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mount-util: Add mount_fd_clone() helper

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The kernel prevents you from open_tree()'ing an open_tree() fd unless it
was created from the caller's mount namespace. For various use cases, we
want to be able to open_tree() arbitrary mount file descriptors. Turns
out there's a way go get around it by mounting the mount file descriptor
in a throw-away mount namespace and then open_tree()'ing the mount file
descriptor. Let's implement this as a new helper mount_fd_clone() and
add a test for it.

Because move_mount()'ing the original fd makes it pretty useless as it
can't be move_mount()'ed again, we optionally make a second clone which
can replace the original fd so it can be cloned again later.
This commit is contained in:
Daan De Meyer
2025-12-14 11:31:59 +01:00
parent 0859fe3f32
commit 273c6bc045
3 changed files with 165 additions and 0 deletions
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src/test/test-mount-util.c
+63
View File
@@ -19,6 +19,7 @@
#include "process-util.h"
#include "random-util.h"
#include "rm-rf.h"
#include "socket-util.h"
#include "string-util.h"
#include "strv.h"
#include "tests.h"
@@ -541,4 +542,66 @@ TEST(umountat) {
ASSERT_ERROR(umountat_detach_verbose(LOG_ERR, dfd, "foo"), EINVAL);
}
TEST(mount_fd_clone) {
_cleanup_(rm_rf_physical_and_freep) char *t = NULL;
_cleanup_close_pair_ int fds[2] = EBADF_PAIR;
int r;
CHECK_PRIV;
ASSERT_OK(mkdtemp_malloc(NULL, &t));
/* Set up a socket pair to transfer the mount fd from the child (in a different mountns) to us. */
ASSERT_OK_ERRNO(socketpair(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC, 0, fds));
r = ASSERT_OK(safe_fork_full(
"(mount-fd-clone-setup)",
/* stdio_fds= */ NULL,
&fds[1], 1,
FORK_COMMON_FLAGS,
NULL));
if (r == 0) {
/* Create a tmpfs mount in this child's mountns. */
ASSERT_OK(mount_nofollow_verbose(LOG_ERR, "tmpfs", t, "tmpfs", 0, NULL));
/* Create a file in it to verify the mount later. */
_cleanup_free_ char *marker = ASSERT_NOT_NULL(path_join(t, "marker"));
ASSERT_OK(touch(marker));
/* Clone the mount as a detached mount fd. */
_cleanup_close_ int mount_fd = ASSERT_OK_ERRNO(open_tree(AT_FDCWD, t, OPEN_TREE_CLONE|OPEN_TREE_CLOEXEC));
/* Send the mount fd to the parent. */
ASSERT_OK(send_one_fd(fds[1], mount_fd, 0));
_exit(EXIT_SUCCESS);
}
fds[1] = safe_close(fds[1]);
/* Parent: Receive the mount fd, clone it with mount_fd_clone(), and verify we can attach it. */
_cleanup_close_ int foreign_mount_fd = ASSERT_OK(receive_one_fd(fds[0], 0));
_cleanup_close_ int first_clone = ASSERT_OK(
mount_fd_clone(foreign_mount_fd, /* recursive= */ true, &foreign_mount_fd));
_cleanup_close_ _unused_ int second_clone = ASSERT_OK(
mount_fd_clone(foreign_mount_fd, /* recursive= */ true, /* replacement_fd= */ NULL));
_cleanup_free_ char *target = ASSERT_NOT_NULL(path_join(t, "target"));
ASSERT_OK_ERRNO(mkdir(target, 0755));
r = ASSERT_OK(safe_fork_full(
"(mount-fd-clone-verify)",
/* stdio_fds= */ NULL,
&first_clone, 1,
FORK_COMMON_FLAGS,
NULL));
if (r == 0) {
ASSERT_OK_ERRNO(move_mount(first_clone, "", AT_FDCWD, target, MOVE_MOUNT_F_EMPTY_PATH));
_cleanup_free_ char *marker = ASSERT_NOT_NULL(path_join(target, "marker"));
ASSERT_OK_ERRNO(access(marker, F_OK));
_exit(EXIT_SUCCESS);
}
}
DEFINE_TEST_MAIN(LOG_DEBUG);