ptrace: implement PTRACE_LISTEN

The previous patch implemented async notification for ptrace but it
only worked while trace is running.  This patch introduces
PTRACE_LISTEN which is suggested by Oleg Nestrov.

It's allowed iff tracee is in STOP trap and puts tracee into
quasi-running state - tracee never really runs but wait(2) and
ptrace(2) consider it to be running.  While ptracer is listening,
tracee is allowed to re-enter STOP to notify an async event.
Listening state is cleared on the first notification.  Ptracer can
also clear it by issuing INTERRUPT - tracee will re-trap into STOP
with listening state cleared.

This allows ptracer to monitor group stop state without running tracee
- use INTERRUPT to put tracee into STOP trap, issue LISTEN and then
wait(2) to wait for the next group stop event.  When it happens,
PTRACE_GETSIGINFO provides information to determine the current state.

Test program follows.

  #define PTRACE_SEIZE		0x4206
  #define PTRACE_INTERRUPT	0x4207
  #define PTRACE_LISTEN		0x4208

  #define PTRACE_SEIZE_DEVEL	0x80000000

  static const struct timespec ts1s = { .tv_sec = 1 };

  int main(int argc, char **argv)
  {
	  pid_t tracee, tracer;
	  int i;

	  tracee = fork();
	  if (!tracee)
		  while (1)
			  pause();

	  tracer = fork();
	  if (!tracer) {
		  siginfo_t si;

		  ptrace(PTRACE_SEIZE, tracee, NULL,
			 (void *)(unsigned long)PTRACE_SEIZE_DEVEL);
		  ptrace(PTRACE_INTERRUPT, tracee, NULL, NULL);
	  repeat:
		  waitid(P_PID, tracee, NULL, WSTOPPED);

		  ptrace(PTRACE_GETSIGINFO, tracee, NULL, &si);
		  if (!si.si_code) {
			  printf("tracer: SIG %d\n", si.si_signo);
			  ptrace(PTRACE_CONT, tracee, NULL,
				 (void *)(unsigned long)si.si_signo);
			  goto repeat;
		  }
		  printf("tracer: stopped=%d signo=%d\n",
			 si.si_signo != SIGTRAP, si.si_signo);
		  if (si.si_signo != SIGTRAP)
			  ptrace(PTRACE_LISTEN, tracee, NULL, NULL);
		  else
			  ptrace(PTRACE_CONT, tracee, NULL, NULL);
		  goto repeat;
	  }

	  for (i = 0; i < 3; i++) {
		  nanosleep(&ts1s, NULL);
		  printf("mother: SIGSTOP\n");
		  kill(tracee, SIGSTOP);
		  nanosleep(&ts1s, NULL);
		  printf("mother: SIGCONT\n");
		  kill(tracee, SIGCONT);
	  }
	  nanosleep(&ts1s, NULL);

	  kill(tracer, SIGKILL);
	  kill(tracee, SIGKILL);
	  return 0;
  }

This is identical to the program to test TRAP_NOTIFY except that
tracee is PTRACE_LISTEN'd instead of PTRACE_CONT'd when group stopped.
This allows ptracer to monitor when group stop ends without running
tracee.

  # ./test-listen
  tracer: stopped=0 signo=5
  mother: SIGSTOP
  tracer: SIG 19
  tracer: stopped=1 signo=19
  mother: SIGCONT
  tracer: stopped=0 signo=5
  tracer: SIG 18
  mother: SIGSTOP
  tracer: SIG 19
  tracer: stopped=1 signo=19
  mother: SIGCONT
  tracer: stopped=0 signo=5
  tracer: SIG 18
  mother: SIGSTOP
  tracer: SIG 19
  tracer: stopped=1 signo=19
  mother: SIGCONT
  tracer: stopped=0 signo=5
  tracer: SIG 18

-v2: Moved JOBCTL_LISTENING check in wait_task_stopped() into
     task_stopped_code() as suggested by Oleg.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>

diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index ad754d1e0b135d0a67c4e3f7c7c389eba083bb17..4f224f1695240859e810921fe5f3ee0ea4de4265 100644 (file)
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -49,6 +49,7 @@
 
 #define PTRACE_SEIZE           0x4206
 #define PTRACE_INTERRUPT       0x4207
+#define PTRACE_LISTEN          0x4208
 
 /* flags in @data for PTRACE_SEIZE */
 #define PTRACE_SEIZE_DEVEL     0x80000000 /* temp flag for development */

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1854def284f5b0f4e1a89ad8f1591638b58a74ce..87f7ca7ed6f6c11e59d49e87fdc7a00bc2450d25 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1813,6 +1813,7 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *
 #define JOBCTL_TRAP_STOP_BIT   19      /* trap for STOP */
 #define JOBCTL_TRAP_NOTIFY_BIT 20      /* trap for NOTIFY */
 #define JOBCTL_TRAPPING_BIT    21      /* switching to TRACED */
+#define JOBCTL_LISTENING_BIT   22      /* ptracer is listening for events */
 
 #define JOBCTL_STOP_DEQUEUED   (1 << JOBCTL_STOP_DEQUEUED_BIT)
 #define JOBCTL_STOP_PENDING    (1 << JOBCTL_STOP_PENDING_BIT)
@@ -1820,6 +1821,7 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *
 #define JOBCTL_TRAP_STOP       (1 << JOBCTL_TRAP_STOP_BIT)
 #define JOBCTL_TRAP_NOTIFY     (1 << JOBCTL_TRAP_NOTIFY_BIT)
 #define JOBCTL_TRAPPING                (1 << JOBCTL_TRAPPING_BIT)
+#define JOBCTL_LISTENING       (1 << JOBCTL_LISTENING_BIT)
 
