[Xen-devel] Re: [PATCH 0/5] Collected vdso/vsyscall fixes for 3.1

[Andrew Lutomirski <luto@xxxxxxx>]

On Wed, Jul 27, 2011 at 11:43 AM, Konrad Rzeszutek Wilk
<konrad.wilk@xxxxxxxxxx> wrote:
> On Wed, Jul 27, 2011 at 11:34:21AM -0400, Andrew Lutomirski wrote:
>> On Wed, Jul 27, 2011 at 11:30 AM, Konrad Rzeszutek Wilk
>> <konrad.wilk@xxxxxxxxxx> wrote:
>> >> > Anyhow, removed the benchmark code and ran it on 64-bit:
>> >> >
>> >> > sh-4.1# /test_vsyscall  test
>> >> > Testing gettimeofday...
>> >> > [  109.552261] test_vsyscall[2462] trap invalid opcode ip:400c8d 
>> >> > sp:7fff84fab470 error:0 in test_vsyscall[400000+2000]
>> >> > Illegal instruction
>> >> > sh-4.1# /test_vsyscall  intcc
>> >> > About to execute int 0xcc from RIP = 400959
>> >> > [  114.137150] test_vsyscall[2463] illegal int 0xcc (exploit attempt?) 
>> >> > ip:400959 cs:e033 sp:7fff8b328310 ax:2c si:0 di:7fff8b3280f0
>> >> > Caught SIGSEGV: Segmentation fault (Signal sent by the kernel 
>> >> > [(nil)])RIP = 400959
>> >> >
>> >> > [This is on 
>> >> > git://git.kernel.org/pub/scm/linux/kernel/git/konrad/xen.git #testing, 
>> >> > which
>> >> > has todays linus/master and your patchset]
>> >> >
>> >>
>> >> I'll set up Xen.  Something's clearly still buggy.
>> >
>> > You sure? This is what I get when I boot baremetal:
>> >
>> > sh-4.1#
>> > sh-4.1# xen-detect
>> > Not running on Xen.
>> > sh-4.1# /test_vsyscall test
>> > Testing gettimeo[   84.442819] test_vsyscall[3175] trap invalid opcode 
>> > ip:400c8d sp:7fffa8a72dc0 error:0fday...
>> >  in test_vsyscall[400000+2000]
>>
>> $ test_vsyscall test
>> Testing gettimeofday...
>>   vDSO offset = 0.000001s
>>   vsyscall offset = 0.000001s
>>
>> Testing time...
>>   vDSO offset = 0
>>   vsyscall offset = 0
>> Testing getcpu...
>>   ok!  cpu=6 node=0
>>
>> Can you send me your test_vsyscall binary so I can disassemble it?
>
> Here it is (also including source since I uncommented parts of it).
>
> One extra thing - I've been using AMD machines for this - I hadn't
> tried this on an Intel box.
>

Whoops!  The offending instruction is:

400c8d:       c4 e1 f3 2a c8          vcvtsi2sd %rax,%xmm1,%xmm1

which is unlikely to work on AMD unless you're the lucky owner of a
prerelease Bulldozer chip.  I

I bet if you pull a new copy or remove -mavx from Makefile it will
work.  I got a grossly hacked-up Xen domU booted and everything seems
to work.

(Testing native kernels is really fun with qemu-kvm -kernel <image>
-initrd <my silly initramfs>.  But Xen doesn't seem to support that.)

--Andy

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