linux-kernel - Re: [3.1 patch] x86: default to vsyscall=native

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Date:	Mon, 10 Oct 2011 13:19:12 +0200
From:	richard -rw- weinberger <richard.weinberger@...il.com>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Adrian Bunk <bunk@...sta.de>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [3.1 patch] x86: default to vsyscall=native

On Fri, Oct 7, 2011 at 2:48 AM, Andrew Lutomirski <luto@....edu> wrote:
> On Thu, Oct 6, 2011 at 11:34 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>> On Thu, Oct 6, 2011 at 11:16 AM, Andrew Lutomirski <luto@....edu> wrote:
>>>
>>> Fixing it will be annoying because the attached fancier version needs
>>> to work, too.  I could implement the whole mess in software, but it
>>> might be nicer to arrange for uaccess errors to stash some information
>>> somewhere (like in the thread_struct cr2 variable).
>>
>> That should be easy enough to do. Just add it to the
>> "fixup_exception()" case in no_context().
>
> This code is rather messy.  We stash the cr2, err, and trap fields of
> sigcontext in thread_struct and we *never* reset them until the next
> segfault.  So userspace sees stale garbage on every signal that isn't
> a (genuine) segfault.  I can imagine this breaking UML is remarkably
> bizarre ways even without vsyscall emulation because UML actually
> seems to rely on that stuff to determine the source of a signal.
>

>From UML's point of view the current situation is odd.
UML will no longer run on top of a default 3.1 kernel.

Why is this odd?
One of the major reasons why people are still using UML is because you
can run it as non-privileged user on any x86 Linux host.
An user which has root privileges can setup and use KVM which is much
nicer than UML...

-- 
Thanks,
//richard
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