Re: [PATCH v3] ublk: improve detection and handling of ublk server exit

Uday Shankar
2025-04-07T19:16:37Z

On Sat, Apr 05, 2025 at 10:59:29PM +0800, Ming Lei wrote:
> On Thu, Apr 03, 2025 at 06:05:57PM -0600, Uday Shankar wrote:
> > There are currently two ways in which ublk server exit is detected by
> > ublk_drv:
> >
> > 1. uring_cmd cancellation. If there are any outstanding uring_cmds which
> > have not been completed to the ublk server when it exits, io_uring
> > calls the uring_cmd callback with a special cancellation flag as the
> > issuing task is exiting.
> > 2. I/O timeout. This is needed in addition to the above to handle the
> > "saturated queue" case, when all I/Os for a given queue are in the
> > ublk server, and therefore there are no outstanding uring_cmds to
> > cancel when the ublk server exits.
> >
> > There are a couple of issues with this approach:
> >
> > - It is complex and inelegant to have two methods to detect the same
> > condition
> > - The second method detects ublk server exit only after a long delay
> > (~30s, the default timeout assigned by the block layer). This delays
> > the nosrv behavior from kicking in and potential subsequent recovery
> > of the device.
> >
> > The second issue is brought to light with the new test_generic_04. It
> > fails before this fix:
> >
> > selftests: ublk: test_generic_04.sh
> > dev id is 0
> > dd: error writing '/dev/ublkb0': Input/output error
> > 1+0 records in
> > 0+0 records out
> > 0 bytes copied, 30.0611 s, 0.0 kB/s
> > DEAD
> > dd took 31 seconds to exit (>= 5s tolerance)!
> > generic_04 : [FAIL]
> >
> > Fix this by instead detecting and handling ublk server exit in the
> > character file release callback. This has several advantages:
> >
> > - This one place can handle both saturated and unsaturated queues. Thus,
> > it replaces both preexisting methods of detecting ublk server exit.
> > - It runs quickly on ublk server exit - there is no 30s delay.
> > - It starts the process of removing task references in ublk_drv. This is
> > needed if we want to relax restrictions in the driver like letting
> > only one thread serve each queue
> >
> > There is also the disadvantage that the character file release callback
> > can also be triggered by intentional close of the file, which is a
> > significant behavior change. Preexisting ublk servers (libublksrv) are
> > dependent on the ability to open/close the file multiple times. To
> > address this, only transition to a nosrv state if the file is released
> > while the ublk device is live. This allows for programs to open/close
> > the file multiple times during setup. It is still a behavior change if a
> > ublk server decides to close/reopen the file while the device is LIVE
> > (i.e. while it is responsible for serving I/O), but that would be highly
> > unusual. This behavior is in line with what is done by FUSE, which is
> > very similar to ublk in that a userspace daemon is providing services
> > traditionally provided by the kernel.
