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    SubjectRe: [patch 20/24] perfmon: system calls interface
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    > > +	/*
    > > + * cannot attach to a kernel thread
    > > + */
    > > + if (!task->mm) {

    This should test (task->flags & PF_KTHREAD). (But it's superfluous anyway.)

    > > + /*
    > > + * cannot attach to a zombie task
    > > + */
    > > + if (task->exit_state == EXIT_ZOMBIE || task->exit_state == EXIT_DEAD) {

    The usual test to write here is just "if (task->exit_state)".
    (But it's superfluous anyway.)

    > Thirdly, the check for ->exit_state in pfm_task_incompatible() is not
    > needed: we've just passed ptrace_check_attach() so we know we just
    > transitioned the task to task->state == TASK_TRACED.

    Correct.

    > If you _ever_ see a task exit TASK_TRACED and go zombie or dead from
    > there without this code allowing it that means the whole state machine
    > with ptrace is borked up by perfmon.

    Is always possible for a TASK_TRACED task to suddenly die via SIGKILL, and
    it's even possible it will be reaped (EXIT_DEAD) despite ptrace, in the
    de_thread() (MT exec) case. This race is tolerable for everything ptrace
    does (it holds a task ref, and the arch code copes). It must also be
    tolerated by the perfmon code.

    > For example i dont see where the perfmon-control task parents itself as
    > the exclusive debugger (parent) of the debuggee-task.

    ptrace_check_attach() ensures that this is so.


    Thanks,
    Roland


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