Re: Updated version of RD/WR FS/GS BASE patchkit

[Andy Lutomirski]

On Mon, Mar 21, 2016 at 3:41 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
>> Imagine that some brilliant lightweight threading library does:
>>
>>  - set GS to nonzero (by whatever means -- arch_prctl(ARCH_SET_GS,
>> whatever) on a pre-IVB host followed by migration, some modify_ldt
>> garbage, simple bloody-mindedness, whatever);
>
> Migration is only possible when the CPUID flags match.
>
>>  - WRGSBASE
>>  - Use GS for a bit
>>
>> This will work most of the time until it gets unlucky with preemption.
>
> As soon as a kernel thread or something else schedules the value
> will be lost.
>
>> And yes, runtime library authors really do mess up in amazing ways.
>>
>> It's an issue.  It needs conscious design.
>
> Ok. So your only objection is the order of the context switch
> updates?

No.  My objection is that there needs to be an explicit statement what
the semantics are.  If the agreed-upon semantics are "undefined
behavior if GS != 0 and GSBASE doesn't match the descriptor", so be
it, but this needs to be a conscious decision and needs to be weighed
against the alternatives.

The actual implementation details are just details.  They need to
match the intended semantics, of course.

--Andy