Adding upstream version 255.4.upstream/255.4

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1209 insertions, 0 deletions

diff --git a/src/shared/loop-util.c b/src/shared/loop-util.c
new file mode 100644
index 0000000..5860303
--- /dev/null
+++ b/src/shared/loop-util.c
@@ -0,0 +1,1209 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#if HAVE_VALGRIND_MEMCHECK_H
+#include <valgrind/memcheck.h>
+#endif
+
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/blkpg.h>
+#include <linux/fs.h>
+#include <linux/loop.h>
+#include <sys/file.h>
+#include <sys/ioctl.h>
+#include <unistd.h>
+
+#include "sd-device.h"
+
+#include "alloc-util.h"
+#include "blockdev-util.h"
+#include "data-fd-util.h"
+#include "device-util.h"
+#include "devnum-util.h"
+#include "dissect-image.h"
+#include "env-util.h"
+#include "errno-util.h"
+#include "fd-util.h"
+#include "fs-util.h"
+#include "fileio.h"
+#include "loop-util.h"
+#include "missing_loop.h"
+#include "parse-util.h"
+#include "path-util.h"
+#include "random-util.h"
+#include "stat-util.h"
+#include "stdio-util.h"
+#include "string-util.h"
+#include "tmpfile-util.h"
+
+static void cleanup_clear_loop_close(int *fd) {
+        if (*fd < 0)
+                return;
+
+        (void) ioctl(*fd, LOOP_CLR_FD);
+        (void) safe_close(*fd);
+}
+
+static int loop_is_bound(int fd) {
+        struct loop_info64 info;
+
+        if (ioctl(ASSERT_FD(fd), LOOP_GET_STATUS64, &info) < 0) {
+                if (errno == ENXIO)
+                        return false; /* not bound! */
+
+                return -errno;
+        }
+
+        return true; /* bound! */
+}
+
+static int get_current_uevent_seqnum(uint64_t *ret) {
+        _cleanup_free_ char *p = NULL;
+        int r;
+
+        r = read_full_virtual_file("/sys/kernel/uevent_seqnum", &p, NULL);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to read current uevent sequence number: %m");
+
+        r = safe_atou64(strstrip(p), ret);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to parse current uevent sequence number: %s", p);
+
+        return 0;
+}
+
+static int open_lock_fd(int primary_fd, int operation) {
+        _cleanup_close_ int lock_fd = -EBADF;
+
+        assert(IN_SET(operation & ~LOCK_NB, LOCK_SH, LOCK_EX));
+
+        lock_fd = fd_reopen(ASSERT_FD(primary_fd), O_RDONLY|O_CLOEXEC|O_NONBLOCK|O_NOCTTY);
+        if (lock_fd < 0)
+                return lock_fd;
+
+        if (flock(lock_fd, operation) < 0)
+                return -errno;
+
+        return TAKE_FD(lock_fd);
+}
+
+static int loop_configure_verify_direct_io(int fd, const struct loop_config *c) {
+        assert(fd >= 0);
+        assert(c);
+
+        if (FLAGS_SET(c->info.lo_flags, LO_FLAGS_DIRECT_IO)) {
+                struct loop_info64 info;
+
+                if (ioctl(fd, LOOP_GET_STATUS64, &info) < 0)
+                        return log_debug_errno(errno, "Failed to issue LOOP_GET_STATUS64: %m");
+
+#if HAVE_VALGRIND_MEMCHECK_H
+                VALGRIND_MAKE_MEM_DEFINED(&info, sizeof(info));
+#endif
+
+                /* On older kernels (<= 5.3) it was necessary to set the block size of the loopback block
+                 * device to the logical block size of the underlying file system. Since there was no nice
+                 * way to query the value, we are not bothering to do this however. On newer kernels the
+                 * block size is propagated automatically and does not require intervention from us. We'll
+                 * check here if enabling direct IO worked, to make this easily debuggable however.
+                 *
+                 * (Should anyone really care and actually wants direct IO on old kernels: it might be worth
+                 * enabling direct IO with iteratively larger block sizes until it eventually works.)
+                 *
+                 * On older kernels (e.g.: 5.10) when this is attempted on a file stored on a dm-crypt
+                 * backed partition the kernel will start returning I/O errors when accessing the mounted
+                 * loop device, so return a recognizable error that causes the operation to be started
+                 * from scratch without the LO_FLAGS_DIRECT_IO flag. */
+                if (!FLAGS_SET(info.lo_flags, LO_FLAGS_DIRECT_IO))
+                        return log_debug_errno(
+                                        SYNTHETIC_ERRNO(ENOANO),
+                                        "Could not enable direct IO mode, retrying in buffered IO mode.");
+        }
+
+        return 0;
+}
+
+static int loop_configure_verify(int fd, const struct loop_config *c) {
+        bool broken = false;
+        int r;
+
+        assert(fd >= 0);
+        assert(c);
+
+        if (c->block_size != 0) {
+                uint32_t ssz;
+
+                r = blockdev_get_sector_size(fd, &ssz);
+                if (r < 0)
+                        return r;
+
+                if (ssz != c->block_size) {
+                        log_debug("LOOP_CONFIGURE didn't honour requested block size %" PRIu32 ", got %" PRIu32 " instead. Ignoring.", c->block_size, ssz);
+                        broken = true;
+                }
+        }
+
+        if (c->info.lo_sizelimit != 0) {
+                /* Kernel 5.8 vanilla doesn't properly propagate the size limit into the
+                 * block device. If it's used, let's immediately check if it had the desired
+                 * effect hence. And if not use classic LOOP_SET_STATUS64. */
+                uint64_t z;
+
+                if (ioctl(fd, BLKGETSIZE64, &z) < 0)
+                        return -errno;
+
+                if (z != c->info.lo_sizelimit) {
+                        log_debug("LOOP_CONFIGURE is broken, doesn't honour .info.lo_sizelimit. Falling back to LOOP_SET_STATUS64.");
+                        broken = true;
+                }
+        }
+
+        if (FLAGS_SET(c->info.lo_flags, LO_FLAGS_PARTSCAN)) {
+                /* Kernel 5.8 vanilla doesn't properly propagate the partition scanning flag
+                 * into the block device. Let's hence verify if things work correctly here
+                 * before returning. */
+
+                r = blockdev_partscan_enabled(fd);
+                if (r < 0)
+                        return r;
+                if (r == 0) {
+                        log_debug("LOOP_CONFIGURE is broken, doesn't honour LO_FLAGS_PARTSCAN. Falling back to LOOP_SET_STATUS64.");
+                        broken = true;
+                }
+        }
+
+        r = loop_configure_verify_direct_io(fd, c);
+        if (r < 0)
+                return r;
+
+        return !broken;
+}
+
+static int loop_configure_fallback(int fd, const struct loop_config *c) {
+        struct loop_info64 info_copy;
+        int r;
+
+        assert(fd >= 0);
+        assert(c);
+
+        /* Only some of the flags LOOP_CONFIGURE can set are also settable via LOOP_SET_STATUS64, hence mask
+         * them out. */
+        info_copy = c->info;
+        info_copy.lo_flags &= LOOP_SET_STATUS_SETTABLE_FLAGS;
+
+        /* Since kernel commit 5db470e229e22b7eda6e23b5566e532c96fb5bc3 (kernel v5.0) the LOOP_SET_STATUS64
