linux-kernel - Re: [patch 09/31] x86/fpu: Do not inherit FPU context for CLONE

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Message-ID: <87r1cq9iee.ffs@tglx>
Date:   Tue, 12 Oct 2021 21:01:45 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Borislav Petkov <bp@...en8.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [patch 09/31] x86/fpu: Do not inherit FPU context for CLONE_THREAD

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