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H A D | ptrace.c | 7b0fe136 Thu May 05 12:25:36 CDT 2022 Eric W. Biederman <ebiederm@xmission.com> ptrace: Document that wait_task_inactive can't fail
After ptrace_freeze_traced succeeds it is known that the tracee has a __state value of __TASK_TRACED and that no __ptrace_unlink will happen because the tracer is waiting for the tracee, and the tracee is in ptrace_stop.
The function ptrace_freeze_traced can succeed at any point after ptrace_stop has set TASK_TRACED and dropped siglock. The read_lock on tasklist_lock only excludes ptrace_attach.
This means that the !current->ptrace which executes under a read_lock of tasklist_lock will never see a ptrace_freeze_trace as the tracer must have gone away before the tasklist_lock was taken and ptrace_attach can not occur until the read_lock is dropped. As ptrace_freeze_traced depends upon ptrace_attach running before it can run that excludes ptrace_freeze_traced until __state is set to TASK_RUNNING. This means that task_is_traced will fail in ptrace_freeze_attach and ptrace_freeze_attached will fail.
On the current->ptrace branch of ptrace_stop which will be reached any time after ptrace_freeze_traced has succeed it is known that __state is __TASK_TRACED and schedule() will be called with that state.
Use a WARN_ON_ONCE to document that wait_task_inactive(TASK_TRACED) should never fail. Remove the stale comment about may_ptrace_stop.
Strictly speaking this is not true because if PREEMPT_RT is enabled wait_task_inactive can fail because __state can be changed. I don't see this as a problem as the ptrace code is currently broken on PREMPT_RT, and this is one of the issues. Failing and warning when the assumptions of the code are broken is good.
Tested-by: Kees Cook <keescook@chromium.org> Reviewed-by: Oleg Nesterov <oleg@redhat.com> Link: https://lkml.kernel.org/r/20220505182645.497868-8-ebiederm@xmission.com Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
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