> >
> > With this change in, the new test (and all other selftests, and all
> > ublksrv tests) pass:
> >
> > selftests: ublk: test_generic_04.sh
> > dev id is 0
> > dd: error writing '/dev/ublkb0': Input/output error
> > 1+0 records in
> > 0+0 records out
> > 0 bytes copied, 0.0376731 s, 0.0 kB/s
> > DEAD
> > generic_04 : [PASS]
> >
> > Signed-off-by: Uday Shankar <ushankar@purestorage.com>
> > ---
> > Changes in v3:
> > - Quiesce queue earlier to avoid concurrent cancellation and "normal"
> > completion of io_uring cmds (Ming Lei)
> > - Fix del_gendisk hang, found by test_stress_02
> > - Remove unnecessary parameters in fault_inject target (Ming Lei)
> > - Fix delay implementation to have separate per-I/O delay instead of
> > blocking the whole thread (Ming Lei)
> > - Add delay_us to docs
> > - Link to v2: https://lore.kernel.org/r/20250402-ublk_timeout-v2-1-249bc5523000@purestorage.com
> >
> > Changes in v2:
> > - Leave null ublk selftests target untouched, instead create new
> > fault_inject target for injecting per-I/O delay (Ming Lei)
> > - Allow multiple open/close of ublk character device with some
> > restrictions
> > - Drop patches which made it in separately at https://lore.kernel.org/r/20250401-ublk_selftests-v1-1-98129c9bc8bb@purestorage.com
> > - Consolidate more nosrv logic in ublk character device release, and
> > associated code cleanup
> > - Link to v1: https://lore.kernel.org/r/20250325-ublk_timeout-v1-0-262f0121a7bd@purestorage.com
> > ---
> > drivers/block/ublk_drv.c | 228 +++++++++---------------
> > tools/testing/selftests/ublk/Makefile | 4 +-
> > tools/testing/selftests/ublk/fault_inject.c | 72 ++++++++
> > tools/testing/selftests/ublk/kublk.c | 6 +-
> > tools/testing/selftests/ublk/kublk.h | 4 +
> > tools/testing/selftests/ublk/test_generic_04.sh | 43 +++++
> > 6 files changed, 215 insertions(+), 142 deletions(-)
> >
> > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> > index 2fd05c1bd30b03343cb6f357f8c08dd92ff47af9..73baa9d22ccafb00723defa755a0b3aab7238934 100644
> > --- a/drivers/block/ublk_drv.c
> > +++ b/drivers/block/ublk_drv.c
> > @@ -162,7 +162,6 @@ struct ublk_queue {
> >
> > bool force_abort;
> > bool timeout;
> > - bool canceling;
> > bool fail_io; /* copy of dev->state == UBLK_S_DEV_FAIL_IO */
> > unsigned short nr_io_ready; /* how many ios setup */
> > spinlock_t cancel_lock;
> > @@ -199,8 +198,6 @@ struct ublk_device {
> > struct completion completion;
> > unsigned int nr_queues_ready;
> > unsigned int nr_privileged_daemon;
> > -
> > - struct work_struct nosrv_work;
> > };
> >
> > /* header of ublk_params */
> > @@ -209,8 +206,9 @@ struct ublk_params_header {
> > __u32 types;
> > };
> >
> > -static bool ublk_abort_requests(struct ublk_device *ub, struct ublk_queue *ubq);
> > -
> > +static void ublk_stop_dev_unlocked(struct ublk_device *ub);
> > +static void ublk_abort_queue(struct ublk_device *ub, struct ublk_queue *ubq);
> > +static void __ublk_quiesce_dev(struct ublk_device *ub);
