[PATCH 2/2] ptrace: ptrace_detach() can no longer race with SIGKILL

Oleg Nesterov
2015-03-24T18:56:54Z

ptrace_detach() re-checks ->ptrace under tasklist lock and calls
release_task() if __ptrace_detach() returns true. This was needed
because the __TASK_TRACED tracee could be killed/untraced, and it
could even pass exit_notify() before we take tasklist_lock.

But this is no longer possible after 9899d11f6544 "ptrace: ensure
arch_ptrace/ptrace_request can never race with SIGKILL". We can turn
these checks into WARN_ON() and remove release_task().

While at it, document the setting of child->exit_code.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
kernel/ptrace.c | 19 +++++++++----------
1 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 5009263..23be9dd 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -456,8 +456,6 @@ static bool __ptrace_detach(struct task_struct *tracer, struct task_struct *p)

static int ptrace_detach(struct task_struct *child, unsigned int data)
{
- bool dead = false;
-
if (!valid_signal(data))
return -EIO;

@@ -467,18 +465,19 @@ static int ptrace_detach(struct task_struct *child, unsigned int data)

write_lock_irq(&tasklist_lock);
/*
- * This child can be already killed. Make sure de_thread() or
- * our sub-thread doing do_wait() didn't do release_task() yet.
+ * We rely on ptrace_freeze_traced(). It can't be killed and
+ * untraced by another thread, it can't be a zombie.
*/
- if (child->ptrace) {
- child->exit_code = data;
- dead = __ptrace_detach(current, child);
- }
+ WARN_ON(!child->ptrace || child->exit_state);
+ /*
+ * tasklist_lock avoids the race with wait_task_stopped(), see
+ * the comment in ptrace_resume().
+ */
+ child->exit_code = data;
+ __ptrace_detach(current, child);
write_unlock_irq(&tasklist_lock);

proc_ptrace_connector(child, PTRACE_DETACH);
- if (unlikely(dead))
- release_task(child);

return 0;
}
--
1.5.5.1