Re: [patch 09/31] x86/fpu: Do not inherit FPU context for CLONE_THREAD

Thomas Gleixner
2021-10-12T19:01:49Z

On Tue, Oct 12 2021 at 20:52, Thomas Gleixner wrote:

> On Tue, Oct 12 2021 at 18:10, Borislav Petkov wrote:
>
>> On Tue, Oct 12, 2021 at 02:00:11AM +0200, Thomas Gleixner wrote:
>>> CLONE_THREAD does not have the guarantee of a true fork to inherit all
>>> state. Especially the FPU state is meaningless for CLONE_THREAD.
>>>
>>> Just wipe out the minimal required state so restore on return to user space
>>> let's the thread start with a clean FPU.
>>
>> This sentence reads weird, needs massaging.
>
> The patch is wrong and needs to be removed. I just double checked
> pthread_create() again and it says:
>
> The new thread inherits the calling thread's floating-point environment
> (fenv(3))
>
> No idea where I was looking at a few days ago. :(

But fenv(3) is not the FPU state. Duh!

Anyway. It's an optimization which we can do later still and not
required for the cleanups here.

Thanks,

tglx