> > static inline struct request *__ublk_check_and_get_req(struct ublk_device *ub,
> > struct ublk_queue *ubq, int tag, size_t offset);
> > static inline unsigned int ublk_req_build_flags(struct request *req);
> > @@ -1314,8 +1312,6 @@ static void ublk_queue_cmd_list(struct ublk_queue *ubq, struct rq_list *l)
> > static enum blk_eh_timer_return ublk_timeout(struct request *rq)
> > {
> > struct ublk_queue *ubq = rq->mq_hctx->driver_data;
> > - unsigned int nr_inflight = 0;
> > - int i;
> >
> > if (ubq->flags & UBLK_F_UNPRIVILEGED_DEV) {
> > if (!ubq->timeout) {
> > @@ -1326,26 +1322,6 @@ static enum blk_eh_timer_return ublk_timeout(struct request *rq)
> > return BLK_EH_DONE;
> > }
> >
> > - if (!ubq_daemon_is_dying(ubq))
> > - return BLK_EH_RESET_TIMER;
> > -
> > - for (i = 0; i < ubq->q_depth; i++) {
> > - struct ublk_io *io = &ubq->ios[i];
> > -
> > - if (!(io->flags & UBLK_IO_FLAG_ACTIVE))
> > - nr_inflight++;
> > - }
> > -
> > - /* cancelable uring_cmd can't help us if all commands are in-flight */
> > - if (nr_inflight == ubq->q_depth) {
> > - struct ublk_device *ub = ubq->dev;
> > -
> > - if (ublk_abort_requests(ub, ubq)) {
> > - schedule_work(&ub->nosrv_work);
> > - }
> > - return BLK_EH_DONE;
> > - }
> > -
> > return BLK_EH_RESET_TIMER;
> > }
> >
> > @@ -1356,19 +1332,16 @@ static blk_status_t ublk_prep_req(struct ublk_queue *ubq, struct request *rq)
> > if (unlikely(ubq->fail_io))
> > return BLK_STS_TARGET;
> >
> > - /* With recovery feature enabled, force_abort is set in
> > - * ublk_stop_dev() before calling del_gendisk(). We have to
> > - * abort all requeued and new rqs here to let del_gendisk()
> > - * move on. Besides, we cannot not call io_uring_cmd_complete_in_task()
> > - * to avoid UAF on io_uring ctx.
> > + /*
> > + * force_abort is set in ublk_stop_dev() before calling
> > + * del_gendisk(). We have to abort all requeued and new rqs here
> > + * to let del_gendisk() move on. Besides, we cannot not call
> > + * io_uring_cmd_complete_in_task() to avoid UAF on io_uring ctx.
> > *
> > * Note: force_abort is guaranteed to be seen because it is set
> > * before request queue is unqiuesced.
> > */
> > - if (ublk_nosrv_should_queue_io(ubq) && unlikely(ubq->force_abort))
> > - return BLK_STS_IOERR;
> > -
> > - if (unlikely(ubq->canceling))
> > + if (unlikely(ubq->force_abort))
> > return BLK_STS_IOERR;
> >
> > /* fill iod to slot in io cmd buffer */
> > @@ -1391,16 +1364,6 @@ static blk_status_t ublk_queue_rq(struct blk_mq_hw_ctx *hctx,
> > if (res != BLK_STS_OK)
> > return res;
> >
> > - /*
> > - * ->canceling has to be handled after ->force_abort and ->fail_io
> > - * is dealt with, otherwise this request may not be failed in case
> > - * of recovery, and cause hang when deleting disk
> > - */
> > - if (unlikely(ubq->canceling)) {
> > - __ublk_abort_rq(ubq, rq);
> > - return BLK_STS_OK;
> > - }
> > -
>
> If `ubq->canceling` is removed, how to handle recovery? The request has to
> be requeued in case of ublk_nosrv_dev_should_queue_io().

