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

Sven Schnelle
2022-07-05T17:28:49Z

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Sven Schnelle <svens@linux.ibm.com> writes:

> Steven Rostedt <rostedt@goodmis.org> writes:
>
>> On Tue, 21 Jun 2022 17:15:47 +0200
>> Alexander Gordeev <agordeev@linux.ibm.com> wrote:
>>
>>> So I assume (checked actually) the return 0 below from kernel/sched/core.c:
>>> wait_task_inactive() is where it bails out:
>>>
>>> 3303 while (task_running(rq, p)) {
>>> 3304 if (match_state && unlikely(READ_ONCE(p->__state) != match_state))
>>> 3305 return 0;
>>> 3306 cpu_relax();
>>> 3307 }
>>>
>>> Yet, the child task is always found in __TASK_TRACED state (as seen
>>> in crash dumps):
>>>
>>> > 101447 11342 13 ce3a8100 RU 0.0 10040 4412 strace
>>> 101450 101447 0 bb04b200 TR 0.0 2272 1136 kill_child
>>> 108261 101447 2 d0b10100 TR 0.0 2272 532 kill_child
>>> crash> task bb04b200 __state
>>> PID: 101450 TASK: bb04b200 CPU: 0 COMMAND: "kill_child"
>>> __state = 8,
>>>
>>> crash> task d0b10100 __state
>>> PID: 108261 TASK: d0b10100 CPU: 2 COMMAND: "kill_child"
>>> __state = 8,
>>
>> If you are using crash, can you enable all trace events?
>>
>> Then you should be able to extract the ftrace ring buffer from crash using
>> the trace.so extend (https://github.com/fujitsu/crash-trace)
>>
>> I guess it should still work with s390.
>>
>> Then you can see the events that lead up to the crash.

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=17 errno=0 code=4 comm=strace pid=300542 grp=1 res=1
[ 86.218496] kill_chi-300545 2d.... 79990136us : sched_stat_runtime: comm=kill_child pid=300545 runtime=3058 [ns] vruntime=606165713178 [ns]
[ 86.218500] kill_chi-300545 2d.... 79990136us : sched_switch: prev_comm=kill_child prev_pid=300545 prev_prio=120 prev_state=t ==> next_comm=swapper/2 next_pid=0 next_prio=120
[ 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 pid=343805 prio=120 target_cpu=006
[ 86.218519] <idle>-0 6d.... 79990140us : sched_switch: prev_comm=swapper/6 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kill_child next_pid=343805 next_prio=120
[ 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?
[ 86.218554] kill_chi-343805 6d.... 79990141us : ptrace_stop: state 8
[ 86.218558] kill_chi-343805 6d.... 79990142us : signal_generate: sig=17 errno=0 code=4 comm=strace pid=300542 grp=1 res=1
[ 86.218562] kill_chi-343805 6d.... 79990142us : sched_stat_runtime: comm=kill_child pid=343805 runtime=2135 [ns] vruntime=556109013931 [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_pid=343805 prev_prio=120 prev_state=t ==>next_comm=swapper/6 next_pid=0 next_prio=120

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diff --git a/include/linux/sched.h b/include/linux/sched.h
index c46f3a63b758..c2ddc47271b8 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -154,8 +154,8 @@ struct task_group;
} while (0)

#else
-# define debug_normal_state_change(cond) do { } while (0)
-# define debug_special_state_change(cond) do { } while (0)
+# define debug_normal_state_change(cond) do { trace_printk("state %d\n", cond); } while (0)
+# define debug_special_state_change(cond) do { trace_printk("state %d\n", cond); } while (0)
# define debug_rtlock_wait_set_state() do { } while (0)
# define debug_rtlock_wait_restore_state() do { } while (0)
#endif
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 156a99283b11..2cb2ae8acf23 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -202,6 +202,7 @@ static bool ptrace_freeze_traced(struct task_struct *task)
spin_lock_irq(&task->sighand->siglock);
if (task_is_traced(task) && !looks_like_a_spurious_pid(task) &&
!__fatal_signal_pending(task)) {
+ trace_printk("task_is_traced: %d, fatal signal pending: %d\n", task_is_traced(task), __fatal_signal_pending(task));
task->jobctl |= JOBCTL_PTRACE_FROZEN;
ret = true;
}
@@ -263,8 +264,10 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
* child->sighand can't be NULL, release_task()
* does ptrace_unlink() before __exit_signal().
*/
- if (ignore_state || ptrace_freeze_traced(child))
+ if (ignore_state || ptrace_freeze_traced(child)) {
+ trace_printk("child->pid = %d, child->__flags=%d\n", child->pid, child->__state);
ret = 0;
+ }
}
read_unlock(&tasklist_lock);

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index da0bf6fe9ecd..73bb4c7882d0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3301,8 +3301,10 @@ unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state
* is actually now running somewhere else!
*/
while (task_running(rq, p)) {
- if (match_state && unlikely(READ_ONCE(p->__state) != match_state))
+ if (match_state && unlikely(READ_ONCE(p->__state) != match_state)) {
+ trace_printk("NO MATCH: state %d, match_state %d, pid %d\n", p->__state, match_state, p->pid);
return 0;
+ }
cpu_relax();
}

diff --git a/kernel/signal.c b/kernel/signal.c
index edb1dc9b00dc..0ea8e6b6a641 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2232,6 +2232,8 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
if (!current->ptrace || __fatal_signal_pending(current))
return exit_code;

+ if (current->jobctl & JOBCTL_TRACED)
+ trace_printk("JOBCTL_TRACED already set, state=%d\n", current->__state);
set_special_state(TASK_TRACED);
current->jobctl |= JOBCTL_TRACED;

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I'll continue debugging tomorrow, but maybe this helps already.