+         * ioctl can return EAGAIN in case we change the info.lo_offset field, if someone else is accessing the
+         * block device while we try to reconfigure it. This is a pretty common case, since udev might
+         * instantly start probing the device as soon as we attach an fd to it. Hence handle it in two ways:
+         * first, let's take the BSD lock to ensure that udev will not step in between the point in
+         * time where we attach the fd and where we reconfigure the device. Secondly, let's wait 50ms on
+         * EAGAIN and retry. The former should be an efficient mechanism to avoid we have to wait 50ms
+         * needlessly if we are just racing against udev. The latter is protection against all other cases,
+         * i.e. peers that do not take the BSD lock. */
+
+        for (unsigned n_attempts = 0;;) {
+                if (ioctl(fd, LOOP_SET_STATUS64, &info_copy) >= 0)
+                        break;
+
+                if (errno != EAGAIN || ++n_attempts >= 64)
+                        return log_debug_errno(errno, "Failed to configure loopback block device: %m");
+
+                /* Sleep some random time, but at least 10ms, at most 250ms. Increase the delay the more
+                 * failed attempts we see */
+                (void) usleep_safe(UINT64_C(10) * USEC_PER_MSEC +
+                              random_u64_range(UINT64_C(240) * USEC_PER_MSEC * n_attempts/64));
+        }
+
+        /* Work around a kernel bug, where changing offset/size of the loopback device doesn't correctly
+         * invalidate the buffer cache. For details see:
+         *
+         *     https://android.googlesource.com/platform/system/apex/+/bef74542fbbb4cd629793f4efee8e0053b360570
+         *
+         * This was fixed in kernel 5.0, see:
+         *
+         *     https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5db470e229e22b7eda6e23b5566e532c96fb5bc3
+         *
+         * We'll run the work-around here in the legacy LOOP_SET_STATUS64 codepath. In the LOOP_CONFIGURE
+         * codepath above it should not be necessary. */
+        if (c->info.lo_offset != 0 || c->info.lo_sizelimit != 0)
+                if (ioctl(fd, BLKFLSBUF, 0) < 0)
+                        log_debug_errno(errno, "Failed to issue BLKFLSBUF ioctl, ignoring: %m");
+
+        /* If a block size is requested then try to configure it. If that doesn't work, ignore errors, but
+         * afterwards, let's validate what is in effect, and if it doesn't match what we want, fail */
+        if (c->block_size != 0) {
+                uint32_t ssz;
+
+                if (ioctl(fd, LOOP_SET_BLOCK_SIZE, (unsigned long) c->block_size) < 0)
+                        log_debug_errno(errno, "Failed to set sector size, ignoring: %m");
+
+                r = blockdev_get_sector_size(fd, &ssz);
+                if (r < 0)
+                        return log_debug_errno(r, "Failed to read sector size: %m");
+                if (ssz != c->block_size)
+                        return log_debug_errno(SYNTHETIC_ERRNO(EIO), "Sector size of loopback device doesn't match what we requested, refusing.");
+        }
+
+        /* LO_FLAGS_DIRECT_IO is a flags we need to configure via explicit ioctls. */
+        if (FLAGS_SET(c->info.lo_flags, LO_FLAGS_DIRECT_IO))
+                if (ioctl(fd, LOOP_SET_DIRECT_IO, 1UL) < 0)
+                        log_debug_errno(errno, "Failed to enable direct IO mode, ignoring: %m");
+
+        return loop_configure_verify_direct_io(fd, c);
+}
+
+static int loop_configure(
+                int nr,
+                int open_flags,
+                int lock_op,
+                const struct loop_config *c,
+                LoopDevice **ret) {
+
+        static bool loop_configure_broken = false;
+
+        _cleanup_(sd_device_unrefp) sd_device *dev = NULL;
+        _cleanup_(cleanup_clear_loop_close) int loop_with_fd = -EBADF; /* This must be declared before lock_fd. */
+        _cleanup_close_ int fd = -EBADF, lock_fd = -EBADF;
+        _cleanup_free_ char *node = NULL;
+        uint64_t diskseq = 0, seqnum = UINT64_MAX;
+        usec_t timestamp = USEC_INFINITY;
+        dev_t devno;
+        int r;
+
+        assert(nr >= 0);
+        assert(c);
+        assert(ret);
+
+        if (asprintf(&node, "/dev/loop%i", nr) < 0)
+                return log_oom_debug();
+
+        r = sd_device_new_from_devname(&dev, node);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to create sd_device object for \"%s\": %m", node);
+
+        r = sd_device_get_devnum(dev, &devno);
+        if (r < 0)
+                return log_device_debug_errno(dev, r, "Failed to get devnum: %m");
+
+        fd = sd_device_open(dev, O_CLOEXEC|O_NONBLOCK|O_NOCTTY|open_flags);
+        if (fd < 0)
+                return log_device_debug_errno(dev, fd, "Failed to open device: %m");
+
+        /* Let's lock the device before we do anything. We take the BSD lock on a second, separately opened
+         * fd for the device. udev after all watches for close() events (specifically IN_CLOSE_WRITE) on
+         * block devices to reprobe them, hence by having a separate fd we will later close() we can ensure
+         * we trigger udev after everything is done. If we'd lock our own fd instead and keep it open for a
+         * long time udev would possibly never run on it again, even though the fd is unlocked, simply
+         * because we never close() it. It also has the nice benefit we can use the _cleanup_close_ logic to
+         * automatically release the lock, after we are done. */
+        lock_fd = open_lock_fd(fd, LOCK_EX);
+        if (lock_fd < 0)
+                return log_device_debug_errno(dev, lock_fd, "Failed to acquire lock: %m");
+
+        log_device_debug(dev, "Acquired exclusive lock.");
+
+        /* Let's see if backing file is really unattached. Someone may already attach a backing file without
+         * taking BSD lock. */
+        r = loop_is_bound(fd);
+        if (r < 0)
+                return log_device_debug_errno(dev, r, "Failed to check if the loopback block device is bound: %m");
+        if (r > 0)
+                return log_device_debug_errno(dev, SYNTHETIC_ERRNO(EBUSY),
+                                              "The loopback block device is already bound, ignoring.");
+
+        /* Let's see if the device is really detached, i.e. currently has no associated partition block
+         * devices. On various kernels (such as 5.8) it is possible to have a loopback block device that
+         * superficially is detached but still has partition block devices associated for it. Let's then
+         * manually remove the partitions via BLKPG, and tell the caller we did that via EUCLEAN, so they try
+         * again. */
+        r = block_device_remove_all_partitions(dev, fd);
+        if (r < 0)
+                return log_device_debug_errno(dev, r, "Failed to remove partitions on the loopback block device: %m");
+        if (r > 0)
+                /* Removed all partitions. Let's report this to the caller, to try again, and count this as
+                 * an attempt. */
+                return log_device_debug_errno(dev, SYNTHETIC_ERRNO(EUCLEAN),
