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

Ming Lei
2025-04-05T14:59:48Z

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().

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

> 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.

...

> --- /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.

thanks,
Ming