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

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Message-ID: <20111003173359.GA3072@localhost.pp.htv.fi>
Date:	Mon, 3 Oct 2011 20:33:59 +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 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.

> --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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