ptrace: implement PTRACE_INTERRUPT

Currently, there's no way to trap a running ptracee short of sending a
signal which has various side effects.  This patch implements
PTRACE_INTERRUPT which traps ptracee without any signal or job control
related side effect.

The implementation is almost trivial.  It uses the group stop trap -
SIGTRAP | PTRACE_EVENT_STOP << 8.  A new trap flag
JOBCTL_TRAP_INTERRUPT is added, which is set on PTRACE_INTERRUPT and
cleared when any trap happens.  As INTERRUPT should be useable
regardless of the current state of tracee, task_is_traced() test in
ptrace_check_attach() is skipped for INTERRUPT.

PTRACE_INTERRUPT is available iff tracee is attached with
PTRACE_SEIZE.

Test program follows.

  #define PTRACE_SEIZE		0x4206
  #define PTRACE_INTERRUPT	0x4207

  #define PTRACE_SEIZE_DEVEL	0x80000000

  static const struct timespec ts100ms = { .tv_nsec = 100000000 };
  static const struct timespec ts1s = { .tv_sec = 1 };
  static const struct timespec ts3s = { .tv_sec = 3 };

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

	  tracee = fork();
	  if (tracee == 0) {
		  nanosleep(&ts100ms, NULL);
		  while (1) {
			  printf("tracee: alive pid=%d\n", getpid());
			  nanosleep(&ts1s, NULL);
		  }
	  }

	  if (argc > 1)
		  kill(tracee, SIGSTOP);

	  nanosleep(&ts100ms, NULL);

	  ptrace(PTRACE_SEIZE, tracee, NULL,
		 (void *)(unsigned long)PTRACE_SEIZE_DEVEL);
	  if (argc > 1) {
		  waitid(P_PID, tracee, NULL, WSTOPPED);
		  ptrace(PTRACE_CONT, tracee, NULL, NULL);
	  }
	  nanosleep(&ts3s, NULL);

	  printf("tracer: INTERRUPT and DETACH\n");
	  ptrace(PTRACE_INTERRUPT, tracee, NULL, NULL);
	  waitid(P_PID, tracee, NULL, WSTOPPED);
	  ptrace(PTRACE_DETACH, tracee, NULL, NULL);
	  nanosleep(&ts3s, NULL);

	  printf("tracer: exiting\n");
	  kill(tracee, SIGKILL);
	  return 0;
  }

When called without argument, tracee is seized from running state,
interrupted and then detached back to running state.

  # ./test-interrupt
  tracee: alive pid=4546
  tracee: alive pid=4546
  tracee: alive pid=4546
  tracer: INTERRUPT and DETACH
  tracee: alive pid=4546
  tracee: alive pid=4546
  tracee: alive pid=4546
  tracer: exiting

When called with argument, tracee is seized from stopped state,
continued, interrupted and then detached back to stopped state.

  # ./test-interrupt  1
  tracee: alive pid=4548
  tracee: alive pid=4548
  tracee: alive pid=4548
  tracer: INTERRUPT and DETACH
  tracer: exiting

Before PTRACE_INTERRUPT, once the tracee was running, there was no way
to trap tracee and do PTRACE_DETACH without causing side effect.

-v2: Updated to use task_set_jobctl_pending() so that it doesn't end
     up scheduling TRAP_STOP if child is dying which may make the
     child unkillable.  Spotted by Oleg.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>

include/linux/ptrace.h

@@ -48,6 +48,7 @@
 #define PTRACE_SETREGSET	0x4205
 
 #define PTRACE_SEIZE		0x4206
+#define PTRACE_INTERRUPT	0x4207
 
 /* flags in @data for PTRACE_SEIZE */
 #define PTRACE_SEIZE_DEVEL	0x80000000 /* temp flag for development */

kernel/ptrace.c

@@ -658,10 +658,12 @@ static int ptrace_regset(struct task_struct *task, int req, unsigned int type,
 int ptrace_request(struct task_struct *child, long request,
 		   unsigned long addr, unsigned long data)
 {
+	bool seized = child->ptrace & PT_SEIZED;
 	int ret = -EIO;
 	siginfo_t siginfo;
 	void __user *datavp = (void __user *) data;
 	unsigned long __user *datalp = datavp;
+	unsigned long flags;
 
 	switch (request) {
 	case PTRACE_PEEKTEXT:
@@ -694,6 +696,27 @@ int ptrace_request(struct task_struct *child, long request,
 			ret = ptrace_setsiginfo(child, &siginfo);
 		break;
 
+	case PTRACE_INTERRUPT:
+		/*
+		 * Stop tracee without any side-effect on signal or job
+		 * control.  At least one trap is guaranteed to happen
+		 * after this request.  If @child is already trapped, the
+		 * current trap is not disturbed and another trap will
+		 * happen after the current trap is ended with PTRACE_CONT.
+		 *
+		 * The actual trap might not be PTRACE_EVENT_STOP trap but
+		 * the pending condition is cleared regardless.
+		 */
+		if (unlikely(!seized || !lock_task_sighand(child, &flags)))
+			break;
+
+		if (likely(task_set_jobctl_pending(child, JOBCTL_TRAP_STOP)))
+			signal_wake_up(child, 0);
+
+		unlock_task_sighand(child, &flags);
+		ret = 0;
+		break;
+
 	case PTRACE_DETACH:	 /* detach a process that was attached. */
 		ret = ptrace_detach(child, data);
 		break;
@@ -819,7 +842,8 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
 		goto out_put_task_struct;
 	}
 
-	ret = ptrace_check_attach(child, request == PTRACE_KILL);
+	ret = ptrace_check_attach(child, request == PTRACE_KILL ||
+				  request == PTRACE_INTERRUPT);
 	if (ret < 0)
 		goto out_put_task_struct;
 
@@ -961,7 +985,8 @@ asmlinkage long compat_sys_ptrace(compat_long_t request, compat_long_t pid,
 		goto out_put_task_struct;
 	}
 
-	ret = ptrace_check_attach(child, request == PTRACE_KILL);
+	ret = ptrace_check_attach(child, request == PTRACE_KILL ||
+				  request == PTRACE_INTERRUPT);
 	if (!ret)
 		ret = compat_arch_ptrace(child, request, addr, data);