linux-kernel - Re: [x86, vdso] BUG: unable to handle kernel paging request at d34bd000

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Date:	Fri, 7 Mar 2014 15:07:13 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Stefani Seibold <stefani@...bold.net>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [x86, vdso] BUG: unable to handle kernel paging request at d34bd000

On Fri, Mar 7, 2014 at 1:53 PM, Stefani Seibold <stefani@...bold.net> wrote:
>
> Am Freitag, den 07.03.2014, 10:56 -0800 schrieb Andy Lutomirski:
>> On Thu, Mar 6, 2014 at 11:21 PM, Stefani Seibold <stefani@...bold.net> wrote:
>> > Hi Fengguang,
>> >
>> > i have build a kernel with the config, but my kvm is unable to start it.
>> > I will try to find a way to test your kernek config.
>> >
>> > One thing is the crash point:
>> >
>> > The function sysenter_setup was modified by Andy, maybe he has an idea
>> > what fails.
>>
>> *sigh*
>>
>> My host kernel is currently fscked up and won't run KVM.  Also, I want
>> to confirm that I'm reproducing exactly what you're seeing, and I
>> think it depends on the toolchain.  Can you (Fenguang) do:
>>
>> $ ls -l arch/x86/vdso/vdso32*.so
>> -rwxrwxr-x. 1 luto luto 4096 Mar  7 10:19 arch/x86/vdso/vdso32-int80.so
>> -rwxrwxr-x. 1 luto luto 4116 Mar  7 10:19 arch/x86/vdso/vdso32-sysenter.so
>>
>> (Of course, triggering this depends on which image gets selected.)
>>
>
> Yes, that what i also figured out. There are two culprits:
> CONFIG_OPTIMIZE_INLINING and CONFIG_X86_PPRO_FENCE. Each of them
> increase the size of the code by about 500 bytes.
>
> When i add to file arch/x86/vdso/vdso32/vclock_gettime.c
>
> #undef CONFIG_OPTIMIZE_INLINING
> #undef CONFIG_X86_PPRO_FENCE
>
> this will solve the issue.
>
>> Note that we have a .so file that exceeds 4k, i.e. one page.  Then
>> read the relevant code and wonder what everyone was smoking when they
>> wrote it.  There are so many buffer overflows, screwed up
>> initializations, unnecessary and incorrect copies, etc, that I don't
>> even want to speculate on what the first failure will be when the
>> image is bigger than a page.
>>
>
> Right. So the above one will not really solve it. At least when
> __vdso_getcpu() code will also become a part of the 32 bit VDSO.
>
>> It's easy enough to fix, but someone should figure out what the impact
>> will be on the compat vdso case.
>>
>> I wonder how hard it would be to change the compat vdso do be a dummy
>> image a la the x86_64 fake vsyscall page so that old code can keep
>> working (maybe with a performance hit) and new code can use a sane
>> image.
>>
>
> That is exactly what i wrote one week ago:
>
> Move the VDSO code before the VDSO compat fixmap area and create a kind
> of helper VDSO for the VDSO compat fixmap page, which only calls the
> real VDSO. But this would result in a performance regression for the
> VDSO compat mode.

I think that regressing performance for compat_vdso (only) users is
fine.  We need to figure out what those users are.  I have a vague
recollection that it's a particular version of SuSE or OpenSuSE.

--Andy
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