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

From: Andrew Lutomirski
Date: Mon Oct 03 2011 - 14:06:46 EST

On Mon, Oct 3, 2011 at 10:33 AM, Adrian Bunk <bunk@xxxxxxxxx> 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@xxxxxxxxx> 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-[3800] vsyscall fault (exploit attempt?) ip:ffffffffff600000 cs:33 sp:7fbfb9c498 ax:ffffffffff600000 si:0 di:606790
>> >  linux-[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 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 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?

No -- I actually meant to try running that revision or to try with the
vsyscall= patch reverted.

>> What's the .config for your UML binary?  I'd like to see if I can
>> reproduce this.
> It's attached.

I'll play around with it.

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