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Subject: Re: [PATCH 24/30] x86, kaiser: disable native VSYSCALL
To: Andy Lutomirski <luto@kernel.org>
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From: Dave Hansen <dave.hansen@linux.intel.com>
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Date: Fri, 10 Nov 2017 15:04:07 -0800
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On 11/10/2017 02:06 PM, Andy Lutomirski wrote:
> On Thu, Nov 9, 2017 at 10:31 PM, Dave Hansen
> <dave.hansen@linux.intel.com> wrote:
>> On 11/09/2017 06:25 PM, Andy Lutomirski wrote:
>>> Here are two proposals to address this without breaking vsyscalls.
>>>
>>> 1. Set NX on low mappings that are _PAGE_USER.  Don't set NX on high
>>> mappings but, optionally, warn if you see _PAGE_USER on any address
>>> that isn't the vsyscall page.
>>>
>>> 2. Ignore _PAGE_USER entirely and just mark the EFI mm as special so
>>> KAISER doesn't muck with it.
>>
>> These are totally doable.  But, what's the big deal with breaking native
>> vsyscall?  We can still do the emulation so nothing breaks: it is just slow.
> 
> I have nothing against disabling native.  I object to breaking the
> weird binary tracing behavior in the emulation mode, especially if
> it's tangled up with KAISER.  I got all kinds of flak in an earlier
> version of the vsyscall emulation patches when I broke that use case.
> KAISER may get very widely backported -- let's not make changes that
> are already known to break things.

Is the thing that broke a "user mode program that actually looks at the
vsyscall page"?  Like Linus is referring to here:

> http://lkml.kernel.org/r/CA+55aFyijHb4WnDMKgeXekTZHYT8pajqSAu2peo3O4EKiZbYPA@mail.gmail.com