linux-kernel - Re: New vsyscall emulation breaks JITs

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Message-ID: <CA+55aFyV65BNjdsmS9oTaRwWzyJZS3BDEDJBx5Poy-vGp5iN_Q@mail.gmail.com>
Date:	Tue, 9 Aug 2011 15:36:06 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Greg Lueck <lueckintel@...oo.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kimwooyoung@...il.com" <kimwooyoung@...il.com>
Subject: Re: New vsyscall emulation breaks JITs

On Tue, Aug 9, 2011 at 2:04 PM, Andrew Lutomirski <luto@....edu> wrote:
>
> Here's a different proposal, then:
>
> What if the kernel had the sequence:
>
> mov $__NR_whatever,%eax
> syscall
> ret
>
> in the vsyscall page but marked the vsyscall page NX.

This sounds like a sound idea. And then the difference between "fast
and native" and "slow and trapping" ends up literally being just the
NX bit.

                        Linus
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