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Subject[PATCH 06/19] job control: introduce task_set_jobctl_pending()
Date
task->jobctl currently hosts JOBCTL_STOP_PENDING and will host TRAP
pending bits too. Setting pending conditions on a dying task may make
the task unkillable. Currently, each setting site is responsible for
checking for the condition but with to-be-added job control traps this
becomes too fragile.

This patch adds task_set_jobctl_pending() which should be used when
setting task->jobctl bits to schedule a stop or trap. The function
performs the followings to ease setting pending bits.

* Sanity checks.

* If fatal signal is pending or PF_EXITING is set, no bit is set.

* STOP_SIGMASK is automatically cleared if new value is being set.

do_signal_stop() and ptrace_attach() are updated to use
task_set_jobctl_pending() instead of setting STOP_PENDING explicitly.
The surrounding structures around setting are changed to fit
task_set_jobctl_pending() better but there should be no userland
visible behavior difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
---
include/linux/sched.h | 2 ++
kernel/ptrace.c | 5 +++--
kernel/signal.c | 46 ++++++++++++++++++++++++++++++++++++++++------
3 files changed, 45 insertions(+), 8 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index e0ce64c..8519614 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1803,6 +1803,8 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *

#define JOBCTL_PENDING_MASK JOBCTL_STOP_PENDING

+extern bool task_set_jobctl_pending(struct task_struct *task,
+ unsigned int mask);
extern void task_clear_jobctl_pending(struct task_struct *task,
unsigned int mask);

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index abf6383..b44dc43 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -257,8 +257,9 @@ static int ptrace_attach(struct task_struct *task)
* The following task_is_stopped() test is safe as both transitions
* in and out of STOPPED are protected by siglock.
*/
- if (task_is_stopped(task)) {
- task->jobctl |= JOBCTL_STOP_PENDING | JOBCTL_TRAPPING;
+ if (task_is_stopped(task) &&
+ task_set_jobctl_pending(task,
+ JOBCTL_STOP_PENDING | JOBCTL_TRAPPING)) {
signal_wake_up(task, 1);
wait_trap = true;
}
diff --git a/kernel/signal.c b/kernel/signal.c
index 16826c4..17feedf 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -224,6 +224,39 @@ static inline void print_dropped_signal(int sig)
}

/**
+ * task_set_jobctl_pending - set jobctl pending bits
+ * @task: target task
+ * @mask: pending bits to set
+ *
+ * Clear @mask from @task->jobctl. @mask must be subset of
+ * %JOBCTL_PENDING_MASK | %JOBCTL_STOP_CONSUME | %JOBCTL_STOP_SIGMASK |
+ * %JOBCTL_TRAPPING. If stop signo is being set, the existing signo is
+ * cleared. If @task is already being killed or exiting, this function
+ * becomes noop.
+ *
+ * CONTEXT:
+ * Must be called with @task->sighand->siglock held.
+ *
+ * RETURNS:
+ * %true if @mask is set, %false if made noop because @task was dying.
+ */
+bool task_set_jobctl_pending(struct task_struct *task, unsigned int mask)
+{
+ BUG_ON(mask & ~(JOBCTL_PENDING_MASK | JOBCTL_STOP_CONSUME |
+ JOBCTL_STOP_SIGMASK | JOBCTL_TRAPPING));
+ BUG_ON((mask & JOBCTL_TRAPPING) && !(mask & JOBCTL_PENDING_MASK));
+
+ if (unlikely(fatal_signal_pending(task) || (task->flags & PF_EXITING)))
+ return false;
+
+ if (mask & JOBCTL_STOP_SIGMASK)
+ task->jobctl &= ~JOBCTL_STOP_SIGMASK;
+
+ task->jobctl |= mask;
+ return true;
+}
+
+/**
* task_clear_jobctl_trapping - clear jobctl trapping bit
* @task: target task
*
@@ -1902,19 +1935,20 @@ static int do_signal_stop(int signr)
else
WARN_ON_ONCE(!task_ptrace(current));

- current->jobctl &= ~JOBCTL_STOP_SIGMASK;
- current->jobctl |= signr | gstop;
- sig->group_stop_count = 1;
+ sig->group_stop_count = 0;
+
+ if (task_set_jobctl_pending(current, signr | gstop))
+ sig->group_stop_count++;
+
for (t = next_thread(current); t != current;
t = next_thread(t)) {
- t->jobctl &= ~JOBCTL_STOP_SIGMASK;
/*
* Setting state to TASK_STOPPED for a group
* stop is always done with the siglock held,
* so this check has no races.
*/
- if (!(t->flags & PF_EXITING) && !task_is_stopped(t)) {
- t->jobctl |= signr | gstop;
+ if (!task_is_stopped(t) &&
+ task_set_jobctl_pending(t, signr | gstop)) {
sig->group_stop_count++;
signal_wake_up(t, 0);
}
--
1.7.1


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