Re: [uml-devel] [PATCH 1/3] um: extend _fpstate to _xstate

[Eli Cooper <elicooper@gmx.com>]

Hi Richard,

On 2016/3/13 15:58, Richard Weinberger wrote:
> Eli,
>
> Am 12.03.2016 um 08:08 schrieb Eli Cooper:
>> > Hi Richard,
>> > 
>> > On 2016/3/10 4:44, Richard Weinberger wrote:
>>> >> Hmm, this needs rework. Having everything on the stack is not good.
>> > 
>> > Okay, I'll rework the functions whose stack size is greater than the
>> > warning threshold by using kmalloc.
> I fear it is not that easy. Having a kmalloc() per context switch would
> be every expensive. Even for UML.

Actually only two functions' stack frame size exceed kernel's default
warning threshold (1024 bytes) after the _xstate extension, i.e.,
copy_sc_from_user and copy_sc_to_user. That's because they have an
_xstate on stack as well as a sigcontext, which contains another
_xstate. Context switches due to signal handling are rare; thus I think
having a kmalloc() for signal handling is acceptable.
>>> >> Can you also create a selftest such that this bug cannot happen again?
>> > 
>> > It seems that instead of writing a self-test showing this problem cannot
>> > happen again, I wrote a test that manifested another bug that is not
>> > directly related to my patch.
>> > 
>> > Without applying my patch, the current UML should support XMM registers
>> > because those are covered by _fpstate and PTRACE_GETFPREGS. But it
>> > seemed that XMM registers are not restored after the signal handler returns.
>> > 
>> > In the following quick test, the main loop should run indefinitely
>> > despite XMM registers are modified by the signal handler. But in UML,
>> > the loop breaks randomly within a minute or two, showing that the
>> > registers are corrupted. So far I haven't found the cause. Any hints?
> Meh. :(
> Can you figure out whether the issue depends on the host kernel? i.e. try something older
> and Linus' tree.
> UML is a heavy user of ptrace(), maybe the recent FPU cleanup on x86 broke something.
No, it seems that this issue does not depend on the host kernel, UML
kernel, or CPU. I can reproduce this bug on a variety of combinations of
them, with the host kernel ranging from 2.6.32 to 3.19 to 4.5.

Thanks,
Eli
>
> Thanks,
> //richard
>


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