+                                              "Removed partitions on the loopback block device.");
+
+        if (!loop_configure_broken) {
+                /* Acquire uevent seqnum immediately before attaching the loopback device. This allows
+                 * callers to ignore all uevents with a seqnum before this one, if they need to associate
+                 * uevent with this attachment. Doing so isn't race-free though, as uevents that happen in
+                 * the window between this reading of the seqnum, and the LOOP_CONFIGURE call might still be
+                 * mistaken as originating from our attachment, even though might be caused by an earlier
+                 * use. But doing this at least shortens the race window a bit. */
+                r = get_current_uevent_seqnum(&seqnum);
+                if (r < 0)
+                        return log_device_debug_errno(dev, r, "Failed to get the current uevent seqnum: %m");
+
+                timestamp = now(CLOCK_MONOTONIC);
+
+                if (ioctl(fd, LOOP_CONFIGURE, c) < 0) {
+                        /* Do fallback only if LOOP_CONFIGURE is not supported, propagate all other
+                         * errors. Note that the kernel is weird: non-existing ioctls currently return EINVAL
+                         * rather than ENOTTY on loopback block devices. They should fix that in the kernel,
+                         * but in the meantime we accept both here. */
+                        if (!ERRNO_IS_NOT_SUPPORTED(errno) && errno != EINVAL)
+                                return log_device_debug_errno(dev, errno, "ioctl(LOOP_CONFIGURE) failed: %m");
+
+                        loop_configure_broken = true;
+                } else {
+                        loop_with_fd = TAKE_FD(fd);
+
+                        r = loop_configure_verify(loop_with_fd, c);
+                        if (r < 0)
+                                return log_device_debug_errno(dev, r, "Failed to verify if loopback block device is correctly configured: %m");
+                        if (r == 0) {
+                                /* LOOP_CONFIGURE doesn't work. Remember that. */
+                                loop_configure_broken = true;
+
+                                /* We return EBUSY here instead of retrying immediately with LOOP_SET_FD,
+                                 * because LOOP_CLR_FD is async: if the operation cannot be executed right
+                                 * away it just sets the autoclear flag on the device. This means there's a
+                                 * good chance we cannot actually reuse the loopback device right-away. Hence
+                                 * let's assume it's busy, avoid the trouble and let the calling loop call us
+                                 * again with a new, likely unused device. */
+                                return -EBUSY;
+                        }
+                }
+        }
+
+        if (loop_configure_broken) {
+                /* Let's read the seqnum again, to shorten the window. */
+                r = get_current_uevent_seqnum(&seqnum);
+                if (r < 0)
+                        return log_device_debug_errno(dev, r, "Failed to get the current uevent seqnum: %m");
+
+                timestamp = now(CLOCK_MONOTONIC);
+
+                if (ioctl(fd, LOOP_SET_FD, c->fd) < 0)
+                        return log_device_debug_errno(dev, errno, "ioctl(LOOP_SET_FD) failed: %m");
+
+                loop_with_fd = TAKE_FD(fd);
+
+                r = loop_configure_fallback(loop_with_fd, c);
+                if (r < 0)
+                        return r;
+        }
+
+        r = fd_get_diskseq(loop_with_fd, &diskseq);
+        if (r < 0 && r != -EOPNOTSUPP)
+                return log_device_debug_errno(dev, r, "Failed to get diskseq: %m");
+
+        switch (lock_op & ~LOCK_NB) {
+        case LOCK_EX: /* Already in effect */
+                break;
+        case LOCK_SH: /* Downgrade */
+                if (flock(lock_fd, lock_op) < 0)
+                        return log_device_debug_errno(dev, errno, "Failed to downgrade lock level: %m");
+                break;
+        case LOCK_UN: /* Release */
+                lock_fd = safe_close(lock_fd);
+                break;
+        default:
+                assert_not_reached();
+        }
+
+        LoopDevice *d = new(LoopDevice, 1);
+        if (!d)
+                return log_oom_debug();
+
+        *d = (LoopDevice) {
+                .n_ref = 1,
+                .fd = TAKE_FD(loop_with_fd),
+                .lock_fd = TAKE_FD(lock_fd),
+                .node = TAKE_PTR(node),
+                .nr = nr,
+                .devno = devno,
+                .dev = TAKE_PTR(dev),
+                .diskseq = diskseq,
+                .uevent_seqnum_not_before = seqnum,
+                .timestamp_not_before = timestamp,
+                .sector_size = c->block_size,
+        };
+
+        *ret = TAKE_PTR(d);
+        return 0;
+}
+
+static int loop_device_make_internal(
+                const char *path,
+                int fd,
+                int open_flags,
+                uint64_t offset,
+                uint64_t size,
+                uint32_t sector_size,
+                uint32_t loop_flags,
+                int lock_op,
+                LoopDevice **ret) {
+
+        _cleanup_(loop_device_unrefp) LoopDevice *d = NULL;
+        _cleanup_close_ int reopened_fd = -EBADF, control = -EBADF;
+        _cleanup_free_ char *backing_file = NULL;
+        struct loop_config config;
+        int r, f_flags;
+        struct stat st;
+
+        assert(ret);
+        assert(IN_SET(open_flags, O_RDWR, O_RDONLY));
+
+        if (fstat(ASSERT_FD(fd), &st) < 0)
+                return -errno;
+
+        if (S_ISBLK(st.st_mode)) {
+                if (offset == 0 && IN_SET(size, 0, UINT64_MAX))
+                        /* If this is already a block device and we are supposed to cover the whole of it
+                         * then store an fd to the original open device node — and do not actually create an
+                         * unnecessary loopback device for it. */
+                        return loop_device_open_from_fd(fd, open_flags, lock_op, ret);
+        } else {
+                r = stat_verify_regular(&st);
+                if (r < 0)
+                        return r;
+        }
+
+        if (path) {
+                r = path_make_absolute_cwd(path, &backing_file);
+                if (r < 0)
+                        return r;
+
+                path_simplify(backing_file);
+        } else {
+                r = fd_get_path(fd, &backing_file);
+                if (r < 0)
+                        return r;
+        }
+
+        f_flags = fcntl(fd, F_GETFL);
+        if (f_flags < 0)
+                return -errno;
+
+        if (FLAGS_SET(loop_flags, LO_FLAGS_DIRECT_IO) != FLAGS_SET(f_flags, O_DIRECT)) {
+                /* If LO_FLAGS_DIRECT_IO is requested, then make sure we have the fd open with O_DIRECT, as
+                 * that's required. Conversely, if it's off require that O_DIRECT is off too (that's because
+                 * new kernels will implicitly enable LO_FLAGS_DIRECT_IO if O_DIRECT is set).
