Hello,
syzbot found the following crash on:
HEAD commit: 0072a0c14d5b Merge tag 'media/v4.20-4' of git://git.kernel..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14d2796d400000
kernel config: https://syzkaller.appspot.com/x/.config?x=b9cc5a440391cbfd
dashboard link: https://syzkaller.appspot.com/bug?extid=296ea3811c64964727aa
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
userspace arch: i386
Unfortunately, I don't have any reproducer for this crash yet.
IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+296ea3811c6496472...@syzkaller.appspotmail.com
======================================================
WARNING: possible circular locking dependency detected
4.20.0-rc5+ #265 Not tainted
------------------------------------------------------
kworker/0:3/6216 is trying to acquire lock:
000000006d826351 (epmutex){+.+.}, at: ep_free+0xf6/0x300 fs/eventpoll.c:829
but task is already holding lock:
000000005098accc ((delayed_fput_work).work){+.+.}, at:
process_one_work+0xb9a/0x1c40 kernel/workqueue.c:2128
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #4 ((delayed_fput_work).work){+.+.}:
process_one_work+0xc0a/0x1c40 kernel/workqueue.c:2129
worker_thread+0x17f/0x1390 kernel/workqueue.c:2296
kthread+0x35a/0x440 kernel/kthread.c:246
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352
-> #3 ((wq_completion)"events"){+.+.}:
flush_workqueue+0x30a/0x1e10 kernel/workqueue.c:2655
flush_scheduled_work include/linux/workqueue.h:599 [inline]
vim2m_stop_streaming+0x7c/0x2c0 drivers/media/platform/vim2m.c:811
__vb2_queue_cancel+0x171/0xd20
drivers/media/common/videobuf2/videobuf2-core.c:1823
vb2_core_queue_release+0x26/0x80
drivers/media/common/videobuf2/videobuf2-core.c:2229
vb2_queue_release+0x15/0x20
drivers/media/common/videobuf2/videobuf2-v4l2.c:837
v4l2_m2m_ctx_release+0x2a/0x35
drivers/media/v4l2-core/v4l2-mem2mem.c:931
vim2m_release+0xe6/0x150 drivers/media/platform/vim2m.c:977
v4l2_release+0x224/0x3a0 drivers/media/v4l2-core/v4l2-dev.c:456
__fput+0x385/0xa30 fs/file_table.c:278
____fput+0x15/0x20 fs/file_table.c:309
task_work_run+0x1e8/0x2a0 kernel/task_work.c:113
tracehook_notify_resume include/linux/tracehook.h:188 [inline]
exit_to_usermode_loop+0x318/0x380 arch/x86/entry/common.c:166
prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
syscall_return_slowpath arch/x86/entry/common.c:268 [inline]
do_syscall_32_irqs_on arch/x86/entry/common.c:341 [inline]
do_fast_syscall_32+0xcd5/0xfb2 arch/x86/entry/common.c:397
entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
-> #2 (&dev->dev_mutex){+.+.}:
__mutex_lock_common kernel/locking/mutex.c:925 [inline]
__mutex_lock+0x166/0x16f0 kernel/locking/mutex.c:1072
mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1087
v4l2_m2m_fop_poll+0x98/0x120
drivers/media/v4l2-core/v4l2-mem2mem.c:1105
v4l2_poll+0x153/0x200 drivers/media/v4l2-core/v4l2-dev.c:350
vfs_poll include/linux/poll.h:86 [inline]
ep_item_poll.isra.15+0x15c/0x400 fs/eventpoll.c:892
ep_insert+0x781/0x1dd0 fs/eventpoll.c:1464
__do_sys_epoll_ctl fs/eventpoll.c:2121 [inline]
__se_sys_epoll_ctl fs/eventpoll.c:2007 [inline]
__ia32_sys_epoll_ctl+0xeda/0x1080 fs/eventpoll.c:2007
do_syscall_32_irqs_on arch/x86/entry/common.c:326 [inline]
do_fast_syscall_32+0x34d/0xfb2 arch/x86/entry/common.c:397
entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
-> #1 (&ep->mtx){+.+.}:
__mutex_lock_common kernel/locking/mutex.c:925 [inline]
__mutex_lock+0x166/0x16f0 kernel/locking/mutex.c:1072
mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1087
ep_free+0x160/0x300 fs/eventpoll.c:849
ep_eventpoll_release+0x44/0x60 fs/eventpoll.c:869
__fput+0x385/0xa30 fs/file_table.c:278
____fput+0x15/0x20 fs/file_table.c:309
task_work_run+0x1e8/0x2a0 kernel/task_work.c:113
tracehook_notify_resume include/linux/tracehook.h:188 [inline]
exit_to_usermode_loop+0x318/0x380 arch/x86/entry/common.c:166
prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
syscall_return_slowpath arch/x86/entry/common.c:268 [inline]
do_syscall_64+0x6be/0x820 arch/x86/entry/common.c:293
entry_SYSCALL_64_after_hwframe+0x49/0xbe
-> #0 (epmutex){+.+.}:
lock_acquire+0x1ed/0x520 kernel/locking/lockdep.c:3844
