linux-kernel - Re: [PATCH?] uprobes: change uprobe_write

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Message-ID: <20131205184550.GA28159@redhat.com>
Date:	Thu, 5 Dec 2013 19:45:50 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Hugh Dickins <hughd@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH?] uprobes: change uprobe_write_opcode() to modify the
	page directly

On 12/05, Borislav Petkov wrote:
>
> On Thu, Dec 05, 2013 at 06:23:55PM +0100, Oleg Nesterov wrote:
> > This is almost off-topic, but I am wondering if (in the long term) we
> > can avoid this "insert the bp into every mm" altogether.
> >
> > Instead, uprobe_write_opcode() should only unmap this page and set
>
> Ok, sorry if I'm completely off base here but have you guys tried
> unmapping the page from all other VMs,

This is what I meant, but we can't simply clear this pte,

> and causing all
> the VMs to refault

why? it would be better to install the page on demand.

> patching you'd probably need to cause the #PF handler to "loop" until
> patching is complete though.

We can't do this, but I do not think we need to block #PF handler.

However, somehow the #PF handler should know that it should install
the patched page owned by uprobes. That is why I talked about
SWP_UPROBE_ENTRY (or something similar)

But again, in any case this is not trivial.

And perhaps I misundestood you... If you actually want to cause
all the VMs to refault, then why we can't unmap + refault every
mm like the patch I sent does? Just in case, note that we can't
share the same page anyway without more complications.

Oleg.

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