Re: question about save_xstate_sig() - WHY DOES THIS WORK?

[Rik van Riel]

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On 01/23/2015 04:07 PM, H. Peter Anvin wrote:
> On 01/23/2015 11:34 AM, Rik van Riel wrote:
>> While working on a patch series to defer FPU state loading until 
>> kernel -> user space transition, and be more lazy with FPU state 
>> while in the kernel, I came across this code in
>> save_xstate_sig().
>> 
>> Not only is this broken with my new code, but it looks like it
>> may be broken with the current code, too...
>> 
>> Specifically, save_user_xstate() may page fault and sleep. After 
>> returning from the page fault, there is no guarantee that the FPU
>> state will be restored into the CPU, when the system is not 
>> running with eager fpu mode.
>> 
>> In that case, what prevents us from saving random FPU register
>> state to the user's stack frame?  Potentially state containing
>> data from other programs...
>> 
> 
> If the FPU state is not current, we'll have CR0.TS = 1 and the
> XSAVE will cause an #NM exception, which will cause the FPU state
> to be swapped in.

OK, so the code works as it is right now, but I still need
to have the patch in my "defer FPU state loading and CR0.TS
switching to kernel -> user space transition" patch series.

Thanks for explaining why the current code is safe, Peter.

- -- 
All rights reversed
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