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

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Message-ID: <20111005222452.GF14406@localhost.pp.htv.fi>
Date:	Thu, 6 Oct 2011 01:24:52 +0300
From:	Adrian Bunk <bunk@...sta.de>
To:	Andrew Lutomirski <luto@....edu>
Cc:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Wed, Oct 05, 2011 at 03:13:51PM -0700, Andrew Lutomirski wrote:
> On Mon, Oct 3, 2011 at 10:33 AM, Adrian Bunk <bunk@...sta.de> wrote:
> > On Mon, Oct 03, 2011 at 06:04:53AM -0700, Andrew Lutomirski wrote:
> >> On Mon, Oct 3, 2011 at 2:08 AM, Adrian Bunk <bunk@...sta.de> wrote:
> >> > After upgrading a kernel the existing userspace should just work
> >> > (assuming it did work before ;-) ), but when I upgraded my kernel
> >> > from 3.0.4 to 3.1.0-rc8 a UML instance didn't come up properly.
> >> >
> >> > dmesg said:
> >> >  linux-2.6.30.1[3800] vsyscall fault (exploit attempt?) ip:ffffffffff600000 cs:33 sp:7fbfb9c498 ax:ffffffffff600000 si:0 di:606790
> >> >  linux-2.6.30.1[3856] vsyscall fault (exploit attempt?) ip:ffffffffff600000 cs:33 sp:7fbfb13168 ax:ffffffffff600000 si:0 di:606790
> >> >
> >> > Looking throught the changelog I ended up at commit 3ae36655
> >> > ("x86-64: Rework vsyscall emulation and add vsyscall= parameter").
> >> >
> >> > Linus suggested in https://lkml.org/lkml/2011/8/9/376 to default to
> >> > vsyscall=native.
> >> >
> >> > That sounds reasonable to me, and fixes the problem for me.
> >>
> >> At this point in the -rc cycle, this sounds fine.
> >>
> >> That being said, I'd like to fix it for real for 3.2.  This particular
> >> failure is suspicious -- the "vsyscall fault" message means that
> >> sys_gettimeofday returned EFAULT, which means that the old (3.0 and
> >> before) vgettimeofday should *also* have segfaulted.
> >
> > This 2.6.30.1 UML kernel binary from 2009 worked for me for all host
> > kernels from 2.6.30 to 3.0, and with 3.1.0-rc8 and vsyscall=native
> > it also seems to run nicely.
> >
> > Looking deeper into "a UML instance didn't come up properly",
> > the problem is that it comes up in a strange (readonly) state.
> >
> > There are "Using makefile-style concurrent boot in runlevel S."
> > and "Using makefile-style concurrent boot in runlevel 2." in the
> > logs with a Debian userspace, but no output from the init scripts
> > in these broken bootups (normal messages are in non-broken bootups).
> >
> > Perhaps the two the messages I see in dmesg on the host are from the
> > processes running rcS and rc2 failing early?
> >
> > In a working startup with a Debian userspace, I'm getting during rcS
> >  Setting the system clock.
> >  Cannot access the Hardware Clock via any known method.
> >  Use the --debug option to see the details of our search for an access method.
> >  Unable to set System Clock to: Mon Oct 3 17:01:35 UTC 2011 ... (warning).
> >
> >> We do have a bit
> >> of a bug in that the new code doesn't report si_addr properly, but
> >> that sounds unlikely as a culprit.  Did you try with the offending
> >> commit reverted (i.e. fce8dc0)?  I bet that it also fails there.
> >
> > fce8dc0 is "x86-64: Wire up getcpu syscall", is that really the one you
> > want me to revert?
> >
> >> What's the .config for your UML binary?  I'd like to see if I can
> >> reproduce this.
> >
> > It's attached.
> >
> 
> I can't reproduce it.  What distro is running inside the UML instance?

Debian stable.

> --Andy

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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