+                 *
+                 * Our intention here is that LO_FLAGS_DIRECT_IO is the primary knob, and O_DIRECT derived
+                 * from that automatically. */
+
+                reopened_fd = fd_reopen(fd, (FLAGS_SET(loop_flags, LO_FLAGS_DIRECT_IO) ? O_DIRECT : 0)|O_CLOEXEC|O_NONBLOCK|open_flags);
+                if (reopened_fd < 0) {
+                        if (!FLAGS_SET(loop_flags, LO_FLAGS_DIRECT_IO))
+                                return log_debug_errno(reopened_fd, "Failed to reopen file descriptor without O_DIRECT: %m");
+
+                        /* Some file systems might not support O_DIRECT, let's gracefully continue without it then. */
+                        log_debug_errno(reopened_fd, "Failed to enable O_DIRECT for backing file descriptor for loopback device. Continuing without.");
+                        loop_flags &= ~LO_FLAGS_DIRECT_IO;
+                } else
+                        fd = reopened_fd; /* From now on, operate on our new O_DIRECT fd */
+        }
+
+        control = open("/dev/loop-control", O_RDWR|O_CLOEXEC|O_NOCTTY|O_NONBLOCK);
+        if (control < 0)
+                return -errno;
+
+        if (sector_size == 0)
+                /* If no sector size is specified, default to the classic default */
+                sector_size = 512;
+        else if (sector_size == UINT32_MAX) {
+
+                if (S_ISBLK(st.st_mode))
+                        /* If the sector size is specified as UINT32_MAX we'll propagate the sector size of
+                         * the underlying block device. */
+                        r = blockdev_get_sector_size(fd, &sector_size);
+                else {
+                        _cleanup_close_ int non_direct_io_fd = -EBADF;
+                        int probe_fd;
+
+                        assert(S_ISREG(st.st_mode));
+
+                        /* If sector size is specified as UINT32_MAX, we'll try to probe the right sector
+                         * size of the image in question by looking for the GPT partition header at various
+                         * offsets. This of course only works if the image already has a disk label.
+                         *
+                         * So here we actually want to read the file contents ourselves. This is quite likely
+                         * not going to work if we managed to enable O_DIRECT, because in such a case there
+                         * are some pretty strict alignment requirements to offset, size and target, but
+                         * there's no way to query what alignment specifically is actually required. Hence,
+                         * let's avoid the mess, and temporarily open an fd without O_DIRECT for the probing
+                         * logic. */
+
+                        if (FLAGS_SET(loop_flags, LO_FLAGS_DIRECT_IO)) {
+                                non_direct_io_fd = fd_reopen(fd, O_RDONLY|O_CLOEXEC|O_NONBLOCK);
+                                if (non_direct_io_fd < 0)
+                                        return non_direct_io_fd;
+
+                                probe_fd = non_direct_io_fd;
+                        } else
+                                probe_fd = fd;
+
+                        r = probe_sector_size(probe_fd, &sector_size);
+                }
+                if (r < 0)
+                        return r;
+        }
+
+        config = (struct loop_config) {
+                .fd = fd,
+                .block_size = sector_size,
+                .info = {
+                        /* Use the specified flags, but configure the read-only flag from the open flags, and force autoclear */
+                        .lo_flags = (loop_flags & ~LO_FLAGS_READ_ONLY) | ((open_flags & O_ACCMODE) == O_RDONLY ? LO_FLAGS_READ_ONLY : 0) | LO_FLAGS_AUTOCLEAR,
+                        .lo_offset = offset,
+                        .lo_sizelimit = size == UINT64_MAX ? 0 : size,
+                },
+        };
+
+        /* Loop around LOOP_CTL_GET_FREE, since at the moment we attempt to open the returned device it might
+         * be gone already, taken by somebody else racing against us. */
+        for (unsigned n_attempts = 0;;) {
+                usec_t usec;
+                int nr;
+
+                /* Let's take a lock on the control device first. On a busy system, where many programs
+                 * attempt to allocate a loopback device at the same time, we might otherwise keep looping
+                 * around relatively heavy operations: asking for a free loopback device, then opening it,
+                 * validating it, attaching something to it. Let's serialize this whole operation, to make
+                 * unnecessary busywork less likely. Note that this is just something we do to optimize our
+                 * own code (and whoever else decides to use LOCK_EX locks for this), taking this lock is not
+                 * necessary, it just means it's less likely we have to iterate through this loop again and
+                 * again if our own code races against our own code.
