From mboxrd@z Thu Jan  1 00:00:00 1970
From: Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH 05/22] x86/fpu: Remove fpu->initialized usage in
 copy_fpstate_to_sigframe()
Date: Tue, 26 Feb 2019 17:38:22 +0100
Message-ID: <20190226163821.GC21443@redhat.com>
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Cc: Dave Hansen <dave.hansen@intel.com>,
        Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        kvm@vger.kernel.org, "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Rik van Riel <riel@surriel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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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.