loop-util: if a loopback device we want to use still has partitions, do something about it
On current kernels (5.8 for example) under some conditions I don't fully grok it might happen that a detached loopback block device still has partition block devices around. Accessing these partition block devices results in EIO errors (that also fill up dmesg). These devices cannot be claned up with LOOP_CLR_FD (since the main device already is officially detached), nor with LOOP_CTL_DELETE (returns EBUSY as long as the partitions still exist). This is a kernel bug. But it appears to apply to all recent kernels. I cannot really pin down what triggers this, suffice to say our heavy-duty test can trigger it. Either way, let's do something about it: when we notice this state we'll attach an empty file to it, which is guaranteed to have to part table. This makes the partitions go away. After closing/reoping the device we hence are good to go again. ugly workaround, but I think OK enough to use. The net result is: with this commit, we'll guarantee that by the time we attach a file to the loopback device we have zero kernel partitions associated with it. Thus if we then wait for the kernel partitions we need to appear we should have entirely reliable behaviour even if loopback devices by the name are heavily recycled and udev events reach us very late. Fixes: #16858
This commit is contained in:
Lennart Poettering
parent
4ba86848c9
commit
021bf17528
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@ -13,8 +13,11 @@
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#include <sys/ioctl.h>
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#include <unistd.h>
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#include "sd-device.h"
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#include "alloc-util.h"
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#include "blockdev-util.h"
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#include "device-util.h"
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#include "errno-util.h"
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#include "fd-util.h"
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#include "fileio.h"
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@ -24,6 +27,7 @@
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#include "stat-util.h"
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#include "stdio-util.h"
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#include "string-util.h"
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#include "tmpfile-util.h"
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static void cleanup_clear_loop_close(int *fd) {
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if (*fd < 0)
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@ -33,18 +37,134 @@ static void cleanup_clear_loop_close(int *fd) {
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(void) safe_close(*fd);
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}
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static int loop_is_bound(int fd) {
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struct loop_info64 info;
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assert(fd >= 0);
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if (ioctl(fd, LOOP_GET_STATUS64, &info) < 0) {
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if (errno == ENXIO)
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return false; /* not bound! */
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return -errno;
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}
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return true; /* bound! */
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}
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static int device_has_block_children(sd_device *d) {
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_cleanup_(sd_device_enumerator_unrefp) sd_device_enumerator *e = NULL;
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const char *main_sn, *main_ss;
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sd_device *q;
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int r;
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assert(d);
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/* Checks if the specified device currently has block device children (i.e. partition block
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* devices). */
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r = sd_device_get_sysname(d, &main_sn);
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if (r < 0)
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return r;
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r = sd_device_get_subsystem(d, &main_ss);
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if (r < 0)
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return r;
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if (!streq(main_ss, "block"))
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return -EINVAL;
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r = sd_device_enumerator_new(&e);
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if (r < 0)
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return r;
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r = sd_device_enumerator_allow_uninitialized(e);
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if (r < 0)
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return r;
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r = sd_device_enumerator_add_match_parent(e, d);
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if (r < 0)
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return r;
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FOREACH_DEVICE(e, q) {
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const char *ss, *sn;
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r = sd_device_get_subsystem(q, &ss);
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if (r < 0)
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continue;
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if (!streq(ss, "block"))
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continue;
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r = sd_device_get_sysname(q, &sn);
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if (r < 0)
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continue;
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if (streq(sn, main_sn))
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continue;
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return 1; /* we have block device children */
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}
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return 0;
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}
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static int loop_configure(
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int fd,
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int nr,
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const struct loop_config *c,
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bool *try_loop_configure) {
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_cleanup_(sd_device_unrefp) sd_device *d = NULL;
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_cleanup_free_ char *sysname = NULL;
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_cleanup_close_ int lock_fd = -1;
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int r;
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assert(fd >= 0);
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assert(nr >= 0);
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assert(c);
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assert(try_loop_configure);
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if (asprintf(&sysname, "loop%i", nr) < 0)
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return -ENOMEM;
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r = sd_device_new_from_subsystem_sysname(&d, "block", sysname);
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if (r < 0)
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return r;
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/* Let's lock the device before we do anything. We take the BSD lock on a second, separately opened
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* fd for the device. udev after all watches for close() events (specifically IN_CLOSE_WRITE) on
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* block devices to reprobe them, hence by having a separate fd we will later close() we can ensure
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* we trigger udev after everything is done. If we'd lock our own fd instead and keep it open for a
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* long time udev would possibly never run on it again, even though the fd is unlocked, simply
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* because we never close() it. It also has the nice benefit we can use the _cleanup_close_ logic to
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* automatically release the lock, after we are done. */
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lock_fd = fd_reopen(fd, O_RDWR|O_CLOEXEC|O_NONBLOCK|O_NOCTTY);
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if (lock_fd < 0)
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return lock_fd;
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if (flock(lock_fd, LOCK_EX) < 0)
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return -errno;
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/* Let's see if the device is really detached, i.e. currently has no associated partition block
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* devices. On various kernels (such as 5.8) it is possible to have a loopback block device that
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* superficially is detached but still has partition block devices associated for it. They only go
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* away when the device is reattached. (Yes, LOOP_CLR_FD doesn't work then, because officially
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* nothing is attached and LOOP_CTL_REMOVE doesn't either, since it doesn't care about partition
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* block devices. */
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r = device_has_block_children(d);
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if (r < 0)
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return r;
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if (r > 0) {
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r = loop_is_bound(fd);
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if (r < 0)
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return r;
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if (r > 0)
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return -EBUSY;
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return -EUCLEAN; /* Bound but children? Tell caller to reattach something so that the
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* partition block devices are gone too. */
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}
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if (*try_loop_configure) {
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if (ioctl(fd, LOOP_CONFIGURE, c) < 0) {
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/* Do fallback only if LOOP_CONFIGURE is not supported, propagate all other
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@ -115,21 +235,7 @@ static int loop_configure(
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* time where we attach the fd and where we reconfigure the device. Secondly, let's wait 50ms on
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* EAGAIN and retry. The former should be an efficient mechanism to avoid we have to wait 50ms
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* needlessly if we are just racing against udev. The latter is protection against all other cases,
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* i.e. peers that do not take the BSD lock.
