_EQ(sys_move_mount(ovl, "", AT_FDCWD, mnt,
+				 MOVE_MOUNT_F_EMPTY_PATH), 0);
+
+	snprintf(path, sizeof(path), "%s/file", mnt);
+	fh->handle_bytes = MAX_HANDLE_SZ;
+	ASSERT_EQ(name_to_handle_at(AT_FDCWD, path, fh, &mount_id, 0), 0);
+
+	mfd = open(mnt, O_RDONLY | O_DIRECTORY);
+	ASSERT_GE(mfd, 0);
+	fd = open_by_handle_at(mfd, fh, O_RDONLY);
+	EXPECT_EQ(close(mfd), 0);
+	ASSERT_GE(fd, 0);
+
+	ASSERT_EQ(fstat(fd, &st), 0);
+	EXPECT_EQ(st.st_uid, ID_NS + 7);
+	EXPECT_EQ(st.st_gid, ID_NS + 7);
+
+	EXPECT_EQ(close(fd), 0);
+	EXPECT_EQ(close(ovl), 0);
+}
+
+TEST_HARNESS_MAIN

-- 
2.47.3