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    Subject[PATCH 18/23] tracehook: TIF_NOTIFY_RESUME
    Date
    This adds tracehook.h inlines to enable a new arch feature in support
    of user debugging/tracing. This is not used yet, but it lays the
    groundwork for a debugger to be able to wrangle a task that's possibly
    running, without interrupting its syscalls in progress.

    Each arch should define TIF_NOTIFY_RESUME, and in their entry.S code
    treat it much like TIF_SIGPENDING. That is, it causes you to take the
    slow path when returning to user mode, where you get the full user-mode
    state accessible as for signal handling or ptrace. The arch code
    should check TIF_NOTIFY_RESUME after handling TIF_SIGPENDING.
    When it's set, clear it and then call tracehook_notify_resume().

    In future, tracing code will call set_notify_resume() when it
    wants to get a callback in tracehook_notify_resume().

    Signed-off-by: Roland McGrath <roland@redhat.com>
    ---
    include/linux/tracehook.h | 34 ++++++++++++++++++++++++++++++++++
    1 files changed, 34 insertions(+), 0 deletions(-)
    diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
    index 43bc51b..32867ab 100644
    --- a/include/linux/tracehook.h
    +++ b/include/linux/tracehook.h
    @@ -537,4 +537,38 @@ static inline void tracehook_report_death(struct task_struct *task,
    {
    }

    +#ifdef TIF_NOTIFY_RESUME
    +/**
    + * set_notify_resume - cause tracehook_notify_resume() to be called
    + * @task: task that will call tracehook_notify_resume()
    + *
    + * Calling this arranges that @task will call tracehook_notify_resume()
    + * before returning to user mode. If it's already running in user mode,
    + * it will enter the kernel and call tracehook_notify_resume() soon.
    + * If it's blocked, it will not be woken.
    + */
    +static inline void set_notify_resume(struct task_struct *task)
    +{
    + if (!test_and_set_tsk_thread_flag(task, TIF_NOTIFY_RESUME))
    + kick_process(task);
    +}
    +
    +/**
    + * tracehook_notify_resume - report when about to return to user mode
    + * @regs: user-mode registers of @current task
    + *
    + * This is called when %TIF_NOTIFY_RESUME has been set. Now we are
    + * about to return to user mode, and the user state in @regs can be
    + * inspected or adjusted. The caller in arch code has cleared
    + * %TIF_NOTIFY_RESUME before the call. If the flag gets set again
    + * asynchronously, this will be called again before we return to
    + * user mode.
    + *
    + * Called without locks.
    + */
    +static inline void tracehook_notify_resume(struct pt_regs *regs)
    +{
    +}
    +#endif /* TIF_NOTIFY_RESUME */
    +
    #endif /* <linux/tracehook.h> */

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