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Message-Id: <1291654624-6230-13-git-send-email-tj@kernel.org>
Date: Mon, 6 Dec 2010 17:57:00 +0100
From: Tejun Heo <tj@...nel.org>
To: oleg@...hat.com, roland@...hat.com, linux-kernel@...r.kernel.org,
torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
rjw@...k.pl, jan.kratochvil@...hat.com
Cc: Tejun Heo <tj@...nel.org>
Subject: [PATCH 12/16] ptrace: make group stop notification reliable against ptrace
Group stop notifications are unreliable if one or more tasks of the
task group are being ptraced. If a ptraced task ends up finishing a
group stop, the notification is sent to the ptracer and the real
parent never gets notified.
This patch adds a new signal flag SIGNAL_NOTIFY_STOP which is set on
group stop completion and cleared on notification to the real parent
or together with other stopped flags on SIGCONT/KILL. This guarantees
that the real parent is notified correctly regardless of ptrace. If a
ptraced task is the last task to stop, the notification is postponed
till ptrace detach or canceled if SIGCONT/KILL is received inbetween.
Oleg spotted race against ptrace attach/detach in the initial
implementation. This is fixed by moving notification determiniation
into do_notify_parent_cldstop() and performing it while holding both
tasklist_lock and siglock.
Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Roland McGrath <roland@...hat.com>
---
include/linux/sched.h | 2 +
kernel/signal.c | 65 +++++++++++++++++++++++++++++-------------------
2 files changed, 41 insertions(+), 26 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 7045c34..7a26e7d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -653,6 +653,8 @@ struct signal_struct {
#define SIGNAL_UNKILLABLE 0x00000040 /* for init: ignore fatal signals */
+#define SIGNAL_NOTIFY_STOP 0x00000100 /* notify parent of group stop */
+
/* If true, all threads except ->group_exit_task have pending SIGKILL */
static inline int signal_group_exit(const struct signal_struct *sig)
{
diff --git a/kernel/signal.c b/kernel/signal.c
index 7dfbba9..3196367 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -269,7 +269,7 @@ void task_clear_group_stop(struct task_struct *task)
* CONTEXT:
* Must be called with @task->sighand->siglock held.
*/
-static bool task_participate_group_stop(struct task_struct *task)
+static void task_participate_group_stop(struct task_struct *task)
{
struct signal_struct *sig = task->signal;
bool consume = task->group_stop & GROUP_STOP_CONSUME;
@@ -279,18 +279,15 @@ static bool task_participate_group_stop(struct task_struct *task)
task_clear_group_stop(task);
if (!consume)
- return false;
+ return;
task->group_stop &= ~GROUP_STOP_CONSUME;
if (!WARN_ON_ONCE(sig->group_stop_count == 0))
sig->group_stop_count--;
- if (!sig->group_stop_count) {
- sig->flags = SIGNAL_STOP_STOPPED;
- return true;
- }
- return false;
+ if (!sig->group_stop_count)
+ sig->flags = SIGNAL_STOP_STOPPED | SIGNAL_NOTIFY_STOP;
}
/*
@@ -1603,6 +1600,16 @@ int do_notify_parent(struct task_struct *tsk, int sig)
* @why: CLD_{CONTINUED|STOPPED|TRAPPED}
*
* Notifies the parent that @tsk has been continued or is about to stop.
+ * Depending on @why and other conditions, the notification might be
+ * skipped.
+ *
+ * CLD_STOPPED : If ptraced, always notify; otherwise, notify
+ * once if SIGNAL_NOTIFY_STOP is set.
+ *
+ * CLD_TRAPPED : Always notify.
+ *
+ * For notify once cases, the respective NOTIFY flag is consumed and
+ * cleared.
*
* The notify target changes depending on whether @tsk is being ptraced or
* not. If @tsk is being ptraced, it's always the ptracer; otherwise, it's
@@ -1632,9 +1639,26 @@ static void do_notify_parent_cldstop(struct task_struct *tsk, int why)
switch (why) {
case CLD_CONTINUED:
+ notify = why;
+ break;
+
case CLD_STOPPED:
+ /*
+ * If ptraced, always notify; otherwise, notify once if
+ * NOTIFY_STOP is set.
+ */
+ if (task_ptrace(tsk))
+ notify = CLD_STOPPED;
+ else if (sig->flags & SIGNAL_NOTIFY_STOP) {
+ notify = CLD_STOPPED;
+ sig->flags &= ~SIGNAL_NOTIFY_STOP;
+ }
+ break;
+
case CLD_TRAPPED:
- notify = why;
+ /* TRAPPED is possible only while ptraced and always notified */
+ WARN_ON_ONCE(!task_ptrace(tsk));
+ notify = CLD_TRAPPED;
break;
}
@@ -1901,21 +1925,12 @@ retry:
__set_current_state(TASK_STOPPED);
if (likely(!task_ptrace(current))) {
- int notify = 0;
-
- /*
- * If there are no other threads in the group, or if there
- * is a group stop in progress and we are the last to stop,
- * report to the parent.
- */
- if (task_participate_group_stop(current))
- notify = CLD_STOPPED;
-
+ task_participate_group_stop(current);
spin_unlock_irq(¤t->sighand->siglock);
- if (notify) {
+ if (sig->flags & SIGNAL_NOTIFY_STOP) {
read_lock(&tasklist_lock);
- do_notify_parent_cldstop(current, notify);
+ do_notify_parent_cldstop(current, CLD_STOPPED);
read_unlock(&tasklist_lock);
}
@@ -2160,7 +2175,6 @@ relock:
void exit_signals(struct task_struct *tsk)
{
- int group_stop = 0;
struct task_struct *t;
if (thread_group_empty(tsk) || signal_group_exit(tsk->signal)) {
@@ -2185,15 +2199,14 @@ void exit_signals(struct task_struct *tsk)
if (!signal_pending(t) && !(t->flags & PF_EXITING))
recalc_sigpending_and_wake(t);
- if (unlikely(tsk->group_stop & GROUP_STOP_PENDING) &&
- task_participate_group_stop(tsk))
- group_stop = CLD_STOPPED;
+ if (unlikely(tsk->group_stop & GROUP_STOP_PENDING))
+ task_participate_group_stop(tsk);
out:
spin_unlock_irq(&tsk->sighand->siglock);
- if (unlikely(group_stop)) {
+ if (unlikely(tsk->signal->flags & SIGNAL_NOTIFY_STOP)) {
read_lock(&tasklist_lock);
- do_notify_parent_cldstop(tsk, group_stop);
+ do_notify_parent_cldstop(tsk, CLD_STOPPED);
read_unlock(&tasklist_lock);
}
}
--
1.7.1
--
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