__mutex_lock_common kernel/locking/mutex.c:925 [inline]
__mutex_lock+0x166/0x16f0 kernel/locking/mutex.c:1072
mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1087
ep_free+0xf6/0x300 fs/eventpoll.c:829
ep_eventpoll_release+0x44/0x60 fs/eventpoll.c:869
__fput+0x385/0xa30 fs/file_table.c:278
delayed_fput+0x55/0x80 fs/file_table.c:304
process_one_work+0xc90/0x1c40 kernel/workqueue.c:2153
worker_thread+0x17f/0x1390 kernel/workqueue.c:2296
kthread+0x35a/0x440 kernel/kthread.c:246
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352
other info that might help us debug this:
Chain exists of:
epmutex --> (wq_completion)"events" --> (delayed_fput_work).work
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock((delayed_fput_work).work);
lock((wq_completion)"events");
lock((delayed_fput_work).work);
lock(epmutex);
*** DEADLOCK ***
2 locks held by kworker/0:3/6216:
#0: 000000000aab2aee ((wq_completion)"events"){+.+.}, at:
__write_once_size include/linux/compiler.h:209 [inline]
#0: 000000000aab2aee ((wq_completion)"events"){+.+.}, at:
arch_atomic64_set arch/x86/include/asm/atomic64_64.h:34 [inline]
#0: 000000000aab2aee ((wq_completion)"events"){+.+.}, at: atomic64_set
include/asm-generic/atomic-instrumented.h:40 [inline]
#0: 000000000aab2aee ((wq_completion)"events"){+.+.}, at: atomic_long_set
include/asm-generic/atomic-long.h:59 [inline]
#0: 000000000aab2aee ((wq_completion)"events"){+.+.}, at: set_work_data
kernel/workqueue.c:617 [inline]
#0: 000000000aab2aee ((wq_completion)"events"){+.+.}, at:
set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
#0: 000000000aab2aee ((wq_completion)"events"){+.+.}, at:
process_one_work+0xb43/0x1c40 kernel/workqueue.c:2124
#1: 000000005098accc ((delayed_fput_work).work){+.+.}, at:
process_one_work+0xb9a/0x1c40 kernel/workqueue.c:2128
stack backtrace:
CPU: 0 PID: 6216 Comm: kworker/0:3 Not tainted 4.20.0-rc5+ #265
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Workqueue: events delayed_fput
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x244/0x39d lib/dump_stack.c:113
print_circular_bug.isra.35.cold.54+0x1bd/0x27d
kernel/locking/lockdep.c:1221
check_prev_add kernel/locking/lockdep.c:1863 [inline]
check_prevs_add kernel/locking/lockdep.c:1976 [inline]
validate_chain kernel/locking/lockdep.c:2347 [inline]
__lock_acquire+0x3399/0x4c20 kernel/locking/lockdep.c:3341
lock_acquire+0x1ed/0x520 kernel/locking/lockdep.c:3844
__mutex_lock_common kernel/locking/mutex.c:925 [inline]
__mutex_lock+0x166/0x16f0 kernel/locking/mutex.c:1072
mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1087
ep_free+0xf6/0x300 fs/eventpoll.c:829
ep_eventpoll_release+0x44/0x60 fs/eventpoll.c:869
__fput+0x385/0xa30 fs/file_table.c:278
delayed_fput+0x55/0x80 fs/file_table.c:304
process_one_work+0xc90/0x1c40 kernel/workqueue.c:2153
worker_thread+0x17f/0x1390 kernel/workqueue.c:2296
kthread+0x35a/0x440 kernel/kthread.c:246
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352
audit: type=1326 audit(1543921229.079:66): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23841 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=265 compat=1
ip=0xf7f68a29 code=0x50000
kobject: 'loop1' (00000000b08eabe4): kobject_uevent_env
audit: type=1326 audit(1543921229.079:67): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23841 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
kobject: 'loop1' (00000000b08eabe4): fill_kobj_path: path
= '/devices/virtual/block/loop1'
audit: type=1326 audit(1543921229.079:68): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23841 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
kobject: 'loop5' (0000000095b64449): kobject_uevent_env
kobject: 'loop5' (0000000095b64449): fill_kobj_path: path
= '/devices/virtual/block/loop5'
audit: type=1326 audit(1543921229.079:69): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23841 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
audit: type=1326 audit(1543921229.079:70): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23841 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
kobject: 'loop0' (00000000c90ec763): kobject_uevent_env
audit: type=1326 audit(1543921229.079:71): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23841 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
