linux-kernel - Re: [PATCH 05/22] x86/fpu: Remove fpu->initialized usage in copy_fpstate_to

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Message-ID: <20190121112117.GA32538@redhat.com>
Date:   Mon, 21 Jan 2019 12:21:17 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 05/22] x86/fpu: Remove fpu->initialized usage in
 copy_fpstate_to_sigframe()

On 01/18, Dave Hansen wrote:
>
> On 1/18/19 1:14 PM, Sebastian Andrzej Siewior wrote:
> > The kernel saves task's FPU registers on user's signal stack before
> > entering the signal handler. Can we avoid that and have in-kernel memory
> > for that? Does someone rely on the FPU registers from the task in the
> > signal handler?
>
> This is part of our ABI for *sure*.  Inspecting that state is how
> userspace makes sense of MPX or protection keys faults.  We even use
> this in selftests/.

Yes.

And in any case I do not understand the idea to use the second in-kernel struct fpu.
A signal handler can be interrupted by another signal, this will need to save/restore
the FPU state again.

Oleg.