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Subject[PATCH 6/9] job control: make task_clear_jobctl_pending() clear TRAPPING automatically
Date
JOBCTL_TRAPPING indicates that ptracer is waiting for tracee to
(re)transit into TRACED. task_clear_jobctl_pending() must be called
when either tracee enters TRACED or the transition is cancelled for
some reason. The former is achieved by explicitly calling
task_clear_jobctl_pending() in ptrace_stop() and the latter by calling
it at the end of do_signal_stop().

Calling task_clear_jobctl_trapping() at the end of do_signal_stop()
limits the scope TRAPPING can be used and is fragile in that seemingly
unrelated changes to tracee's control flow can lead to stuck TRAPPING.

We already have task_clear_jobctl_pending() calls on those cancelling
events to clear JOBCTL_STOP_PENDING. Cancellations can be handled by
making those call sites use JOBCTL_PENDING_MASK instead and updating
task_clear_jobctl_pending() such that task_clear_jobctl_trapping() is
called automatically if no stop/trap is pending.

This patch makes the above changes and removes the fallback
task_clear_jobctl_trapping() call from do_signal_stop().

Signed-off-by: Tejun Heo <tj@kernel.org>
---
fs/exec.c | 2 +-
kernel/signal.c | 13 ++++++++-----
2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index c654664..6ef8958 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1659,7 +1659,7 @@ static int zap_process(struct task_struct *start, int exit_code)

t = start;
do {
- task_clear_jobctl_pending(t, JOBCTL_STOP_PENDING);
+ task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
if (t != current && t->mm) {
sigaddset(&t->pending.signal, SIGKILL);
signal_wake_up(t, 1);
diff --git a/kernel/signal.c b/kernel/signal.c
index e67eaa0..91e57d3 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -253,6 +253,9 @@ static void task_clear_jobctl_trapping(struct task_struct *task)
* %JOBCTL_PENDING_MASK. If %JOBCTL_STOP_PENDING is being cleared, other
* STOP bits are cleared together.
*
+ * If clearing of @mask leaves no stop or trap pending, this function calls
+ * task_clear_jobctl_trapping().
+ *
* CONTEXT:
* Must be called with @task->sighand->siglock held.
*/
@@ -264,6 +267,9 @@ void task_clear_jobctl_pending(struct task_struct *task, unsigned int mask)
mask |= JOBCTL_STOP_CONSUME | JOBCTL_STOP_DEQUEUED;

task->jobctl &= ~mask;
+
+ if (!(task->jobctl & JOBCTL_PENDING_MASK))
+ task_clear_jobctl_trapping(task);
}

/**
@@ -933,7 +939,7 @@ static void complete_signal(int sig, struct task_struct *p, int group)
signal->group_stop_count = 0;
t = p;
do {
- task_clear_jobctl_pending(t, JOBCTL_STOP_PENDING);
+ task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
sigaddset(&t->pending.signal, SIGKILL);
signal_wake_up(t, 1);
} while_each_thread(p, t);
@@ -1168,7 +1174,7 @@ int zap_other_threads(struct task_struct *p)
p->signal->group_stop_count = 0;

while_each_thread(p, t) {
- task_clear_jobctl_pending(t, JOBCTL_STOP_PENDING);
+ task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
count++;

/* Don't bother with already dead threads */
@@ -1964,9 +1970,6 @@ retry:
goto retry;
}

- /* PTRACE_ATTACH might have raced with task killing, clear trapping */
- task_clear_jobctl_trapping(current);
-
spin_unlock_irq(&current->sighand->siglock);

tracehook_finish_jctl();
--
1.7.1


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