kobject: 'loop0' (00000000c90ec763): fill_kobj_path: path
= '/devices/virtual/block/loop0'
kobject: 'loop3' (00000000287e60d6): kobject_uevent_env
audit: type=1326 audit(1543921229.079:72): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23841 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
kobject: 'loop3' (00000000287e60d6): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop5' (0000000095b64449): kobject_uevent_env
audit: type=1326 audit(1543921229.079:73): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23841 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
kobject: 'loop5' (0000000095b64449): fill_kobj_path: path
= '/devices/virtual/block/loop5'
audit: type=1326 audit(1543921229.079:74): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23841 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
kobject: 'loop2' (00000000a9a0cc13): kobject_uevent_env
audit: type=1326 audit(1543921229.079:75): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23841 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
kobject: 'loop2' (00000000a9a0cc13): fill_kobj_path: path
= '/devices/virtual/block/loop2'
audit: type=1326 audit(1543921229.079:76): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23841 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
kobject: 'loop1' (00000000b08eabe4): kobject_uevent_env
kobject: 'loop1' (00000000b08eabe4): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop2' (00000000a9a0cc13): kobject_uevent_env
kobject: 'loop2' (00000000a9a0cc13): fill_kobj_path: path
= '/devices/virtual/block/loop2'
kobject: 'loop5' (0000000095b64449): kobject_uevent_env
kobject: 'loop5' (0000000095b64449): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop0' (00000000c90ec763): kobject_uevent_env
kobject: 'loop0' (00000000c90ec763): fill_kobj_path: path
= '/devices/virtual/block/loop0'
kobject: 'loop1' (00000000b08eabe4): kobject_uevent_env
kobject: 'loop1' (00000000b08eabe4): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop4' (00000000b69c11d9): kobject_uevent_env
kobject: 'loop4' (00000000b69c11d9): fill_kobj_path: path
= '/devices/virtual/block/loop4'
kobject: 'loop2' (00000000a9a0cc13): kobject_uevent_env
kobject: 'loop2' (00000000a9a0cc13): fill_kobj_path: path
= '/devices/virtual/block/loop2'
kobject: 'loop0' (00000000c90ec763): kobject_uevent_env
kobject: 'loop0' (00000000c90ec763): fill_kobj_path: path
= '/devices/virtual/block/loop0'
kobject: 'loop5' (0000000095b64449): kobject_uevent_env
kobject: 'loop5' (0000000095b64449): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop3' (00000000287e60d6): kobject_uevent_env
kobject: 'loop3' (00000000287e60d6): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop1' (00000000b08eabe4): kobject_uevent_env
kobject: 'loop1' (00000000b08eabe4): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop3' (00000000287e60d6): kobject_uevent_env
kobject: 'loop3' (00000000287e60d6): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop5' (0000000095b64449): kobject_uevent_env
kobject: 'loop5' (0000000095b64449): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop1' (00000000b08eabe4): kobject_uevent_env
kobject: 'loop1' (00000000b08eabe4): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop0' (00000000c90ec763): kobject_uevent_env
kobject: 'loop0' (00000000c90ec763): fill_kobj_path: path
= '/devices/virtual/block/loop0'
kobject: 'loop5' (0000000095b64449): kobject_uevent_env
kobject: 'loop5' (0000000095b64449): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kauditd_printk_skb: 30514 callbacks suppressed
audit: type=1326 audit(1543921234.119:30591): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23893 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
audit: type=1326 audit(1543921234.149:30592): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23893 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
audit: type=1326 audit(1543921234.149:30593): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23893 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
audit: type=1326 audit(1543921234.149:30594): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23893 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