+                 *
+                 * Note: our lock protocol is to take the /dev/loop-control lock first, and the block device
+                 * lock second, if both are taken, and always in this order, to avoid ABBA locking issues. */
+                if (flock(control, LOCK_EX) < 0)
+                        return -errno;
+
+                nr = ioctl(control, LOOP_CTL_GET_FREE);
+                if (nr < 0)
+                        return -errno;
+
+                r = loop_configure(nr, open_flags, lock_op, &config, &d);
+                if (r >= 0)
+                        break;
+
+                /* -ENODEV or friends: Somebody might've gotten the same number from the kernel, used the
+                 * device, and called LOOP_CTL_REMOVE on it. Let's retry with a new number.
+                 * -EBUSY: a file descriptor is already bound to the loopback block device.
+                 * -EUCLEAN: some left-over partition devices that were cleaned up.
+                 * -ENOANO: we tried to use LO_FLAGS_DIRECT_IO but the kernel rejected it. */
+                if (!ERRNO_IS_DEVICE_ABSENT(r) && !IN_SET(r, -EBUSY, -EUCLEAN, -ENOANO))
+                        return r;
+
+                /* OK, this didn't work, let's try again a bit later, but first release the lock on the
+                 * control device */
+                if (flock(control, LOCK_UN) < 0)
+                        return -errno;
+
+                if (++n_attempts >= 64) /* Give up eventually */
+                        return -EBUSY;
+
+                /* If we failed to enable direct IO mode, let's retry without it. We restart the process as
+                 * on some combination of kernel version and storage filesystem, the kernel is very unhappy
+                 * about a failed DIRECT_IO enablement and throws I/O errors. */
+                if (r == -ENOANO && FLAGS_SET(config.info.lo_flags, LO_FLAGS_DIRECT_IO)) {
+                        config.info.lo_flags &= ~LO_FLAGS_DIRECT_IO;
+                        open_flags &= ~O_DIRECT;
+
+                        int non_direct_io_fd = fd_reopen(config.fd, O_CLOEXEC|O_NONBLOCK|open_flags);
+                        if (non_direct_io_fd < 0)
+                                return log_debug_errno(
+                                                non_direct_io_fd,
+                                                "Failed to reopen file descriptor without O_DIRECT: %m");
+
+                        safe_close(reopened_fd);
+                        fd = config.fd = /* For cleanups */ reopened_fd = non_direct_io_fd;
+                }
+
+                /* Wait some random time, to make collision less likely. Let's pick a random time in the
+                 * range 0ms…250ms, linearly scaled by the number of failed attempts. */
+                usec = random_u64_range(UINT64_C(10) * USEC_PER_MSEC +
+                                        UINT64_C(240) * USEC_PER_MSEC * n_attempts/64);
+                log_debug("Trying again after %s.", FORMAT_TIMESPAN(usec, USEC_PER_MSEC));
+                (void) usleep_safe(usec);
+        }
+
+        d->backing_file = TAKE_PTR(backing_file);
+        d->backing_inode = st.st_ino;
+        d->backing_devno = st.st_dev;
+
+        log_debug("Successfully acquired %s, devno=%u:%u, nr=%i, diskseq=%" PRIu64,
+                  d->node,
+                  major(d->devno), minor(d->devno),
+                  d->nr,
+                  d->diskseq);
+
+        *ret = TAKE_PTR(d);
+        return 0;
+}
+
+static uint32_t loop_flags_mangle(uint32_t loop_flags) {
+        int r;
+
+        r = getenv_bool("SYSTEMD_LOOP_DIRECT_IO");
+        if (r < 0 && r != -ENXIO)
+                log_debug_errno(r, "Failed to parse $SYSTEMD_LOOP_DIRECT_IO, ignoring: %m");
+
+        return UPDATE_FLAG(loop_flags, LO_FLAGS_DIRECT_IO, r != 0); /* Turn on LO_FLAGS_DIRECT_IO by default, unless explicitly configured to off. */
+}
+
+int loop_device_make(
+                int fd,
+                int open_flags,
+                uint64_t offset,
+                uint64_t size,
+                uint32_t sector_size,
+                uint32_t loop_flags,
+                int lock_op,
+                LoopDevice **ret) {
+
+        assert(fd >= 0);
+        assert(ret);
+
+        return loop_device_make_internal(
+                        NULL,
+                        fd,
+                        open_flags,
+                        offset,
+                        size,
+                        sector_size,
+                        loop_flags_mangle(loop_flags),
+                        lock_op,
+                        ret);
+}
+
+int loop_device_make_by_path_at(
+                int dir_fd,
+                const char *path,
+                int open_flags,
+                uint32_t sector_size,
+                uint32_t loop_flags,
+                int lock_op,
+                LoopDevice **ret) {
+
+        int r, basic_flags, direct_flags, rdwr_flags;
+        _cleanup_close_ int fd = -EBADF;
+        bool direct = false;
+
+        assert(dir_fd >= 0 || dir_fd == AT_FDCWD);
+        assert(path);
+        assert(ret);
+        assert(open_flags < 0 || IN_SET(open_flags, O_RDWR, O_RDONLY));
+
+        /* Passing < 0 as open_flags here means we'll try to open the device writable if we can, retrying
+         * read-only if we cannot. */
+
+        loop_flags = loop_flags_mangle(loop_flags);
+
+        /* Let's open with O_DIRECT if we can. But not all file systems support that, hence fall back to
+         * non-O_DIRECT mode automatically, if it fails. */
+
+        basic_flags = O_CLOEXEC|O_NONBLOCK|O_NOCTTY;
+        direct_flags = FLAGS_SET(loop_flags, LO_FLAGS_DIRECT_IO) ? O_DIRECT : 0;
+        rdwr_flags = open_flags >= 0 ? open_flags : O_RDWR;
+
+        fd = xopenat(dir_fd, path, basic_flags|direct_flags|rdwr_flags, /* xopen_flags = */ 0, /* mode = */ 0);
+        if (fd < 0 && direct_flags != 0) /* If we had O_DIRECT on, and things failed with that, let's immediately try again without */
+                fd = xopenat(dir_fd, path, basic_flags|rdwr_flags, /* xopen_flags = */ 0, /* mode = */ 0);
+        else
+                direct = direct_flags != 0;
+        if (fd < 0) {
+                r = fd;
+
+                /* Retry read-only? */
