linux-kernel - Re: [PATCHV5 3/3] x86, ras: Add __mcsafe_copy() function to recover from machine checks

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Message-ID: <CALCETrUXhQH=n0_sG2Y2MxpSjQuJ5SFwMjh0f8_w7RSC+d5QTQ@mail.gmail.com>
Date:	Sun, 27 Dec 2015 05:40:14 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Tony Luck <tony.luck@...il.com>,
	linux-nvdimm <linux-nvdimm@...1.01.org>, X86 ML <x86@...nel.org>,
	"elliott@....com" <elliott@....com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Williams, Dan J" <dan.j.williams@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHV5 3/3] x86, ras: Add __mcsafe_copy() function to recover
 from machine checks

On Sun, Dec 27, 2015 at 5:33 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Sun, Dec 27, 2015 at 05:25:45AM -0800, Andy Lutomirski wrote:
>> That could significantly bloat the kernel image.
>
> Yeah, we probably should build an allyesconfig and see how big
> __ex_table is and compute how much actually that bloat would be,
> because...
>
>> Anyway, the bit 31 game isn't so bad IMO because it's localized to the
>> extable macros and the extable reader, whereas the bit 63 thing is all
>> tangled up with the __mcsafe_copy thing, and that's just the first
>> user of a more general mechanism.
>>
>> Did you see this:
>>
>> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=strict_uaccess_fixups/patch_v1&id=16644d9460fc6531456cf510d5efc57f89e5cd34
>
> ... the problem this has is that you have 4 classes, AFAICT. And since
> we're talking about a generic mechanism, the moment the 4 classes are
> not enough, this new scheme fails.
>
> I'm just saying...
>
> 4 classes are probably more than enough but we don't know.

On whatever kernel I just built (defconfig-ish), __ex_table is about
12kB, so the bloat would be 6 kB.

In any case, I can think of a total of three useful classes:

0: normal

1: extended (indirect block with fanciness, as I proposed)

2: uaccess (some day, not yet)

For rare future things, we could shoehorn them into the extended
version's indirect block.

Or someone could think of a clever encoding that makes this work better.

--Andy
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