Messages in this thread Patch in this message | | | Date | Thu, 7 Apr 2022 14:13:40 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] ptrace: fix ptrace vs tasklist_lock race on PREEMPT_RT. |
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On Tue, Apr 05, 2022 at 01:28:16PM +0200, Peter Zijlstra wrote: > On Tue, Apr 05, 2022 at 12:29:03PM +0200, Oleg Nesterov wrote: > > On 04/05, Peter Zijlstra wrote: > > > > > > 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. > > > > Not really. JOBCTL_STOP_PENDING means that the task should stop. > > > > And this flag is cleared right before set_special_state(TASK_STOPPED) > > in do_signal_stop(), see task_participate_group_stop(). > > Argh! More work then :-( Still, I really want to not have p->__state be > actual unique state.
How insane is this?
In addition to solving the whole saved_state issue, it also allows my freezer rewrite to reconstruct __state. If you don't find the below fundamentally broken I can respin that freezer rewrite on top of it.
--- include/linux/sched.h | 8 +++----- include/linux/sched/jobctl.h | 8 ++++++++ include/linux/sched/signal.h | 15 ++++++++++++++- kernel/ptrace.c | 18 ++++++++++++++---- kernel/signal.c | 9 ++++++--- 5 files changed, 45 insertions(+), 13 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h index 67f06f72c50e..6698b97b8e3b 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -118,11 +118,9 @@ 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) +#define task_is_traced(task) ((READ_ONCE(task->jobctl) & JOBCTL_TRACED) != 0) +#define task_is_stopped(task) ((READ_ONCE(task->jobctl) & JOBCTL_STOPPED) != 0) +#define task_is_stopped_or_traced(task) ((READ_ONCE(task->jobctl) & (JOBCTL_STOPPED | JOBCTL_TRACED)) != 0) /* * Special states are those that do not use the normal wait-loop pattern. See diff --git a/include/linux/sched/jobctl.h b/include/linux/sched/jobctl.h index fa067de9f1a9..ec8b312f7506 100644 --- a/include/linux/sched/jobctl.h +++ b/include/linux/sched/jobctl.h @@ -20,6 +20,10 @@ struct task_struct; #define JOBCTL_LISTENING_BIT 22 /* ptracer is listening for events */ #define JOBCTL_TRAP_FREEZE_BIT 23 /* trap for cgroup freezer */ +#define JOBCTL_STOPPED_BIT 24 +#define JOBCTL_TRACED_BIT 25 +#define JOBCTL_TRACED_FROZEN_BIT 26 + #define JOBCTL_STOP_DEQUEUED (1UL << JOBCTL_STOP_DEQUEUED_BIT) #define JOBCTL_STOP_PENDING (1UL << JOBCTL_STOP_PENDING_BIT) #define JOBCTL_STOP_CONSUME (1UL << JOBCTL_STOP_CONSUME_BIT) @@ -29,6 +33,10 @@ struct task_struct; #define JOBCTL_LISTENING (1UL << JOBCTL_LISTENING_BIT) #define JOBCTL_TRAP_FREEZE (1UL << JOBCTL_TRAP_FREEZE_BIT) +#define JOBCTL_STOPPED (1UL << JOBCTL_STOPPED_BIT) +#define JOBCTL_TRACED (1UL << JOBCTL_TRACED_BIT) +#define JOBCTL_TRACED_FROZEN (1UL << JOBCTL_TRACED_FROZEN_BIT) + #define JOBCTL_TRAP_MASK (JOBCTL_TRAP_STOP | JOBCTL_TRAP_NOTIFY) #define JOBCTL_PENDING_MASK (JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK) diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h index 3c8b34876744..3e4a14ed402e 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -294,8 +294,10 @@ static inline int kernel_dequeue_signal(void) static inline void kernel_signal_stop(void) { spin_lock_irq(¤t->sighand->siglock); - if (current->jobctl & JOBCTL_STOP_DEQUEUED) + if (current->jobctl & JOBCTL_STOP_DEQUEUED) { + current->jobctl |= JOBCTL_STOPPED; set_special_state(TASK_STOPPED); + } spin_unlock_irq(¤t->sighand->siglock); schedule(); @@ -437,10 +439,21 @@ extern void signal_wake_up_state(struct task_struct *t, unsigned int state); static inline void signal_wake_up(struct task_struct *t, bool resume) { + lockdep_assert_held(t->sighand->siglock); + + if (resume && !