diff mbox series

[v2,1/5] sched,signal,ptrace: Rework TASK_TRACED, TASK_STOPPED state

Message ID 20220421150654.757693825@infradead.org
State New
Headers show
Series ptrace-vs-PREEMPT_RT and freezer rewrite | expand

Commit Message

Peter Zijlstra April 21, 2022, 3:02 p.m. UTC 
Currently ptrace_stop() / do_signal_stop() rely on the special states
TASK_TRACED and TASK_STOPPED resp. to keep unique state. That is, this
state exists only in task->__state and nowhere else.

There's two spots of bother with this:

 - PREEMPT_RT has task->saved_state which complicates matters,
   meaning task_is_{traced,stopped}() needs to check an additional
   variable.

 - An alternative freezer implementation that itself relies on a
   special TASK state would loose TASK_TRACED/TASK_STOPPED and will
   result in misbehaviour.

As such, add additional state to task->jobctl to track this state
outside of task->__state.

NOTE: this doesn't actually fix anything yet, just adds extra state.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/sched.h        |    8 +++-----
 include/linux/sched/jobctl.h |    6 ++++++
 include/linux/sched/signal.h |    5 ++++-
 kernel/ptrace.c              |   26 +++++++++++++++-----------
 kernel/signal.c              |   16 ++++++++++++----
 5 files changed, 40 insertions(+), 21 deletions(-)

Comments

Eric W. Biederman April 26, 2022, 11:34 p.m. UTC | #1 
Peter Zijlstra <peterz@infradead.org> writes:

> Currently ptrace_stop() / do_signal_stop() rely on the special states
> TASK_TRACED and TASK_STOPPED resp. to keep unique state. That is, this
> state exists only in task->__state and nowhere else.
>
> There's two spots of bother with this:
>
>  - PREEMPT_RT has task->saved_state which complicates matters,
>    meaning task_is_{traced,stopped}() needs to check an additional
>    variable.
>
>  - An alternative freezer implementation that itself relies on a
>    special TASK state would loose TASK_TRACED/TASK_STOPPED and will
>    result in misbehaviour.
>
> As such, add additional state to task->jobctl to track this state
> outside of task->__state.
>
> NOTE: this doesn't actually fix anything yet, just adds extra state.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -770,7 +773,9 @@ void signal_wake_up_state(struct task_st
>  	 * By using wake_up_state, we ensure the process will wake up and
>  	 * handle its death signal.
>  	 */
> -	if (!wake_up_state(t, state | TASK_INTERRUPTIBLE))
> +	if (wake_up_state(t, state | TASK_INTERRUPTIBLE))
> +		t->jobctl &= ~(JOBCTL_STOPPED | JOBCTL_TRACED);
> +	else
>  		kick_process(t);
>  }

This hunk is subtle and I don't think it is actually what we want if the
code is going to be robust against tsk->__state becoming TASK_FROZEN.

I think we want the clearing of JOBCTL_STOPPED and JOBCTL_TRACED
to be independent of what tsk->__state and tsk->saved_state are.

Something like:

static inline void signal_wake_up(struct task_struct *t, bool resume)
{
	unsigned int state = 0;
	if (resume && !(t->jobctl & JOBCTL_DELAY_WAKEKILL)) {
		t->jobctl &= ~(JOBCTL_STOPPED | JOBCTL_TRACED);
		state = TASK_WAKEKILL;
	}
	signal_wake_up_state(t, state);
}

static inline void ptrace_signal_wake_up(struct task_struct *t, bool resume)
{
	unsigned int state = 0;
	if (resume) {
		t->jobctl &= ~JOBCTL_TRACED;
		state = __TASK_TRACED;
	}
	signal_wake_up_state(t, state);
}

That would allow __set_task_special in the final patch to look like:

/*
 * The special task states (TASK_STOPPED, TASK_TRACED) keep their canonical
 * state in p->jobctl. If either of them got a wakeup that was missed because
 * TASK_FROZEN, then their canonical state reflects that and the below will
 * refuse to restore the special state and instead issue the wakeup.
 */
static int __set_task_special(struct task_struct *p, void *arg)
{
        unsigned int state = 0;

	if (p->jobctl & JOBCTL_TRACED)
        	state = TASK_TRACED;

	else if (p->jobctl & JOBCTL_STOPPED)
		state = TASK_STOPPED;

	if (state)
		WRITE_ONCE(p->__state, state);

	return state;
}


With no need to figure out if a wake_up was dropped and reverse engineer
what the wakeup was.

