Re: [patch 2/2] x86 NMI-safe INT3 and Page Fault

From: Peter Zijlstra
Date: Wed Aug 04 2010 - 05:47:51 EST

On Fri, 2010-07-16 at 11:05 -0700, H. Peter Anvin wrote:
> I really hope noone ever gets the idea of touching user space from an
> NMI handler, though, and expecting it to work...

Perf actually already does that to unwind user-space stacks... ;-)

See arch/x86/kernel/cpu/perf_event.c:copy_from_user_nmi() and its users.

What we do is a manual page table walk (using __get_user_pages_fast) and
simply bail when the page is not available.

That said, I think that the thing that started the whole
per-cpu-per-context temp stack-frame storage story also means that that
function is now broken and can lead to kmap_atomic corruption.

I really should brush up that stack based kmap_atomic thing, last time I
got stuck on FRV wanting things.

Linus should I refresh that whole series and give a FRV a slow but
working implementation and then let David Howells sort out things if he
cares about that?

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