From 39796254f4458a532dcef8197ec84b7c80ad2b18 Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Fri, 2 Oct 2026 15:52:43 +0200 Subject: [PATCH] selftests/filesystems: check that an automount below an overlay layer is refused Make sure that we don't automount on top of internal things. Link: https://patch.msgid.link/20261002-work-mount-fixes-4-v1-12-dd44b89d44ce@kernel.org Signed-off-by: Christian Brauner (Amutable) --- .../filesystems/overlayfs/.gitignore | 1 + .../selftests/filesystems/overlayfs/Makefile | 1 + .../overlayfs/automount_in_layer.c | 174 ++++++++++++++++++ 3 files changed, 176 insertions(+) create mode 100644 tools/testing/selftests/filesystems/overlayfs/automount_in_layer.c diff --git a/tools/testing/selftests/filesystems/overlayfs/.gitignore b/tools/testing/selftests/filesystems/overlayfs/.gitignore index 077f7a128168..b343cc430051 100644 --- a/tools/testing/selftests/filesystems/overlayfs/.gitignore +++ b/tools/testing/selftests/filesystems/overlayfs/.gitignore @@ -2,3 +2,4 @@ dev_in_maps set_layers_via_fds idmapped_mounts +automount_in_layer diff --git a/tools/testing/selftests/filesystems/overlayfs/Makefile b/tools/testing/selftests/filesystems/overlayfs/Makefile index b3185f684add..382a59bda5e7 100644 --- a/tools/testing/selftests/filesystems/overlayfs/Makefile +++ b/tools/testing/selftests/filesystems/overlayfs/Makefile @@ -9,6 +9,7 @@ LOCAL_HDRS += ../wrappers.h log.h TEST_GEN_PROGS := dev_in_maps TEST_GEN_PROGS += set_layers_via_fds TEST_GEN_PROGS += idmapped_mounts +TEST_GEN_PROGS += automount_in_layer include ../../lib.mk diff --git a/tools/testing/selftests/filesystems/overlayfs/automount_in_layer.c b/tools/testing/selftests/filesystems/overlayfs/automount_in_layer.c new file mode 100644 index 000000000000..0476c4582bdf --- /dev/null +++ b/tools/testing/selftests/filesystems/overlayfs/automount_in_layer.c @@ -0,0 +1,174 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * A layer of an overlay is a private clone of the mount it was given and + * belongs to no mount namespace. fanotify hands out descriptors on it. An + * automount triggered through one has no namespace to go into: the open has + * to fail, not oops with namespace_sem held. + */ +#define _GNU_SOURCE +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include + +#include "../../kselftest_harness.h" + +#define DIR_LEN 64 +#define PATH_LEN 192 + +static bool have_fs(const char *name) +{ + char line[128]; + bool found = false; + FILE *f; + + f = fopen("/proc/filesystems", "re"); + if (!f) + return false; + while (fgets(line, sizeof(line), f)) { + char *nl = strchr(line, '\n'); + char *tab = strchr(line, '\t'); + + if (nl) + *nl = 0; + if (tab && !strcmp(tab + 1, name)) + found = true; + } + fclose(f); + return found; +} + +static int mnt_id_of(int fd) +{ + char path[64], buf[4096], *p; + ssize_t n; + int info; + + snprintf(path, sizeof(path), "/proc/self/fdinfo/%d", fd); + info = open(path, O_RDONLY | O_CLOEXEC); + if (info < 0) + return -1; + n = read(info, buf, sizeof(buf) - 1); + close(info); + if (n <= 0) + return -1; + buf[n] = 0; + p = strstr(buf, "mnt_id:"); + return