[PATCH 12/16] ptrace: Stop protecting ptrace_set_signr with tasklist_lock

Eric W. Biederman
2022-05-18T22:53:51Z

Now that ptrace_set_signr no longer sets task->exit_code the race
documented in commit b72c186999e6 ("ptrace: fix race between
ptrace_resume() and wait_task_stopped()") is no longer possible, as
task->exit_code is only updated by wait during a ptrace_stop.

As there is no possibilty of a race and ptrace_freeze_traced is
all of the protection ptrace_set_signr needs to operate without
contention move ptrace_set_signr outside of tasklist_lock
and remove the documentation about the race that is no more.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
kernel/ptrace.c | 19 ++++---------------
1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index e0ecb1536dfc..d0527b6e2b29 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -595,17 +595,14 @@ static int ptrace_detach(struct task_struct *child, unsigned int data)
/* Architecture-specific hardware disable .. */
ptrace_disable(child);

+ ptrace_set_signr(child, data);
+
write_lock_irq(&tasklist_lock);
/*
* We rely on ptrace_freeze_traced(). It can't be killed and
* untraced by another thread, it can't be a zombie.
*/
WARN_ON(!child->ptrace || child->exit_state);
- /*
- * tasklist_lock avoids the race with wait_task_stopped(), see
- * the comment in ptrace_resume().
- */
- ptrace_set_signr(child, data);
__ptrace_detach(current, child);
write_unlock_irq(&tasklist_lock);

@@ -869,17 +866,9 @@ static int ptrace_resume(struct task_struct *child, long request,
user_disable_single_step(child);
}

- /*
- * Change ->exit_code and ->state under siglock to avoid the race
- * with wait_task_stopped() in between; a non-zero ->exit_code will
- * wrongly look like another report from tracee.
- *
- * Note that we need siglock even if ->exit_code = data and/or this
- * status was not reported yet, the new status must not be cleared by
- * wait_task_stopped() after resume.
- */
- spin_lock_irq(&child->sighand->siglock);
ptrace_set_signr(child, data);
+
+ spin_lock_irq(&child->sighand->siglock);
child->jobctl &= ~JOBCTL_TRACED;
wake_up_state(child, __TASK_TRACED);
spin_unlock_irq(&child->sighand->siglock);
--
2.35.3