+                if (open_flags >= 0 || !(ERRNO_IS_PRIVILEGE(r) || r == -EROFS))
+                        return r;
+
+                fd = xopenat(dir_fd, path, basic_flags|direct_flags|O_RDONLY, /* xopen_flags = */ 0, /* mode = */ 0);
+                if (fd < 0 && direct_flags != 0) /* as above */
+                        fd = xopenat(dir_fd, path, basic_flags|O_RDONLY, /* xopen_flags = */ 0, /* mode = */ 0);
+                else
+                        direct = direct_flags != 0;
+                if (fd < 0)
+                        return r; /* Propagate original error */
+
+                open_flags = O_RDONLY;
+        } else if (open_flags < 0)
+                open_flags = O_RDWR;
+
+        log_debug("Opened '%s' in %s access mode%s, with O_DIRECT %s%s.",
+                  path,
+                  open_flags == O_RDWR ? "O_RDWR" : "O_RDONLY",
+                  open_flags != rdwr_flags ? " (O_RDWR was requested but not allowed)" : "",
+                  direct ? "enabled" : "disabled",
+                  direct != (direct_flags != 0) ? " (O_DIRECT was requested but not supported)" : "");
+
+        return loop_device_make_internal(
+                        dir_fd == AT_FDCWD ? path : NULL,
+                        fd,
+                        open_flags,
+                        /* offset = */ 0,
+                        /* size = */ 0,
+                        sector_size,
+                        loop_flags,
+                        lock_op,
+                        ret);
+}
+
+int loop_device_make_by_path_memory(
+                const char *path,
+                int open_flags,
+                uint32_t sector_size,
+                uint32_t loop_flags,
+                int lock_op,
+                LoopDevice **ret) {
+
+        _cleanup_close_ int fd = -EBADF, mfd = -EBADF;
+        _cleanup_free_ char *fn = NULL;
+        struct stat st;
+        int r;
+
+        assert(path);
+        assert(IN_SET(open_flags, O_RDWR, O_RDONLY));
+        assert(ret);
+
+        loop_flags &= ~LO_FLAGS_DIRECT_IO; /* memfds don't support O_DIRECT, hence LO_FLAGS_DIRECT_IO can't be used either */
+
+        fd = open(path, O_CLOEXEC|O_NONBLOCK|O_NOCTTY|O_RDONLY);
+        if (fd < 0)
+                return -errno;
+
+        if (fstat(fd, &st) < 0)
+                return -errno;
+
+        if (!S_ISREG(st.st_mode) && !S_ISBLK(st.st_mode))
+                return -EBADF;
+
+        r = path_extract_filename(path, &fn);
+        if (r < 0)
+                return r;
+
+        mfd = memfd_clone_fd(fd, fn, open_flags|O_CLOEXEC);
+        if (mfd < 0)
+                return mfd;
+
+        fd = safe_close(fd); /* Let's close the original early */
+
+        return loop_device_make_internal(NULL, mfd, open_flags, 0, 0, sector_size, loop_flags, lock_op, ret);
+}
+
+static LoopDevice* loop_device_free(LoopDevice *d) {
+        _cleanup_close_ int control = -EBADF;
+        int r;
+
+        if (!d)
+                return NULL;
+
+        /* Release any lock we might have on the device first. We want to open+lock the /dev/loop-control
+         * device below, but our lock protocol says that if both control and block device locks are taken,
+         * the control lock needs to be taken first, the block device lock second — in order to avoid ABBA
+         * locking issues. Moreover, we want to issue LOOP_CLR_FD on the block device further down, and that
+         * would fail if we had another fd open to the device. */
+        d->lock_fd = safe_close(d->lock_fd);
+
+        /* Let's open the control device early, and lock it, so that we can release our block device and
+         * delete it in a synchronized fashion, and allocators won't needlessly see the block device as free
+         * while we are about to delete it. */
+        if (!LOOP_DEVICE_IS_FOREIGN(d) && !d->relinquished) {
+                control = open("/dev/loop-control", O_RDWR|O_CLOEXEC|O_NOCTTY|O_NONBLOCK);
+                if (control < 0)
+                        log_debug_errno(errno, "Failed to open loop control device, cannot remove loop device '%s', ignoring: %m", strna(d->node));
+                else if (flock(control, LOCK_EX) < 0)
+                        log_debug_errno(errno, "Failed to lock loop control device, ignoring: %m");
+        }
+
+        /* Then let's release the loopback block device */
+        if (d->fd >= 0) {
+                /* Implicitly sync the device, since otherwise in-flight blocks might not get written */
+                if (fsync(d->fd) < 0)
+                        log_debug_errno(errno, "Failed to sync loop block device, ignoring: %m");
+
+                if (!LOOP_DEVICE_IS_FOREIGN(d) && !d->relinquished) {
+                        /* We are supposed to clear the loopback device. Let's do this synchronously: lock
+                         * the device, manually remove all partitions and then clear it. This should ensure
+                         * udev doesn't concurrently access the devices, and we can be reasonably sure that
+                         * once we are done here the device is cleared and all its partition children
+                         * removed. Note that we lock our primary device fd here (and not a separate locking
+                         * fd, as we do during allocation, since we want to keep the lock all the way through
+                         * the LOOP_CLR_FD, but that call would fail if we had more than one fd open.) */
+
+                        if (flock(d->fd, LOCK_EX) < 0)
+                                log_debug_errno(errno, "Failed to lock loop block device, ignoring: %m");
+
+                        r = block_device_remove_all_partitions(d->dev, d->fd);
+                        if (r < 0)
+                                log_debug_errno(r, "Failed to remove partitions of loopback block device, ignoring: %m");
+
+                        if (ioctl(d->fd, LOOP_CLR_FD) < 0)
+                                log_debug_errno(errno, "Failed to clear loop device, ignoring: %m");
+                }
+
+                safe_close(d->fd);
+        }
+
+        /* Now that the block device is released, let's also try to remove it */
+        if (control >= 0) {
+                useconds_t delay = 5 * USEC_PER_MSEC;  /* A total delay of 5090 ms between 39 attempts,
+                                                        * (4*5 + 5*10 + 5*20 + … + 3*640) = 5090. */
