Infinite recursion when automounting snapshots

[mmaybee]

System information

Type	Version/Name
Distribution Name	Ubuntu
Distribution Version	20.04
Kernel Version	5.15.0-1066
Architecture	x86_64
OpenZFS Version	zfs-2.2.99

Describe the problem you're observing

After upgrading ZFS to a version which includes a fix to propogate dataset properties into mount options (commit
34118ea), automounts of snapshots will hang indefinitely. This problem is only present until the system is rebooted into the updated kernel (with the new ZFS version).

The following text from the commit indicates the reason for this issue:

Furthermore, don't run mount.zfs(8) helper for automounting snapshot.
The above change to make mount.zfs(8) to call 'zfs_mount_at'
apparently caused it to trigger an automount for the snapshot
directory. When the helper was invoked as a result of a snapshot
automount, an infinite recursion will occur.

The fix for this issue that was included in the commit was to add the -i flag to mounts requested from zfsctl_snapshot_mount in the kernel. This prevents mount.zfs from being invoked. However, before the new kernel is running we will not have this flag present and the userland changes made by this commit will cause this issue to occur.

Describe how to reproduce the problem

See above description of the problem.

Include any warning/errors/backtraces from the system logs

Here is an example of the issue as it manifests in the kernel logs:

[ 2659.491665] INFO: task mount.zfs:121090 blocked for more than 120 seconds.
[ 2659.493543]       Tainted: P           OE     5.15.0-1063-dx2024080801-aeedf7247-oracle #69~20.04.1
[ 2659.495864] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 2659.498040] task:mount.zfs       state:D stack:    0 pid:121090 ppid:121089 flags:0x00000000
[ 2659.498044] Call Trace:
[ 2659.498045]  <TASK>
[ 2659.498046]  __schedule+0x2c8/0x860
[ 2659.498048]  ? __smp_call_single_queue+0x59/0x90
[ 2659.498050]  ? usleep_range_state+0x90/0x90
[ 2659.498052]  schedule+0x69/0x110
[ 2659.498054]  schedule_timeout+0x20b/0x2d0
[ 2659.498055]  ? try_to_wake_up+0x216/0x5b0
[ 2659.498057]  ? zio_destroy+0x5d/0x90 [zfs]
[ 2659.498190]  ? zio_done+0x69a/0xea0 [zfs]
[ 2659.498318]  ? usleep_range_state+0x90/0x90
[ 2659.498320]  __wait_for_common+0xa8/0x160
[ 2659.498322]  wait_for_completion+0x24/0x30
[ 2659.498324]  call_usermodehelper_exec+0x14c/0x180
[ 2659.498326]  call_usermodehelper+0x93/0xc0
[ 2659.498328]  zfsctl_snapshot_mount+0x1bc/0x360 [zfs]
[ 2659.498461]  zpl_snapdir_automount+0x10/0x40 [zfs]
[ 2659.498607]  __traverse_mounts+0x8c/0x230
[ 2659.498609]  ? __legitimize_path.isra.0+0x50/0x70
[ 2659.498611]  step_into+0x251/0x3b0
[ 2659.498614]  walk_component+0x70/0x1c0
[ 2659.498616]  path_lookupat.isra.0+0x6e/0x150
[ 2659.498618]  filename_lookup+0xcf/0x1a0
[ 2659.498620]  ? mntget+0x18/0x30
[ 2659.498622]  ? path_get+0x27/0x30
[ 2659.498624]  ? audit_alloc_name+0x125/0x150
[ 2659.498626]  ? getname_flags+0xc2/0x1f0
[ 2659.498628]  user_path_at_empty+0x3f/0x60
[ 2659.498629]  user_statfs+0x48/0xb0
[ 2659.498632]  __do_sys_statfs+0x28/0x60
[ 2659.498634]  ? __audit_syscall_entry+0xdb/0x130
[ 2659.498636]  ? syscall_trace_enter.isra.0+0x143/0x1c0
[ 2659.498638]  __x64_sys_statfs+0x16/0x20
[ 2659.498640]  x64_sys_call+0x1cbd/0x1fa0
[ 2659.498642]  do_syscall_64+0x54/0xb0
[ 2659.498644]  ? exit_to_user_mode_prepare+0x3c/0x1d0
[ 2659.498646]  ? syscall_exit_to_user_mode+0x2c/0x50
[ 2659.498648]  ? do_syscall_64+0x61/0xb0