Messages in this thread | | | Date | Tue, 5 Apr 2022 12:10:26 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] ptrace: fix ptrace vs tasklist_lock race on PREEMPT_RT. |
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On Thu, Mar 31, 2022 at 04:25:42PM +0200, Sebastian Andrzej Siewior wrote: > As explained by Alexander Fyodorov <halcy@yandex.ru>: > > |read_lock(&tasklist_lock) in ptrace_stop() is converted to sleeping > |lock on a PREEMPT_RT kernel, and it can remove __TASK_TRACED from > |task->__state (by moving it to task->saved_state). If parent does > |wait() on child followed by a sys_ptrace call, the following race can > |happen: > | > |- child sets __TASK_TRACED in ptrace_stop() > |- parent does wait() which eventually calls wait_task_stopped() and returns > | child's pid > |- child blocks on read_lock(&tasklist_lock) in ptrace_stop() and moves > | __TASK_TRACED flag to saved_state > |- parent calls sys_ptrace, which calls ptrace_check_attach() and > | wait_task_inactive() > > The patch is based on his initial patch where an additional check is > added in case the __TASK_TRACED moved to ->saved_state. The pi_lock is > acquired to have stable view on ->__state and ->saved_state. > > wait_task_inactive() needs to check both task states while waiting for the > expected task state. Should the expected task state be in ->saved_state then > the task is blocked on a sleeping lock. In this case wait_task_inactive() needs > to wait until the lock situtation has been resolved (the expected state is in > ->__state). This ensures that the task is idle and does not wakeup as part of > lock resolving and races for instance with __switch_to_xtra() while the > debugger clears TIF_BLOCKSTEP() (noted by Oleg Nesterov). > > [ Fix for ptrace_unfreeze_traced() by Oleg Nesterov ] > > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > --- > v1…v2: > - Use also ->saved_state in task_state_match_and_set(). > - Wait in wait_task_inactive() until the desired task state is in > ->__state so that the task won't wake up a as part of lock > resolving. Pointed out by Oleg Nesterov. > > include/linux/sched.h | 128 ++++++++++++++++++++++++++++++++++++++++++++++++-- > kernel/ptrace.c | 25 +++++---- > kernel/sched/core.c | 11 +++- > 3 files changed, 146 insertions(+), 18 deletions(-) > > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -118,12 +118,8 @@ struct task_group; > > #define task_is_running(task) (READ_ONCE((task)->__state) == TASK_RUNNING) > > -#define task_is_traced(task) ((READ_ONCE(task->__state) & __TASK_TRACED) != 0) > - > #define task_is_stopped(task) ((READ_ONCE(task->__state) & __TASK_STOPPED) != 0) > > -#define task_is_stopped_or_traced(task) ((READ_ONCE(task->__state) & (__TASK_STOPPED | __TASK_TRACED)) != 0) > - > /* > * Special states are those that do not use the normal wait-loop pattern. See > * the comment with set_special_state().
Urgh, so I have reworking all this somewhere on my todo list as well. Except I mean to move it away from using p->__state entirely. We should not be keeping canonical state in there.
As is, I think we can write task_is_stopped() like:
#define task_is_stopped(task) ((task)->jobctl & JOBCTL_STOP_PENDING)
Because jobctl is in fact the canonical state. I'm still not sure if we can do the same with task_is_traced(), ideally that would be expressed in terms of (task)->ptrace. But ptrace_stop() hurts my brain. All that stuff is entirely to involved.
Anyway, let me see if I can page some of that back..
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