+
+                for (unsigned attempt = 1;; attempt++) {
+                        if (ioctl(control, LOOP_CTL_REMOVE, d->nr) >= 0)
+                                break;
+                        if (errno != EBUSY || attempt > 38) {
+                                log_debug_errno(errno, "Failed to remove device %s: %m", strna(d->node));
+                                break;
+                        }
+                        if (attempt % 5 == 0) {
+                                log_debug("Device is still busy after %u attempts…", attempt);
+                                delay *= 2;
+                        }
+
+                        (void) usleep_safe(delay);
+                }
+        }
+
+        free(d->node);
+        sd_device_unref(d->dev);
+        free(d->backing_file);
+        return mfree(d);
+}
+
+DEFINE_TRIVIAL_REF_UNREF_FUNC(LoopDevice, loop_device, loop_device_free);
+
+void loop_device_relinquish(LoopDevice *d) {
+        assert(d);
+
+        /* Don't attempt to clean up the loop device anymore from this point on. Leave the clean-ing up to the kernel
+         * itself, using the loop device "auto-clear" logic we already turned on when creating the device. */
+
+        d->relinquished = true;
+}
+
+void loop_device_unrelinquish(LoopDevice *d) {
+        assert(d);
+        d->relinquished = false;
+}
+
+int loop_device_open(
+                sd_device *dev,
+                int open_flags,
+                int lock_op,
+                LoopDevice **ret) {
+
+        _cleanup_close_ int fd = -EBADF, lock_fd = -EBADF;
+        _cleanup_free_ char *node = NULL, *backing_file = NULL;
+        dev_t devnum, backing_devno = 0;
+        struct loop_info64 info;
+        ino_t backing_inode = 0;
+        uint64_t diskseq = 0;
+        LoopDevice *d;
+        const char *s;
+        int r, nr = -1;
+
+        assert(dev);
+        assert(IN_SET(open_flags, O_RDWR, O_RDONLY));
+        assert(ret);
+
+        /* Even if fd is provided through the argument in loop_device_open_from_fd(), we reopen the inode
+         * here, instead of keeping just a dup() clone of it around, since we want to ensure that the
+         * O_DIRECT flag of the handle we keep is off, we have our own file index, and have the right
+         * read/write mode in effect. */
+        fd = sd_device_open(dev, O_CLOEXEC|O_NONBLOCK|O_NOCTTY|open_flags);
+        if (fd < 0)
+                return fd;
+
+        if ((lock_op & ~LOCK_NB) != LOCK_UN) {
+                lock_fd = open_lock_fd(fd, lock_op);
+                if (lock_fd < 0)
+                        return lock_fd;
+        }
+
+        if (ioctl(fd, LOOP_GET_STATUS64, &info) >= 0) {
+#if HAVE_VALGRIND_MEMCHECK_H
+                /* Valgrind currently doesn't know LOOP_GET_STATUS64. Remove this once it does */
+                VALGRIND_MAKE_MEM_DEFINED(&info, sizeof(info));
+#endif
+                nr = info.lo_number;
+
+                if (sd_device_get_sysattr_value(dev, "loop/backing_file", &s) >= 0) {
+                        backing_file = strdup(s);
+                        if (!backing_file)
+                                return -ENOMEM;
+                }
+
+                backing_devno = info.lo_device;
+                backing_inode = info.lo_inode;
+        }
+
+        r = fd_get_diskseq(fd, &diskseq);
+        if (r < 0 && r != -EOPNOTSUPP)
+                return r;
+
+        uint32_t sector_size;
+        r = blockdev_get_sector_size(fd, &sector_size);
+        if (r < 0)
+                return r;
+
+        r = sd_device_get_devnum(dev, &devnum);
+        if (r < 0)
+                return r;
+
+        r = sd_device_get_devname(dev, &s);
+        if (r < 0)
+                return r;
+
+        node = strdup(s);
+        if (!node)
+                return -ENOMEM;
+
+        d = new(LoopDevice, 1);
+        if (!d)
+                return -ENOMEM;
+
+        *d = (LoopDevice) {
+                .n_ref = 1,
+                .fd = TAKE_FD(fd),
+                .lock_fd = TAKE_FD(lock_fd),
+                .nr = nr,
+                .node = TAKE_PTR(node),
+                .dev = sd_device_ref(dev),
+                .backing_file = TAKE_PTR(backing_file),
+                .backing_inode = backing_inode,
+                .backing_devno = backing_devno,
+                .relinquished = true, /* It's not ours, don't try to destroy it when this object is freed */
+                .devno = devnum,
+                .diskseq = diskseq,
+                .uevent_seqnum_not_before = UINT64_MAX,
+                .timestamp_not_before = USEC_INFINITY,
+                .sector_size = sector_size,
+        };
+
+        *ret = d;
+        return 0;
+}
+
+int loop_device_open_from_fd(
+                int fd,
+                int open_flags,
+                int lock_op,
+                LoopDevice **ret) {
+
+        _cleanup_(sd_device_unrefp) sd_device *dev = NULL;
+        int r;
+
+        r = block_device_new_from_fd(ASSERT_FD(fd), 0, &dev);
+        if (r < 0)
+                return r;
+
+        return loop_device_open(dev, open_flags, lock_op, ret);
+}
+
+int loop_device_open_from_path(
+                const char *path,
+                int open_flags,
+                int lock_op,
+                LoopDevice **ret) {
+
+        _cleanup_(sd_device_unrefp) sd_device *dev = NULL;
+        int r;
+
+        assert(path);
+
+        r = block_device_new_from_path(path, 0, &dev);
+        if (r < 0)
+                return r;
+
+        return loop_device_open(dev, open_flags, lock_op, ret);
+}
+
+static int resize_partition(int partition_fd, uint64_t offset, uint64_t size) {
+        char sysfs[STRLEN("/sys/dev/block/:/partition") + 2*DECIMAL_STR_MAX(dev_t) + 1];
+        _cleanup_free_ char *buffer = NULL;
+        uint64_t current_offset, current_size, partno;
+        _cleanup_close_ int whole_fd = -EBADF;
+        struct stat st;
+        dev_t devno;
+        int r;
+
+        /* Resizes the partition the loopback device refer to (assuming it refers to one instead of an actual
+         * loopback device), and changes the offset, if needed. This is a fancy wrapper around
+         * BLKPG_RESIZE_PARTITION. */
+
+        if (fstat(ASSERT_FD(partition_fd), &st) < 0)
+                return -errno;
+