 #define JOBCTL_TRAP_MASK       (JOBCTL_TRAP_STOP | JOBCTL_TRAP_NOTIFY)
 #define JOBCTL_PENDING_MASK    (JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK)

diff --git a/kernel/exit.c b/kernel/exit.c
index 20a406471525af2087cf914d8569d401306b3a5b..289f59d686bfc212eb8e03e36e8f17fa1edc855c 100644 (file)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1368,7 +1368,8 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
 static int *task_stopped_code(struct task_struct *p, bool ptrace)
 {
        if (ptrace) {
-               if (task_is_stopped_or_traced(p))
+               if (task_is_stopped_or_traced(p) &&
+                   !(p->jobctl & JOBCTL_LISTENING))
                        return &p->exit_code;
        } else {
                if (p->signal->flags & SIGNAL_STOP_STOPPED)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 6852c0f4a91691f99a2ff7911d18d76535ee52d8..e18966c1c0da4d2724e72dad79da8324a8a971ed 100644 (file)
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -146,7 +146,8 @@ int ptrace_check_attach(struct task_struct *child, bool ignore_state)
                 */
                spin_lock_irq(&child->sighand->siglock);
                WARN_ON_ONCE(task_is_stopped(child));
-               if (task_is_traced(child) || ignore_state)
+               if (ignore_state || (task_is_traced(child) &&
+                                    !(child->jobctl & JOBCTL_LISTENING)))
                        ret = 0;
                spin_unlock_irq(&child->sighand->siglock);
        }
@@ -660,7 +661,7 @@ int ptrace_request(struct task_struct *child, long request,
 {
        bool seized = child->ptrace & PT_SEIZED;
        int ret = -EIO;
-       siginfo_t siginfo;
+       siginfo_t siginfo, *si;
        void __user *datavp = (void __user *) data;
        unsigned long __user *datalp = datavp;
        unsigned long flags;
@@ -710,8 +711,43 @@ int ptrace_request(struct task_struct *child, long request,
                if (unlikely(!seized || !lock_task_sighand(child, &flags)))
                        break;
 
+               /*
+                * INTERRUPT doesn't disturb existing trap sans one
+                * exception.  If ptracer issued LISTEN for the current
+                * STOP, this INTERRUPT should clear LISTEN and re-trap
+                * tracee into STOP.
+                */
                if (likely(task_set_jobctl_pending(child, JOBCTL_TRAP_STOP)))
-                       signal_wake_up(child, 0);
+                       signal_wake_up(child, child->jobctl & JOBCTL_LISTENING);
+
+               unlock_task_sighand(child, &flags);
+               ret = 0;
+               break;
+
+       case PTRACE_LISTEN:
+               /*
+                * Listen for events.  Tracee must be in STOP.  It's not
+                * resumed per-se but is not considered to be in TRACED by
+                * wait(2) or ptrace(2).  If an async event (e.g. group
+                * stop state change) happens, tracee will enter STOP trap
+                * again.  Alternatively, ptracer can issue INTERRUPT to
+                * finish listening and re-trap tracee into STOP.
+                */
+               if (unlikely(!seized || !lock_task_sighand(child, &flags)))
+                       break;
+
+               si = child->last_siginfo;
+               if (unlikely(!si || si->si_code >> 8 != PTRACE_EVENT_STOP))
+                       break;
+
+               child->jobctl |= JOBCTL_LISTENING;
+
+               /*
+                * If NOTIFY is set, it means event happened between start
+                * of this trap and now.  Trigger re-trap immediately.
+                */
+               if (child->jobctl & JOBCTL_TRAP_NOTIFY)
+                       signal_wake_up(child, true);
 
                unlock_task_sighand(child, &flags);
                ret = 0;

diff --git a/kernel/signal.c b/kernel/signal.c
index 06177e2b391705fa4792c5c70886e234f5c67a2c..97e575a3387ef57a2b53088446adee7cdae75394 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -825,9 +825,11 @@ static int check_kill_permission(int sig, struct siginfo *info,
  * TRAP_STOP to notify ptracer of an event.  @t must have been seized by
  * ptracer.
  *
- * If @t is running, STOP trap will be taken.  If already trapped, STOP
- * trap will be eventually taken without returning to userland after the
- * existing traps are finished by PTRACE_CONT.
+ * If @t is running, STOP trap will be taken.  If trapped for STOP and
+ * ptracer is listening for events, tracee is woken up so that it can
+ * re-trap for the new event.  If trapped otherwise, STOP trap will be
+ * eventually taken without returning to userland after the existing traps
+ * are finished by PTRACE_CONT.
  *
  * CONTEXT:
  * Must be called with @task->sighand->siglock held.
@@ -838,7 +840,7 @@ static void ptrace_trap_notify(struct task_struct *t)
        assert_spin_locked(&t->sighand->siglock);
 
        task_set_jobctl_pending(t, JOBCTL_TRAP_NOTIFY);
-       signal_wake_up(t, 0);
+       signal_wake_up(t, t->jobctl & JOBCTL_LISTENING);
 }
 
 /*
@@ -1894,6 +1896,9 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
        spin_lock_irq(&current->sighand->siglock);
        current->last_siginfo = NULL;
 
+       /* LISTENING can be set only during STOP traps, clear it */
+       current->jobctl &= ~JOBCTL_LISTENING;
+
        /*
         * Queued signals ignored us while we were stopped for tracing.
         * So check for any that we should take before resuming user mode.