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Subject[PATCH 17/17] ptrace: implement PTRACE_LISTEN
Date
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) {
int stopped;
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;
}
stopped = !!si.si_status;
printf("tracer: stopped=%d signo=%d\n", stopped, si.si_signo);
if (stopped)
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

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
---
include/linux/ptrace.h | 1 +
include/linux/sched.h | 2 ++
kernel/exit.c | 2 +-
kernel/ptrace.c | 42 +++++++++++++++++++++++++++++++++++++++---
kernel/signal.c | 13 +++++++++----
5 files changed, 52 insertions(+), 8 deletions(-)
diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index 43f762d..abcfb2b 100644
--- 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 161658f..58ee03e 100644
--- 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 20a4064..b3bb3c2 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1416,7 +1416,7 @@ static int wait_task_stopped(struct wait_opts *wo,
spin_lock_irq(&p->sighand->siglock);

p_code = task_stopped_code(p, ptrace);
- if (unlikely(!p_code))
+ if (unlikely(!p_code) || p->jobctl & JOBCTL_LISTENING)
goto unlock_sig;

exit_code = *p_code;
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index f1efe07..3862142 100644
--- 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);
}
@@ -672,7 +673,7 @@ 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;
@@ -722,8 +723,43 @@ 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 != PTRACE_STOP_SI_CODE))
+ 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 5a72324..2ec1d08 100644
--- 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.
--
1.7.5.2


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