+        assert(S_ISBLK(st.st_mode));
+
+        xsprintf(sysfs, "/sys/dev/block/" DEVNUM_FORMAT_STR "/partition", DEVNUM_FORMAT_VAL(st.st_rdev));
+        r = read_one_line_file(sysfs, &buffer);
+        if (r == -ENOENT) /* not a partition, cannot resize */
+                return -ENOTTY;
+        if (r < 0)
+                return r;
+        r = safe_atou64(buffer, &partno);
+        if (r < 0)
+                return r;
+
+        xsprintf(sysfs, "/sys/dev/block/" DEVNUM_FORMAT_STR "/start", DEVNUM_FORMAT_VAL(st.st_rdev));
+
+        buffer = mfree(buffer);
+        r = read_one_line_file(sysfs, &buffer);
+        if (r < 0)
+                return r;
+        r = safe_atou64(buffer, &current_offset);
+        if (r < 0)
+                return r;
+        if (current_offset > UINT64_MAX/512U)
+                return -EINVAL;
+        current_offset *= 512U;
+
+        if (ioctl(partition_fd, BLKGETSIZE64, &current_size) < 0)
+                return -EINVAL;
+
+        if (size == UINT64_MAX && offset == UINT64_MAX)
+                return 0;
+        if (current_size == size && current_offset == offset)
+                return 0;
+
+        xsprintf(sysfs, "/sys/dev/block/" DEVNUM_FORMAT_STR "/../dev", DEVNUM_FORMAT_VAL(st.st_rdev));
+
+        buffer = mfree(buffer);
+        r = read_one_line_file(sysfs, &buffer);
+        if (r < 0)
+                return r;
+        r = parse_devnum(buffer, &devno);
+        if (r < 0)
+                return r;
+
+        whole_fd = r = device_open_from_devnum(S_IFBLK, devno, O_RDWR|O_CLOEXEC|O_NONBLOCK|O_NOCTTY, NULL);
+        if (r < 0)
+                return r;
+
+        return block_device_resize_partition(
+                        whole_fd,
+                        partno,
+                        offset == UINT64_MAX ? current_offset : offset,
+                        size == UINT64_MAX ? current_size : size);
+}
+
+int loop_device_refresh_size(LoopDevice *d, uint64_t offset, uint64_t size) {
+        struct loop_info64 info;
+
+        assert(d);
+        assert(d->fd >= 0);
+
+        /* Changes the offset/start of the loop device relative to the beginning of the underlying file or
+         * block device. If this loop device actually refers to a partition and not a loopback device, we'll
+         * try to adjust the partition offsets instead.
+         *
+         * If either offset or size is UINT64_MAX we won't change that parameter. */
+
+        if (d->nr < 0) /* not a loopback device */
+                return resize_partition(d->fd, offset, size);
+
+        if (ioctl(d->fd, LOOP_GET_STATUS64, &info) < 0)
+                return -errno;
+
+#if HAVE_VALGRIND_MEMCHECK_H
+        /* Valgrind currently doesn't know LOOP_GET_STATUS64. Remove this once it does */
+        VALGRIND_MAKE_MEM_DEFINED(&info, sizeof(info));
+#endif
+
+        if (size == UINT64_MAX && offset == UINT64_MAX)
+                return 0;
+        if (info.lo_sizelimit == size && info.lo_offset == offset)
+                return 0;
+
+        if (size != UINT64_MAX)
+                info.lo_sizelimit = size;
+        if (offset != UINT64_MAX)
+                info.lo_offset = offset;
+
+        return RET_NERRNO(ioctl(d->fd, LOOP_SET_STATUS64, &info));
+}
+
+int loop_device_flock(LoopDevice *d, int operation) {
+        assert(IN_SET(operation & ~LOCK_NB, LOCK_UN, LOCK_SH, LOCK_EX));
+        assert(d);
+
+        /* When unlocking just close the lock fd */
+        if ((operation & ~LOCK_NB) == LOCK_UN) {
+                d->lock_fd = safe_close(d->lock_fd);
+                return 0;
+        }
+
+        /* If we had no lock fd so far, create one and lock it right-away */
+        if (d->lock_fd < 0) {
+                d->lock_fd = open_lock_fd(ASSERT_FD(d->fd), operation);
+                if (d->lock_fd < 0)
+                        return d->lock_fd;
+
+                return 0;
+        }
+
+        /* Otherwise change the current lock mode on the existing fd */
+        return RET_NERRNO(flock(d->lock_fd, operation));
+}
+
+int loop_device_sync(LoopDevice *d) {
+        assert(d);
+
+        /* We also do this implicitly in loop_device_unref(). Doing this explicitly here has the benefit that
+         * we can check the return value though. */
+
+        return RET_NERRNO(fsync(ASSERT_FD(d->fd)));
+}
+
+int loop_device_set_autoclear(LoopDevice *d, bool autoclear) {
+        struct loop_info64 info;
+
+        assert(d);
+
+        if (ioctl(ASSERT_FD(d->fd), LOOP_GET_STATUS64, &info) < 0)
+                return -errno;
+
+        if (autoclear == FLAGS_SET(info.lo_flags, LO_FLAGS_AUTOCLEAR))
+                return 0;
+
+        SET_FLAG(info.lo_flags, LO_FLAGS_AUTOCLEAR, autoclear);
+
+        if (ioctl(d->fd, LOOP_SET_STATUS64, &info) < 0)
+                return -errno;
+
+        return 1;
+}
+
+int loop_device_set_filename(LoopDevice *d, const char *name) {
+        struct loop_info64 info;
+
+        assert(d);
+
+        /* Sets the .lo_file_name of the loopback device. This is supposed to contain the path to the file
+         * backing the block device, but is actually just a free-form string you can pass to the kernel. Most
+         * tools that actually care for the backing file path use the sysfs attribute file loop/backing_file
+         * which is a kernel generated string, subject to file system namespaces and such.
+         *
+         * .lo_file_name is useful since userspace can select it freely when creating a loopback block
+         * device, and we can use it for /dev/disk/by-loop-ref/ symlinks, and similar, so that apps can
+         * recognize their own loopback files. */
+
+        if (name && strlen(name) >= sizeof(info.lo_file_name))
+                return -ENOBUFS;
+
+        if (ioctl(ASSERT_FD(d->fd), LOOP_GET_STATUS64, &info) < 0)
+                return -errno;
+
+        if (strneq((char*) info.lo_file_name, strempty(name), sizeof(info.lo_file_name)))
+                return 0;
+
+        if (name) {
+                strncpy((char*) info.lo_file_name, name, sizeof(info.lo_file_name)-1);
+                info.lo_file_name[sizeof(info.lo_file_name)-1] = 0;
+        } else
+                memzero(info.lo_file_name, sizeof(info.lo_file_name));
+
+        if (ioctl(d->fd, LOOP_SET_STATUS64, &info) < 0)
+                return -errno;
+
+        return 1;
+}