Eric
Peter Zijlstra April 28, 2022, 10 a.m. UTC | #2 
On Tue, Apr 26, 2022 at 06:34:09PM -0500, Eric W. Biederman wrote:
> Peter Zijlstra <peterz@infradead.org> writes:
> 
> > Currently ptrace_stop() / do_signal_stop() rely on the special states
> > TASK_TRACED and TASK_STOPPED resp. to keep unique state. That is, this
> > state exists only in task->__state and nowhere else.
> >
> > There's two spots of bother with this:
> >
> >  - PREEMPT_RT has task->saved_state which complicates matters,
> >    meaning task_is_{traced,stopped}() needs to check an additional
> >    variable.
> >
> >  - An alternative freezer implementation that itself relies on a
> >    special TASK state would loose TASK_TRACED/TASK_STOPPED and will
> >    result in misbehaviour.
> >
> > As such, add additional state to task->jobctl to track this state
> > outside of task->__state.
> >
> > NOTE: this doesn't actually fix anything yet, just adds extra state.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> 
> > --- a/kernel/signal.c
> > +++ b/kernel/signal.c
> > @@ -770,7 +773,9 @@ void signal_wake_up_state(struct task_st
> >  	 * By using wake_up_state, we ensure the process will wake up and
> >  	 * handle its death signal.
> >  	 */
> > -	if (!wake_up_state(t, state | TASK_INTERRUPTIBLE))
> > +	if (wake_up_state(t, state | TASK_INTERRUPTIBLE))
> > +		t->jobctl &= ~(JOBCTL_STOPPED | JOBCTL_TRACED);
> > +	else
> >  		kick_process(t);
> >  }
> 
> This hunk is subtle and I don't think it is actually what we want if the
> code is going to be robust against tsk->__state becoming TASK_FROZEN.

Oooh, indeed. Yes, let me go back to that resume based thing as you
suggest.

But first, let me go read all your patches :-)
diff mbox series

Patch

--- 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
--- a/include/linux/sched/jobctl.h
+++ b/include/linux/sched/jobctl.h
@@ -20,6 +20,9 @@  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	/* do_signal_stop() */
+#define JOBCTL_TRACED_BIT	25	/* ptrace_stop() */
+
 #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 +32,9 @@  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_TRAP_MASK	(JOBCTL_TRAP_STOP | JOBCTL_TRAP_NOTIFY)
 #define JOBCTL_PENDING_MASK	(JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK)
 
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -294,8 +294,10 @@  static inline int kernel_dequeue_signal(
 static inline void kernel_signal_stop(void)
 {
 	spin_lock_irq(&current->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(&current->sighand->siglock);
 
 	schedule();
@@ -439,6 +441,7 @@  static inline void signal_wake_up(struct
 {
 	signal_wake_up_state(t, resume ? TASK_WAKEKILL : 0);
 }
+
 static inline void ptrace_signal_wake_up(struct task_struct *t, bool resume)
 {
 	signal_wake_up_state(t, resume ? __TASK_TRACED : 0);
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -185,7 +185,12 @@  static bool looks_like_a_spurious_pid(st
 	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;
@@ -218,9 +223,10 @@  static void ptrace_unfreeze_traced(struc
 	 */
 	spin_lock_irq(&task->sighand->siglock);
 	if (READ_ONCE(task->__state) == __TASK_TRACED) {
-		if (__fatal_signal_pending(task))
+		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 +481,10 @@  static int ptrace_attach(struct task_str
 	 * 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);
 
@@ -850,8 +858,6 @@  static long ptrace_get_rseq_configuratio
 static int ptrace_resume(struct task_struct *child, long request,
 			 unsigned long data)
 {
-	bool need_siglock;
-
 	if (!valid_signal(data))
 		return -EIO;
 
@@ -892,13 +898,11 @@  static int ptrace_resume(struct task_str
 	 * status and clears the code too; this can't race with the tracee, it
 	 * takes siglock after resume.
 	 */
-	need_siglock = data && !thread_group_empty(current);
-	if (need_siglock)
-		spin_lock_irq(&child->sighand->siglock);
+	spin_lock_irq(&child->sighand->siglock);
 	child->exit_code = data;
+	child->jobctl &= ~JOBCTL_TRACED;
 	wake_up_state(child, __TASK_TRACED);
-	if (need_siglock)
-		spin_unlock_irq(&child->sighand->siglock);
+	spin_unlock_irq(&child->sighand->siglock);
 
 	return 0;
 }
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -762,7 +762,10 @@  static int dequeue_synchronous_signal(ke
  */
 void signal_wake_up_state(struct task_struct *t, unsigned int state)
 {
+	lockdep_assert_held(&t->sighand->siglock);
+
 	set_tsk_thread_flag(t, TIF_SIGPENDING);
+
 	/*
 	 * TASK_WAKEKILL also means wake it up in the stopped/traced/killable
 	 * case. We don't check t->state here because there is a race with it
@@ -770,7 +773,9 @@  void signal_wake_up_state(struct task_st
 	 * By using wake_up_state, we ensure the process will wake up and
 	 * handle its death signal.
 	 */
-	if (!wake_up_state(t, state | TASK_INTERRUPTIBLE))
+	if (wake_up_state(t, state | TASK_INTERRUPTIBLE))
+		t->jobctl &= ~(JOBCTL_STOPPED | JOBCTL_TRACED);
+	else
 		kick_process(t);
 }
 
@@ -884,7 +889,7 @@  static int check_kill_permission(int sig
 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 +935,10 @@  static bool prepare_signal(int sig, stru
 		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 +2225,7 @@  static int ptrace_stop(int exit_code, in
 	 * 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 +2467,7 @@  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(&current->sighand->siglock);