linux-kernel - Re: [REGRESSION] 4.2-rc2: early boot memory corruption from FPU rework

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Message-ID: <CA+55aFzMmZtstS7EqLr0qALzeXxFxu3MCWHxKfrf_RdsgMHzcA@mail.gmail.com>
Date:	Wed, 15 Jul 2015 19:39:26 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Hansen <dave.hansen@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [REGRESSION] 4.2-rc2: early boot memory corruption from FPU rework

On Wed, Jul 15, 2015 at 5:34 PM, Dave Hansen
<dave.hansen@...ux.intel.com> wrote:
>
> The old code sized the buffer in a fully architectural way and it
> worked.  The CPU *tells* you how much memory the 'xsave' instruction is
> going to scribble on.  The new code just merrily calls it and let it
> scribble away.  This is as clear-cut a regression as I've ever seen.

Yes, I think we'll need to revert it, or do something else drastic
like make that initial fp state allocation *much* bigger and then have
a "disable xsaves if if it's still not big enough".

setup_xstate_features() should be able to easily just say "this was
the maximum offset+size we saw", and we can take that to either do a
proper allocation, or verify that the static allocation is indeed big
enough.

Apparently a straight revert doesn't work, if only because things in
that area have been renamed very aggressively (both files and
functions and variables). Ingo?

            Linus
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