Re: [uml-devel] [RFC] weird crap with vdso on uml/i386

[Richard Weinberger <richard@nod.at>]

Am 20.08.2011 23:26, schrieb Andrew Lutomirski:
> On Sat, Aug 20, 2011 at 4:55 PM, Richard Weinberger<richard@nod.at>  wrote:
>> Am 20.08.2011 22:14, schrieb Al Viro:
>>>
>>> On Sat, Aug 20, 2011 at 05:22:23PM +0200, Richard Weinberger wrote:
>>>
>>>> Hmmm, very strange.
>>>> Sadly I cannot reproduce the issue. :(
>>>> Everything works fine within UML.
>>>> (Of course I've applied your vDSO/i386 patches)
>>>>
>>>> My test setup:
>>>> Host kernel: 2.6.37 and 3.0.1
>>>> Distro: openSUSE 11.4/x86_64
>>>>
>>>> UML kernel: 3.1-rc2
>>>> Distro: openSUSE 11.1/i386
>>>>
>>>> Does the problem also occur with another host kernel or a different
>>>> guest image?
>>>
>>> Could you check what you get in __kernel_vsyscall()?  On iAMD64 box
>>> where that sucker contains sysenter-based variant the bug is not
>>> present.  IOW, it's sensitive to syscall vs. systenter vs. int 0x80
>>> differences.
>>
>> OK, this explains why I cannot reproduce it.
>> My Intel Core2 box is sysenter-based.
>>
>> (gdb) disass __kernel_vsyscall
>> 0xffffe420<__kernel_vsyscall+0>:       push   %ecx
>> 0xffffe421<__kernel_vsyscall+1>:       push   %edx
>> 0xffffe422<__kernel_vsyscall+2>:       push   %ebp
>> 0xffffe423<__kernel_vsyscall+3>:       mov    %esp,%ebp
>> 0xffffe425<__kernel_vsyscall+5>:       sysenter
>> 0xffffe427<__kernel_vsyscall+7>:       nop
>> 0xffffe428<__kernel_vsyscall+8>:       nop
>> 0xffffe429<__kernel_vsyscall+9>:       nop
>> 0xffffe42a<__kernel_vsyscall+10>:      nop
>> 0xffffe42b<__kernel_vsyscall+11>:      nop
>> 0xffffe42c<__kernel_vsyscall+12>:      nop
>> 0xffffe42d<__kernel_vsyscall+13>:      nop
>> 0xffffe42e<__kernel_vsyscall+14>:      jmp 0xffffe423<__kernel_vsyscall+3>
>> 0xffffe430<__kernel_vsyscall+16>:      pop    %ebp
>> 0xffffe431<__kernel_vsyscall+17>:      pop    %edx
>> 0xffffe432<__kernel_vsyscall+18>:      pop    %ecx
>> 0xffffe433<__kernel_vsyscall+19>:      ret
>>
>>> I can throw the trimmed-down fs image your way, BTW (66MB of bzipped ext2
>>> ;-/)
>>> if you want to see if that gets reproduced on your box.  I'll drop it on
>>> anonftp if you are interested.  FWIW, the same kernel binary/same image
>>> result in
>>>         * K7 box - no breakage, SYSENTER-based vdso
>>>         * K8 box - breakage as described, SYSCALL-based vdso32
>>>         * P4 box - no breakage, SYSENTER-based vdso32
>>> Hell knows...  In theory that would seem to point towards
>>> ia32_cstar_target(),
>>> so I'm going to RTFS carefully through that animal.
>>
>> Now I'm testing with a Debian fs from:
>> http://fs.devloop.org.uk/filesystems/Debian-Squeeze/
>>
>>> The thing is, whatever happens happens when victim gets resumed inside
>>> vdso page.  I'll try to dump PTRACE_SETREGS and see the values host
>>> kernel asked to set and work from there, but the interesting part is
>>> bloody hard to singlestep through - the victim is back to user mode and
>>> it is already traced by the guest kernel, so it's not as if we could
>>> attach host gdb to it and walk through that crap.  And guest gdb is not
>>> going to be able to set breakpoints in there - vdso page is r/o...
>>
>> [ CC'ing luto@mit.edu ]
>> Andy, do you have an idea?
>> You can find Al's original report here:
>> http://marc.info/?l=linux-kernel&m=131380315624244&w=2
>
> I'm missing a bit of the background.  Is the user-on-UML app calling
> into a vdso entry provided by UML or into a vdso entry provided by the
> host?

UML/i386 reuses the host's vDSO page.
IOW it does not have it's own vDSO like UML/x86_64.

Thanks,
//richard

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