audit: type=1326 audit(1543921234.149:30595): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23893 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
audit: type=1326 audit(1543921234.149:30596): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23893 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
audit: type=1326 audit(1543921234.149:30597): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23893 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
audit: type=1326 audit(1543921234.149:30598): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23893 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
audit: type=1326 audit(1543921234.149:30599): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23893 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
audit: type=1326 audit(1543921234.149:30600): auid=4294967295 uid=0 gid=0
ses=4294967295 subj==unconfined pid=23893 comm="syz-executor4"
exe="/root/syz-executor4" sig=0 arch=40000003 syscall=240 compat=1
ip=0xf7f68a29 code=0x50000
kobject: 'loop4' (00000000b69c11d9): kobject_uevent_env
kobject: 'loop4' (00000000b69c11d9): fill_kobj_path: path
= '/devices/virtual/block/loop4'
kobject: 'loop0' (00000000c90ec763): kobject_uevent_env
kobject: 'loop0' (00000000c90ec763): fill_kobj_path: path
= '/devices/virtual/block/loop0'
kobject: 'loop2' (00000000a9a0cc13): kobject_uevent_env
kobject: 'loop2' (00000000a9a0cc13): fill_kobj_path: path
= '/devices/virtual/block/loop2'
kobject: 'loop5' (0000000095b64449): kobject_uevent_env
kobject: 'loop5' (0000000095b64449): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop1' (00000000b08eabe4): kobject_uevent_env
kobject: 'loop1' (00000000b08eabe4): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop3' (00000000287e60d6): kobject_uevent_env
kobject: 'loop3' (00000000287e60d6): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop5' (0000000095b64449): kobject_uevent_env
kobject: 'loop5' (0000000095b64449): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop0' (00000000c90ec763): kobject_uevent_env
kobject: 'loop0' (00000000c90ec763): fill_kobj_path: path
= '/devices/virtual/block/loop0'
kobject: 'loop5' (0000000095b64449): kobject_uevent_env
kobject: 'loop5' (0000000095b64449): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop1' (00000000b08eabe4): kobject_uevent_env
kobject: 'loop1' (00000000b08eabe4): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop3' (00000000287e60d6): kobject_uevent_env
kobject: 'loop3' (00000000287e60d6): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop4' (00000000b69c11d9): kobject_uevent_env
kobject: 'loop4' (00000000b69c11d9): fill_kobj_path: path
= '/devices/virtual/block/loop4'
kobject: 'loop0' (00000000c90ec763): kobject_uevent_env
kobject: 'loop0' (00000000c90ec763): fill_kobj_path: path
= '/devices/virtual/block/loop0'
kobject: 'loop1' (00000000b08eabe4): kobject_uevent_env
kobject: 'loop1' (00000000b08eabe4): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop2' (00000000a9a0cc13): kobject_uevent_env
kobject: 'loop2' (00000000a9a0cc13): fill_kobj_path: path
= '/devices/virtual/block/loop2'
kobject: 'loop3' (00000000287e60d6): kobject_uevent_env
kobject: 'loop3' (00000000287e60d6): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop5' (0000000095b64449): kobject_uevent_env
kobject: 'loop5' (0000000095b64449): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop3' (00000000287e60d6): kobject_uevent_env
kobject: 'loop3' (00000000287e60d6): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop2' (00000000a9a0cc13): kobject_uevent_env
kobject: 'loop2' (00000000a9a0cc13): fill_kobj_path: path
= '/devices/virtual/block/loop2'
kobject: 'loop5' (0000000095b64449): kobject_uevent_env
kobject: 'loop5' (0000000095b64449): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop0' (00000000c90ec763): kobject_uevent_env
kobject: 'loop0' (00000000c90ec763): fill_kobj_path: path
= '/devices/virtual/block/loop0'
---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkal...@googlegroups.com.
syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#bug-status-tracking for how to communicate with
syzbot.
