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

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@purestorag...

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@purestor...
• 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@purestorag...

---

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