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SubjectRe: [patch 20/24] perfmon: system calls interface
> > +	/*
> > + * 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.


> 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.


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