(t->jobctl & JOBCTL_TRACED_FROZEN)) + t->jobctl &= ~(JOBCTL_STOPPED | JOBCTL_TRACED); + signal_wake_up_state(t, resume ? TASK_WAKEKILL : 0); } + static inline void ptrace_signal_wake_up(struct task_struct *t, bool resume) { + lockdep_assert_held(t->sighand->siglock); + + if (resume) + t->jobctl &= ~JOBCTL_TRACED; + signal_wake_up_state(t, resume ? __TASK_TRACED : 0); } diff --git a/kernel/ptrace.c b/kernel/ptrace.c index ccc4b465775b..626f96d275c7 100644 --- a/kernel/ptrace.c +++ b/kernel/ptrace.c @@ -185,7 +185,12 @@ static bool looks_like_a_spurious_pid(struct task_struct *task) return true; } -/* Ensure that nothing can wake it up, even SIGKILL */ +/* + * Ensure that nothing can wake it up, even SIGKILL + * + * A task is switched to this state while a ptrace operation is in progress; + * such that the ptrace operation is uninterruptible. + */ static bool ptrace_freeze_traced(struct task_struct *task) { bool ret = false; @@ -197,6 +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)) { + task->jobctl |= JOBCTL_TRACED_FROZEN; WRITE_ONCE(task->__state, __TASK_TRACED); ret = true; } @@ -218,9 +224,11 @@ static void ptrace_unfreeze_traced(struct task_struct *task) */ spin_lock_irq(&task->sighand->siglock); if (READ_ONCE(task->__state) == __TASK_TRACED) { - if (__fatal_signal_pending(task)) + task->jobctl &= ~JOBCTL_TRACED_FROZEN; + if (__fatal_signal_pending(task)) { + task->jobctl &= ~JOBCTL_TRACED; wake_up_state(task, __TASK_TRACED); - else + } else WRITE_ONCE(task->__state, TASK_TRACED); } spin_unlock_irq(&task->sighand->siglock); @@ -475,8 +483,10 @@ static int ptrace_attach(struct task_struct *task, long request, * in and out of STOPPED are protected by siglock. */ if (task_is_stopped(task) && - task_set_jobctl_pending(task, JOBCTL_TRAP_STOP | JOBCTL_TRAPPING)) + task_set_jobctl_pending(task, JOBCTL_TRAP_STOP | JOBCTL_TRAPPING)) { + task->jobctl &= ~JOBCTL_STOPPED; signal_wake_up_state(task, __TASK_STOPPED); + } spin_unlock(&task->sighand->siglock); diff --git a/kernel/signal.c b/kernel/signal.c index 30cd1ca43bcd..0aea3f0a8002 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -884,7 +884,7 @@ static int check_kill_permission(int sig, struct kernel_siginfo *info, static void ptrace_trap_notify(struct task_struct *t) { WARN_ON_ONCE(!(t->ptrace & PT_SEIZED)); - assert_spin_locked(&t->sighand->siglock); + lockdep_assert_held(&t->sighand->siglock); task_set_jobctl_pending(t, JOBCTL_TRAP_NOTIFY); ptrace_signal_wake_up(t, t->jobctl & JOBCTL_LISTENING); @@ -930,9 +930,10 @@ static bool prepare_signal(int sig, struct task_struct *p, bool force) for_each_thread(p, t) { flush_sigqueue_mask(&flush, &t->pending); task_clear_jobctl_pending(t, JOBCTL_STOP_PENDING); - if (likely(!(t->ptrace & PT_SEIZED))) + if (likely(!(t->ptrace & PT_SEIZED))) { + t->jobctl &= ~JOBCTL_STOPPED; wake_up_state(t, __TASK_STOPPED); - else + } else ptrace_trap_notify(t); } @@ -2219,6 +2220,7 @@ static int ptrace_stop(int exit_code, int why, int clear_code, * schedule() will not sleep if there is a pending signal that * can awaken the task. */ + current->jobctl |= JOBCTL_TRACED; set_special_state(TASK_TRACED); /* @@ -2460,6 +2462,7 @@ static bool do_signal_stop(int signr) if (task_participate_group_stop(current)) notify = CLD_STOPPED; + current->jobctl |= JOBCTL_STOPPED; set_special_state(TASK_STOPPED); spin_unlock_irq(¤t->sighand->siglock);
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