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

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Message-ID: <33f0e144-1eec-b1a1-8858-58f20d5e477d@intel.com>
Date:   Fri, 18 Jan 2019 13:17:28 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>
Cc:     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 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/.