In the typical case, requests issued after the ublk server dies with
recovery enabled queue because the request_queue is quiesced. This is
the same way things worked before this patch. There is an edge case in
ublk_dispatch_req where we will manually requeue any request that made
it to task_work but I assume that is not what you are talking about
here.

The above test you are highlighting looks like dead code anyways. We
already test ubq->canceling and fail the I/O if it is set in
ublk_prep_req, so we cannot possibly enter the if statement above.

>
> And that should be the reason why 'make test T=generic/004' hangs forever
> after applying this patch.

I am not able to reproduce the hang even after increasing LOOPS to 100
for more repetitions. Can you ensure you have the most recent version of
the patch? I fixed a similar hang between v2-v3.

>
> > ublk_queue_cmd(ubq, rq);
> > return BLK_STS_OK;
> > }
> > @@ -1461,8 +1424,71 @@ static int ublk_ch_open(struct inode *inode, struct file *filp)
> > static int ublk_ch_release(struct inode *inode, struct file *filp)
> > {
> > struct ublk_device *ub = filp->private_data;
> > + int i;
> > +
> > + mutex_lock(&ub->mutex);
> > + /*
> > + * If the device is not live, we will not transition to a nosrv
> > + * state. This protects against:
> > + * - accidental poking of the ublk character device
> > + * - some ublk servers which may open/close the ublk character
> > + * device during startup
> > + */
> > + if (ub->dev_info.state != UBLK_S_DEV_LIVE)
> > + goto out;
> > +
> > + /*
> > + * Since we are releasing the ublk character file descriptor, we
> > + * know that there cannot be any concurrent file-related
> > + * activity (e.g. uring_cmds or reads/writes). However, I/O
> > + * might still be getting dispatched. Quiesce that too so that
> > + * we don't need to worry about anything concurrent.
> > + *
> > + * We may have already quiesced the queue if we canceled any
> > + * uring_cmds, so only quiesce if necessary (quiesce is not
> > + * idempotent, it has an internal counter which we need to
> > + * manage carefully).
> > + */
> > + if (!blk_queue_quiesced(ub->ub_disk->queue))
> > + blk_mq_quiesce_queue(ub->ub_disk->queue);
> > +
> > + /*
> > + * Handle any requests outstanding to the ublk server
> > + */
> > + for (i = 0; i < ub->dev_info.nr_hw_queues; i++)
> > + ublk_abort_queue(ub, ublk_get_queue(ub, i));
> >
> > + /*
> > + * Transition the device to the nosrv state. What exactly this
> > + * means depends on the recovery flags
> > + */
> > + if (ublk_nosrv_should_stop_dev(ub)) {
> > + /*
> > + * Allow any pending/future I/O to pass through quickly
> > + * with an error. This is needed because del_gendisk
> > + * waits for all pending I/O to complete
> > + */
> > + for (i = 0; i < ub->dev_info.nr_hw_queues; i++)
> > + ublk_get_queue(ub, i)->force_abort = true;
> > + blk_mq_unquiesce_queue(ub->ub_disk->queue);
> > +
> > + ublk_stop_dev_unlocked(ub);
> > + } else {
> > + if (ublk_nosrv_dev_should_queue_io(ub)) {
> > + __ublk_quiesce_dev(ub);
>
> Here only inflight IOs are drained, new IO still comes after queue is
> unquiesced, then uring_cmd UAF is triggered.

No, I don't think so. There are three cases here, dependent on recovery
flags:

- no recovery flags: in this case, force_abort is set before queue is
unquiesced, so I/Os will be errored before we try to access the
io_uring_cmd
- just RECOVERY: queue is not actually unquiesced here at all, because
there are two quiesce (one above, one inside __ublk_quiesce_dev), and
only one unquiesce
- RECOVERY|FAIL_IO: similar to no recovery flags case, except we are
setting fail_io instead of force_abort. The two flags are very similar
and we should consolidate in a follow-up patch

In all cases we are protected against using already canceled
io_uring_cmds.

> > --- /dev/null
> > +++ b/tools/testing/selftests/ublk/test_generic_04.sh
> > @@ -0,0 +1,43 @@
> > +#!/bin/bash
> > +# SPDX-License-Identifier: GPL-2.0
> > +
> > +. "$(cd "$(dirname "$0")" && pwd)"/test_common.sh
> > +
> > +TID="generic_04"
> > +ERR_CODE=0
> > +
> > +_prep_test "fault_inject" "fast cleanup when all I/Os of one hctx are in server"
> > +
> > +# configure ublk server to sleep 2s before completing each I/O
> > +dev_id=$(_add_ublk_dev -t fault_inject -q 2 -d 1 --delay_us 2000000)
> > +_check_add_dev $TID $?
> > +
> > +echo "dev id is ${dev_id}"
> > +
> > +STARTTIME=${SECONDS}
> > +
> > +dd if=/dev/urandom of=/dev/ublkb${dev_id} oflag=direct bs=4k count=1 &
>
> stdout/stderr need to be discarded.
>
> Also I'd suggest to make the selftest part as one standalone patch.

Sounds good, that is a nice way to split up this large patch.