Q: perf_event && task->ptrace_bps[]

[Oleg Nesterov]

Hello.

I am trying to understand the usage of hw-breakpoints in arch_ptrace().
ptrace_set_debugreg() and related code looks obviously racy. Nothing
protects us against flush_ptrace_hw_breakpoint() called by the dying
tracee. Afaics we can leak perf_event or use the already freed memory
or both.

Am I missed something?

Looking into the git history, I don't even know which patch should be
blamed (if I am right), there were too many changes. I noticed that
2ebd4ffb6d0cb877787b1e42be8485820158857e "perf events: Split out task
search into helper" moved the PF_EXITING check from find_get_context().
This check coould help if sys_ptrace() races with SIGKILL, but it was
racy anyway.

It is not clear to me what should be done. Looking more, I do not
understand the scope of perf_event/ctx at all, sys_perf_event_open()
looks wrong too, see the next email I am going to send.

Oleg.

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