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

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Date:   Tue, 26 Feb 2019 17:38:22 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        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()

Hi Sebastian,

Sorry, I just noticed your email...

On 02/05, Sebastian Andrzej Siewior wrote:
>
> On 2019-01-21 12:21:17 [+0100], Oleg Nesterov wrote:
> > > 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.
>
> So I assumed that while SIGUSR1 is handled SIGUSR2 will wait until the
> current signal is handled. So no interruption. But then SIGSEGV is
> probably the exception which will interrupt SIGUSR1. So we would need a
> third one…

I guess you do not need my answer, but just in case.

SIGSEGV is not an exception. A SIGUSR1 handler can be interrupted by any other
signal which is not included in sigaction->sa_mask. Even SIGUSR1 can interrupt
the handler if SA_NODEFER was used.


> The idea was to save the FPU state in-kernel so we don't have to
> revalidate everything because userspace had access to it and could do
> things.

I understand, but this simply can't work, see above.

Oleg.