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

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Message-ID: <87tuhm9it4.ffs@tglx>
Date:   Tue, 12 Oct 2021 20:52:55 +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 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. :(

Thanks,

        tglx