p ? atoi(p + strlen("mnt_id:")) : -1; +} + +static bool on_debugfs(int fd) +{ + struct statfs sf; + + return !fstatfs(fd, &sf) && sf.f_type == DEBUGFS_MAGIC; +} + +FIXTURE(layer) { + char base[DIR_LEN]; + int fan; + int evfd; /* on the layer clone of the lower debugfs */ +}; + +FIXTURE_SETUP(layer) +{ + char lower[PATH_LEN], other[PATH_LEN], ovl[PATH_LEN], opts[2 * PATH_LEN + 16]; + struct fanotify_event_metadata *ev; + struct pollfd pfd; + char buf[4096]; + int lfd, lower_id; + ssize_t n; + DIR *d; + + self->fan = -1; + self->evfd = -1; + if (geteuid()) + SKIP(return, "test requires root"); + if (!have_fs("debugfs") || !have_fs("tracefs")) + SKIP(return, "test requires debugfs with the tracefs automount"); + if (!have_fs("overlay")) + SKIP(return, "test requires overlayfs"); + + snprintf(self->base, sizeof(self->base), "/tmp/layer.XXXXXX"); + ASSERT_NE(mkdtemp(self->base), NULL); + ASSERT_EQ(unshare(CLONE_NEWNS), 0); + ASSERT_EQ(mount("", "/", NULL, MS_REC | MS_PRIVATE, NULL), 0); + ASSERT_EQ(mount("tmpfs", self->base, "tmpfs", 0, "mode=0755"), 0); + snprintf(lower, sizeof(lower), "%s/lower", self->base); + snprintf(other, sizeof(other), "%s/other", self->base); + snprintf(ovl, sizeof(ovl), "%s/ovl", self->base); + ASSERT_EQ(mkdir(lower, 0755), 0); + ASSERT_EQ(mkdir(other, 0755), 0); + ASSERT_EQ(mkdir(ovl, 0755), 0); + ASSERT_EQ(mount("debugfs", lower, "debugfs", 0, NULL), 0); + snprintf(opts, sizeof(opts), "lowerdir=%s:%s", lower, other); + ASSERT_EQ(mount("overlay", ovl, "overlay", MS_RDONLY, opts), 0); + + self->fan = fanotify_init(FAN_CLASS_NOTIF | FAN_NONBLOCK | FAN_CLOEXEC, + O_RDONLY | O_CLOEXEC); + ASSERT_GE(self->fan, 0); + ASSERT_EQ(fanotify_mark(self->fan, FAN_MARK_ADD | FAN_MARK_FILESYSTEM, + FAN_OPEN | FAN_ONDIR, AT_FDCWD, lower), 0); + lfd = open(lower, O_RDONLY | O_DIRECTORY | O_CLOEXEC); + ASSERT_GE(lfd, 0); + lower_id = mnt_id_of(lfd); + close(lfd); + + /* the overlay opens its lower directory through the layer clone */ + d = opendir(ovl); + ASSERT_NE(d, NULL); + closedir(d); + pfd.fd = self->fan; + pfd.events = POLLIN; + ASSERT_EQ(poll(&pfd, 1, 5000), 1); + n = read(self->fan, buf, sizeof(buf)); + ASSERT_GT(n, 0); + for (ev = (void *)buf; FAN_EVENT_OK(ev, n); ev = FAN_EVENT_NEXT(ev, n)) { + if (ev->fd < 0) + continue; + if (self->evfd < 0 && on_debugfs(ev->fd) && + mnt_id_of(ev->fd) != lower_id) + self->evfd = ev->fd; + else + close(ev->fd); + } + ASSERT_GE(self->evfd, 0) + TH_LOG("no event on the layer clone"); +} + +FIXTURE_TEARDOWN(layer) +{ + if (self->evfd >= 0) + close(self->evfd); + if (self->fan >= 0) + close(self->fan); + umount2(self->base, MNT_DETACH); + rmdir(self->base); +} + +TEST_F(layer, automount_below_the_clone_is_refused) +{ + struct stat st; + int fd; + + /* the automount point is there */ + ASSERT_EQ(fstatat(self->evfd, "tracing", &st, AT_NO_AUTOMOUNT), 0); + /* the clone is in no namespace, so the automount has nowhere to go */ + fd = openat(self->evfd, "tracing", O_RDONLY | O_DIRECTORY | O_CLOEXEC); + EXPECT_LT(fd, 0); + EXPECT_EQ(errno, EINVAL); + if (fd >= 0) + close(fd); +} + +TEST_HARNESS_MAIN