Re: [PATCH v4 12/12] sched,signal,ptrace: Rework TASK_TRACED, TASK_STOPPED state

Peter Zijlstra
2022-07-05T19:25:58Z

On Tue, Jul 05, 2022 at 07:28:49PM +0200, Sven Schnelle wrote:
> Sven Schnelle <svens@linux.ibm.com> writes:

> I think there's a race in ptrace_check_attach(). It first calls
> ptrace_freeze_task(), which checks whether JOBCTL_TRACED is set.
> If it is (and a few other conditions match) it will set ret = 0.
>
> Later outside of siglock and tasklist_lock it will call
> wait_task_inactive, assuming the target is in TASK_TRACED, but it isn't.
>
> ptrace_stop(), which runs on another CPU, does:
>
> set_special_state(TASK_TRACED);
> current->jobctl |= JOBCTL_TRACED;
>
> which looks ok on first sight, but in this case JOBCTL is already set,
> so the reading CPU will immediately move on to wait_task_inactive(),
> before JOBCTL_TRACED is set. I don't know whether this is a valid
> combination. I never looked into JOBCTL_* semantics, but i guess now
> is a good time to do so. I added some debugging statements, and that
> gives:
>
> [ 86.218488] kill_chi-300545 2d.... 79990135us : ptrace_stop: state 8
> [ 86.218492] kill_chi-300545 2d.... 79990136us : signal_generate: sig\x17 errno=0 code=4 comm=strace pid00542 grp=1 res=1
> [ 86.218496] kill_chi-300545 2d.... 79990136us : sched_stat_runtime: comm=kill_child pid00545 runtime058 [ns] vruntime`6165713178 [ns]
> [ 86.218500] kill_chi-300545 2d.... 79990136us : sched_switch: prev_comm=kill_child prev_pid00545 prev_prio\x120 prev_state=t => next_comm=swapper/2 next_pid=0 next_prio\x120
> [ 86.218504] strace-300542 7..... 79990139us : sys_ptrace -> 0x50
> [ 86.218508] strace-300542 7..... 79990139us : sys_write(fd: 2, buf: 2aa198f7ad0, count: 12)
> [ 86.218512] strace-300542 7..... 79990140us : sys_write -> 0x12
> [ 86.218515] <idle>-0 6dNh.. 79990140us : sched_wakeup: comm=kill_child pid43805 prio\x120 target_cpu\06
> [ 86.218519] <idle>-0 6d.... 79990140us : sched_switch: prev_comm=swapper/6 prev_pid=0 prev_prio\x120 prev_state=R => next_comm=kill_child next_pid43805 next_prio\x120
> [ 86.218524] strace-300542 7..... 79990140us : sys_write(fd: 2, buf: 2aa198f7ad0, count: 19)
> [ 86.218527] strace-300542 7..... 79990141us : sys_write -> 0x19
> [ 86.218531] kill_chi-343805 6..... 79990141us : sys_sched_yield -> 0xffffffffffffffda
> [ 86.218535] strace-300542 7..... 79990141us : sys_ptrace(request: 18, pid: 53efd, addr: 0, data: 0)
> [ 86.218539] kill_chi-343805 6d.... 79990141us : signal_deliver: sig=9 errno=0 code=0 sa_handler=0 sa_flags=0
> [ 86.218543] strace-300542 7d.... 79990141us : ptrace_check_attach: task_is_traced: 1, fatal signal pending: 0
> [ 86.218547] strace-300542 7..... 79990141us : ptrace_check_attach: child->pid = 343805, child->__flags=0
> [ 86.218551] kill_chi-343805 6d.... 79990141us : ptrace_stop: JOBCTL_TRACED already set, state=0 <------ valid combination of flags?

Yeah, that's not supposed to be so. JOBCTL_TRACED is supposed to follow
__TASK_TRACED for now. Set when __TASK_TRACED, cleared when
TASK_RUNNING.

Specifically {ptrace_,}signal_wake_up() in signal.h clear JOBCTL_TRACED
when they would wake a __TASK_TRACED task.

> [ 86.218554] kill_chi-343805 6d.... 79990141us : ptrace_stop: state 8
> [ 86.218558] kill_chi-343805 6d.... 79990142us : signal_generate: sig\x17 errno=0 code=4 comm=strace pid00542 grp=1 res=1
> [ 86.218562] kill_chi-343805 6d.... 79990142us : sched_stat_runtime: comm=kill_child pid43805 runtime!35 [ns] vruntimeU6109013931 [ns]
> [ 86.218566] strace-300542 7..... 79990142us : wait_task_inactive: NO MATCH: state 0, match_state 8, pid 343805
> [ 86.218570] kill_chi-343805 6d.... 79990142us : sched_switch: prev_comm=kill_child prev_pid43805 prev_prio\x120 prev_state=t =>next_comm=swapper/6 next_pid=0 next_prio\x120
>