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Date: Fri, 26 Nov 2010 11:49:28 +0100
From: Tejun Heo <tj@...nel.org>
To: roland@...hat.com, oleg@...hat.com, linux-kernel@...r.kernel.org,
torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
"rjw@...k.plpavel"@ucw.cz
Cc: Tejun Heo <tj@...nel.org>
Subject: [PATCH 13/14] ptrace: make SIGCONT notification reliable against ptrace
Currently, SIGCONT notifications which are pending on ptrace attach or
occur while ptraced are reported to the tracer and never make it to
the real parent.
This patch adds a new signal flag SIGNAL_NOTIFY_CONT which is set when
a task is woken up by SIGCONT and cleared once the event is notified
to the parent. SIGNAL_CLD_MASK bits are no longer cleared after
notification. Combined with clearing SIGNAL_CLD_MASK if
!SIGNAL_NOTIFY_CONT on ptrace attach, these bits are set on ptrace
detach iff the tracee owes a notification to the real parent.
__ptrace_unlink() is updated to check these bits and reschedule
SIGCONT notification if necessary.
This combined with the previous changes makes ptrace attach/detach
mostly transparent with respect to job control signal handling. The
remaining problems are the extra unconditional wake_up_process() from
ptrace_detach() and SIGSTOP generated by ptrace_attach() clearing
pending SIGCONT. These will be dealt with future patches.
Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Roland McGrath <roland@...hat.com>
---
include/linux/sched.h | 1 +
kernel/ptrace.c | 40 +++++++++++++++++++++++++++++++++++++++-
kernel/signal.c | 14 +++++++++-----
3 files changed, 49 insertions(+), 6 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 3e40761..4b7f3ca 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -654,6 +654,7 @@ struct signal_struct {
#define SIGNAL_UNKILLABLE 0x00000040 /* for init: ignore fatal signals */
#define SIGNAL_NOTIFY_STOP 0x00000100 /* notify parent of group stop */
+#define SIGNAL_NOTIFY_CONT 0x00000200 /* notify parent of continuation */
/* 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/ptrace.c b/kernel/ptrace.c
index 71141bf..a6c92ac 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -52,6 +52,7 @@ void __ptrace_link(struct task_struct *child, struct task_struct *new_parent)
void __ptrace_unlink(struct task_struct *child)
{
struct signal_struct *sig = child->signal;
+ bool woken_up = false;
BUG_ON(!child->ptrace);
@@ -66,6 +67,33 @@ void __ptrace_unlink(struct task_struct *child)
if (sig->flags & SIGNAL_STOP_STOPPED || sig->group_stop_count)
child->group_stop |= GROUP_STOP_PENDING;
signal_wake_up(child, 1);
+ woken_up = true;
+ }
+
+ /*
+ * SIGNAL_CLD_MASK is cleared only on a stop signal or, if
+ * notification isn't pending, ptrace attach. If any bit is
+ * set,
+ *
+ * - SIGCONT notification was pending before attach or there
+ * was one or more SIGCONT notifications while tracing.
+ *
+ * - And, there hasn't been any stop signal since the last
+ * pending SIGCONT notification.
+ *
+ * Combined, it means that the tracee owes a SIGCONT
+ * notification to the real parent.
+ */
+ if (sig->flags & SIGNAL_CLD_MASK) {
+ sig->flags |= SIGNAL_NOTIFY_CONT;
+ /*
+ * Force the tracee into signal delivery path so that
+ * the notification is delievered ASAP. This wakeup
+ * is unintrusive as SIGCONT delivery would have
+ * caused the same effect.
+ */
+ if (!woken_up)
+ signal_wake_up(child, 0);
}
child->ptrace = 0;
@@ -198,17 +226,27 @@ int ptrace_attach(struct task_struct *task)
__ptrace_link(task, current);
send_sig_info(SIGSTOP, SEND_SIG_FORCED, task);
+ spin_lock(&task->sighand->siglock);
+
/*
* If the task is already STOPPED, set GROUP_STOP_PENDING and
* kick it so that it transits to TRACED. This is safe as
* both transitions in and out of STOPPED are protected by
* siglock.
*/
- spin_lock(&task->sighand->siglock);
if (task_is_stopped(task)) {
task->group_stop |= GROUP_STOP_PENDING;
signal_wake_up(task, 1);
}
+
+ /*
+ * Clear SIGNAL_CLD_MASK if NOTIFY_CONT is not set. This is
+ * used to preserve SIGCONT notification across ptrace
+ * attach/detach. Read the comment in __ptrace_unlink().
+ */
+ if (!(task->signal->flags & SIGNAL_NOTIFY_CONT))
+ task->signal->flags &= ~SIGNAL_CLD_MASK;
+
spin_unlock(&task->sighand->siglock);
retval = 0;
diff --git a/kernel/signal.c b/kernel/signal.c
index f2da456..735bac5 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -781,7 +781,8 @@ static int prepare_signal(int sig, struct task_struct *p, int from_ancestor_ns)
* will take ->siglock, notice SIGNAL_CLD_MASK, and
* notify its parent. See get_signal_to_deliver().
*/
- signal->flags = why | SIGNAL_STOP_CONTINUED;
+ why |= SIGNAL_STOP_CONTINUED | SIGNAL_NOTIFY_CONT;
+ signal->flags = why;
signal->group_stop_count = 0;
signal->group_exit_code = 0;
} else {
@@ -1895,7 +1896,7 @@ relock:
* we should notify the parent, prepare_signal(SIGCONT) encodes
* the CLD_ si_code into SIGNAL_CLD_MASK bits.
*/
- if (unlikely(signal->flags & SIGNAL_CLD_MASK)) {
+ if (unlikely(signal->flags & SIGNAL_NOTIFY_CONT)) {
int why;
if (signal->flags & SIGNAL_CLD_CONTINUED)
@@ -1903,7 +1904,7 @@ relock:
else
why = CLD_STOPPED;
- signal->flags &= ~SIGNAL_CLD_MASK;
+ signal->flags &= ~SIGNAL_NOTIFY_CONT;
why = tracehook_notify_jctl(why, CLD_CONTINUED);
spin_unlock_irq(&sighand->siglock);
@@ -1942,8 +1943,11 @@ relock:
if (signr != SIGKILL) {
signr = ptrace_signal(signr, info,
regs, cookie);
- if (!signr)
- continue;
+ if (!signr) {
+ /* NOTIFY_CONT might have changed */
+ spin_unlock_irq(&sighand->siglock);
+ goto relock;
+ }
}
ka = &sighand->action[signr-1];
--
1.7.1
--
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