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Subject[PATCH 2/2] ptrace: ensure JOBCTL_STOP_SIGMASK is not zero after detach
This is the temporary simple fix for 3.2, we need more changes in this
area.

1. do_signal_stop() assumes that the running untraced thread in the
stopped thread group is not possible. This was our goal but it is
not yet achieved: a stopped-but-resumed tracee can clone the running
thread which can initiate another group-stop.

Remove WARN_ON_ONCE(!current->ptrace).

2. A new thread always starts with ->jobctl = 0. If it is auto-attached
and this group is stopped, __ptrace_unlink() sets JOBCTL_STOP_PENDING
but JOBCTL_STOP_SIGMASK part is zero, this triggers WANR_ON(!signr)
in do_jobctl_trap() if another debugger attaches.

Change __ptrace_unlink() to set the artificial SIGSTOP for report.

Alternatively we could change ptrace_init_task() to copy signr from
current, but this means we can copy it for no reason and hide the
possible similar problems.

Acked-by: Tejun Heo <tj@kernel.org>
Cc: <stable@kernel.org> [3.1]
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
kernel/ptrace.c | 13 ++++++++++++-
kernel/signal.c | 2 --
2 files changed, 12 insertions(+), 3 deletions(-)
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 24d0447..78ab24a 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -96,9 +96,20 @@ void __ptrace_unlink(struct task_struct *child)
*/
if (!(child->flags & PF_EXITING) &&
(child->signal->flags & SIGNAL_STOP_STOPPED ||
- child->signal->group_stop_count))
+ child->signal->group_stop_count)) {
child->jobctl |= JOBCTL_STOP_PENDING;

+ /*
+ * This is only possible if this thread was cloned by the
+ * traced task running in the stopped group, set the signal
+ * for the future reports.
+ * FIXME: we should change ptrace_init_task() to handle this
+ * case.
+ */
+ if (!(child->jobctl & JOBCTL_STOP_SIGMASK))
+ child->jobctl |= SIGSTOP;
+ }
+
/*
* If transition to TASK_STOPPED is pending or in TASK_TRACED, kick
* @child in the butt. Note that @resume should be used iff @child
diff --git a/kernel/signal.c b/kernel/signal.c
index b3f78d0..2065515 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1994,8 +1994,6 @@ static bool do_signal_stop(int signr)
*/
if (!(sig->flags & SIGNAL_STOP_STOPPED))
sig->group_exit_code = signr;
- else
- WARN_ON_ONCE(!current->ptrace);

sig->group_stop_count = 0;

--
1.5.5.1



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