Messages in this thread | | | Date | Mon, 4 Apr 2022 18:13:39 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH v2] ptrace: fix ptrace vs tasklist_lock race on PREEMPT_RT. |
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Cough. Somehow I can hardly understand v2. For example, if we fix wait_task_inactive() correctly, then why ptrace_freeze_traced() should take saved_state into account? And how can unfreeze_traced hit saved_state == __TASK_TRACED ?
I will read this patch tommorrow again, I have a fever today, quite possibly I missed something...
But in any case, I think you need to update the changelog. Yes sure, ptrace/wait_task_inactive doesn't play well if CONFIG_PREEMPT_RT, but which exactly problem this patch tries to solve? and how?
And btw... What does the "Fix for ptrace_unfreeze_traced() by Oleg Nesterov" below mean? This all looks as if there is something else I am not aware of.
On 03/31, 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(). > @@ -2009,6 +2005,130 @@ static inline int test_tsk_need_resched( > return unlikely(test_tsk_thread_flag(tsk,TIF_NEED_RESCHED)); > } > > +#ifdef CONFIG_PREEMPT_RT > + > +static inline bool task_state_match_and(struct task_struct *tsk, long state) > +{ > + unsigned long flags; > + bool match = false; > + > + raw_spin_lock_irqsave(&tsk->pi_lock, flags); > + if (READ_ONCE(tsk->__state) & state) > + match = true; > + else if (tsk->saved_state & state) > + match = true; > + raw_spin_unlock_irqrestore(&tsk->pi_lock, flags); > + return match; > +} > + > +static inline int __task_state_match_eq(struct task_struct *tsk, long state) > +{ > + int match = 0; > + > + if (READ_ONCE(tsk->__state) == state) > + match = 1; > + else if (tsk->saved_state == state) > + match = -1; > + > + return match; > +} > + > +static inline int task_state_match_eq(struct task_struct *tsk, long state) > +{ > + unsigned long flags; > + int match; > + > + raw_spin_lock_irqsave(&tsk->pi_lock, flags); > + match = __task_state_match_eq(tsk, state); > + raw_spin_unlock_irqrestore(&tsk->pi_lock, flags); > + return match; > +} > + > +static inline bool task_state_match_and_set(struct task_struct *tsk, long state, > + long new_state) > +{ > + unsigned long flags; > + bool match = false; > + > + raw_spin_lock_irqsave(&tsk->pi_lock, flags); > + if (READ_ONCE(tsk->__state) & state) { > + WRITE_ONCE(tsk->__state, new_state); > + match = true; > + } else if (tsk->saved_state & state) { > + tsk->saved_state = new_state; > + match = true; > + } > + raw_spin_unlock_irqrestore(&tsk->pi_lock, flags); > + return match; > +} > + > +static inline bool task_state_match_eq_set(struct task_struct *tsk, long state, > + long new_state) > +{ > + unsigned long flags; > + bool match = false; > + > + raw_spin_lock_irqsave(&tsk->pi_lock, flags); > + if (READ_ONCE(tsk->__state) == state) { > + WRITE_ONCE(tsk->__state, new_state); > + match = true; > + } else if (tsk->saved_state == state) { > + tsk->saved_state = new_state; > + match = true; > + } > + raw_spin_unlock_irqrestore(&tsk->pi_lock, flags); > + return match; > +} > + > +#else > + > +static inline bool task_state_match_and(struct task_struct *tsk, long state) > +{ > + return READ_ONCE(tsk->__state) & state; > +} > + > +static inline int __task_state_match_eq(struct task_struct *tsk, long state) > +{ > + return READ_ONCE(tsk->__state) == state; > +} > + > +static inline int task_state_match_eq(struct task_struct *tsk, long state) > +{ > + return __task_state_match_eq(tsk, state); > +} > + > +static inline bool task_state_match_and_set(struct task_struct *tsk, long state, > + long new_state) > +{ > + if (READ_ONCE(tsk->__state) & state) { > + WRITE_ONCE(tsk->__state, new_state); > + return true; > + } > + return false; > +} > + > +static inline bool task_state_match_eq_set(struct task_struct *tsk, long state, > + long new_state) > +{ > + if (READ_ONCE(tsk->__state) == state) { > + WRITE_ONCE(tsk->__state, new_state); > + return true; > + } > + return false; > +} > + > +#endif > + > +static inline bool task_is_traced(struct task_struct *tsk) > +{ > + return task_state_match_and(tsk, __TASK_TRACED); > +} > + > +static inline bool task_is_stopped_or_traced(struct task_struct *tsk) > +{ > + return task_state_match_and(tsk, __TASK_STOPPED | __TASK_TRACED); > +} > + > /* > * cond_resched() and cond_resched_lock(): latency reduction via > * explicit rescheduling in places that are safe. The return > --- a/kernel/ptrace.c > +++ b/kernel/ptrace.c > @@ -195,10 +195,10 @@ static bool ptrace_freeze_traced(struct > return ret; > > spin_lock_irq(&task->sighand->siglock); > - if (task_is_traced(task) && !looks_like_a_spurious_pid(task) && > - !__fatal_signal_pending(task)) { > - WRITE_ONCE(task->__state, __TASK_TRACED); > - ret = true; > + if (!looks_like_a_spurious_pid(task) && !__fatal_signal_pending(task)) { > + > + ret = task_state_match_and_set(task, __TASK_TRACED, > + __TASK_TRACED); > } > spin_unlock_irq(&task->sighand->siglock); > > @@ -207,7 +207,10 @@ static bool ptrace_freeze_traced(struct > > static void ptrace_unfreeze_traced(struct task_struct *task) > { > - if (READ_ONCE(task->__state) != __TASK_TRACED) > + bool frozen; > + > + if (!IS_ENABLED(CONFIG_PREEMPT_RT) && > + READ_ONCE(task->__state) != __TASK_TRACED) > return; > > WARN_ON(!task->ptrace || task->parent != current); > @@ -217,12 +220,12 @@ static void ptrace_unfreeze_traced(struc > * Recheck state under the lock to close this race. > */ > spin_lock_irq(&task->sighand->siglock); > - if (READ_ONCE(task->__state) == __TASK_TRACED) { > - if (__fatal_signal_pending(task)) > - wake_up_state(task, __TASK_TRACED); > - else > - WRITE_ONCE(task->__state, TASK_TRACED); > - } > + > + frozen = task_state_match_eq_set(task, __TASK_TRACED, TASK_TRACED); > + > + if (frozen && __fatal_signal_pending(task)) > + wake_up_state(task, __TASK_TRACED); > + > spin_unlock_irq(&task->sighand->siglock); > } > > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -3219,6 +3219,8 @@ unsigned long wait_task_inactive(struct > struct rq *rq; > > for (;;) { > + int match_type = 0; > + > /* > * We do the initial early heuristics without holding > * any task-queue locks at all. We'll only try to get > @@ -3239,7 +3241,8 @@ unsigned long wait_task_inactive(struct > * 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(!task_state_match_eq(p, match_state))) > return 0; > cpu_relax(); > } > @@ -3254,7 +3257,9 @@ unsigned long wait_task_inactive(struct > running = task_running(rq, p); > queued = task_on_rq_queued(p); > ncsw = 0; > - if (!match_state || READ_ONCE(p->__state) == match_state) > + if (match_state) > + match_type = __task_state_match_eq(p, match_state); > + if (!match_state || match_type) > ncsw = p->nvcsw | LONG_MIN; /* sets MSB */ > task_rq_unlock(rq, p, &rf); > > @@ -3284,7 +3289,7 @@ unsigned long wait_task_inactive(struct > * running right now), it's preempted, and we should > * yield - it could be a while. > */ > - if (unlikely(queued)) { > + if (unlikely(queued || match_type < 0)) { > ktime_t to = NSEC_PER_SEC / HZ; > > set_current_state(TASK_UNINTERRUPTIBLE);
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