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*
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* We take the BSD lock on a second, separately opened fd for the device. udev after all watches for
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* close() events (specifically IN_CLOSE_WRITE) on block devices to reprobe them, hence by having a
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* separate fd we will later close() we can ensure we trigger udev after everything is done. If we'd
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* lock our own fd instead and keep it open for a long time udev would possibly never run on it
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* again, even though the fd is unlocked, simply because we never close() it. It also has the nice
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* benefit we can use the _cleanup_close_ logic to automatically release the lock, after we are
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* done. */
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lock_fd = fd_reopen(fd, O_RDWR|O_CLOEXEC|O_NONBLOCK|O_NOCTTY);
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if (lock_fd < 0)
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return lock_fd;
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if (flock(lock_fd, LOCK_EX) < 0)
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return -errno;
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* i.e. peers that do not take the BSD lock. */
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if (ioctl(fd, LOOP_SET_FD, c->fd) < 0)
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return -errno;
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@ -152,6 +258,44 @@ fail:
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return r;
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}
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static int attach_empty_file(int loop, int nr) {
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_cleanup_close_ int fd = -1;
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/* So here's the thing: on various kernels (5.8 at least) loop block devices might enter a state
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* where they are detached but nonetheless have partitions, when used heavily. Accessing these
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* partitions results in immediatey IO errors. There's no pretty way to get rid of them
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* again. Neither LOOP_CLR_FD nor LOOP_CTL_REMOVE suffice (see above). What does work is to
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* reassociate them with a new fd however. This is what we do here hence: we associate the devices
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* with an empty file (i.e. an image that definitely has no partitons). We then immediately clear it
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* again. This suffices to make the partitions go away. Ugly but appears to work. */
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log_debug("Found unattached loopback block device /dev/loop%i with partitions. Attaching empty file to remove them.", nr);
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fd = open_tmpfile_unlinkable(NULL, O_RDONLY);
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if (fd < 0)
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return fd;
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if (flock(loop, LOCK_EX) < 0)
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return -errno;
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if (ioctl(loop, LOOP_SET_FD, fd) < 0)
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return -errno;
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if (ioctl(loop, LOOP_SET_STATUS64, &(struct loop_info64) {
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.lo_flags = LO_FLAGS_READ_ONLY|
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LO_FLAGS_AUTOCLEAR|
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LO_FLAGS_PARTSCAN, /* enable partscan, so that the partitions really go away */
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}) < 0)
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return -errno;
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if (ioctl(loop, LOOP_CLR_FD) < 0)
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return -errno;
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/* The caller is expected to immediately close the loopback device after this, so that the BSD lock
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* is released, and udev sees the changes. */
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return 0;
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}
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int loop_device_make(
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int fd,
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int open_flags,
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@ -252,12 +396,17 @@ int loop_device_make(
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if (!IN_SET(errno, ENOENT, ENXIO))
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return -errno;
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} else {
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r = loop_configure(loop, &config, &try_loop_configure);
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r = loop_configure(loop, nr, &config, &try_loop_configure);
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if (r >= 0) {
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loop_with_fd = TAKE_FD(loop);
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break;
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}
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if (r != -EBUSY)
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if (r == -EUCLEAN) {
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/* Make left-over partition disappear hack (see above) */
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r = attach_empty_file(loop, nr);
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if (r < 0 && r != -EBUSY)
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return r;
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} else if (r != -EBUSY)
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return r;
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}
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