linux-kernel - Re: [PATCH] Add a text

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Message-ID: <20131130145245.GA15790@redhat.com>
Date:	Sat, 30 Nov 2013 15:52:45 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, x86@...nel.org,
	Andi Kleen <ak@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] Add a text_poke syscall v2

On 11/29, H. Peter Anvin wrote:
>
> On 11/29/2013 12:05 PM, Oleg Nesterov wrote:
> >
> > Can't we invalidate pte (so that any user will stuck in page fault),
> > update the page(s), restore the pte and drop the locks?
> >
> > This way sys_text_poke() won't be x86-specific, and it will be per-mm.
> >
>
> Hmmm... if we hold mmap_sem() this pretty much will be the net result,
> no?

Yes, down_write(mmap_sem) is enough to block the page faults.

But we need pte-lock anyway, to avoid the races with, say, try_to_unmap().
(and of course, we need to retry if page_check_address() fails).

This actually means that if we want to update a single page we could
use down_read(). But in general we need to update 2 pages. Or even more
if we generalize sys_text_poke(), perhaps it should be renamed in this
case, but this is off-topic.

> That would mean no additional tests needed on the page fault path.

Not sure I understand... but of course we should not change the fault
paths in any case.

>  What I'm not sure of is whether or not it is actually safe to hold
> mmap_sem across all the code we need

Let me repeat, I do not understand vm enough to answer authoritatively.

But I think this should be safe. I do not see why it should not, but
see above.

Oleg.

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