linux-kernel - Re: objtool clac/stac handling change..

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Message-ID: <20200703013328.GQ2786714@ZenIV.linux.org.uk>
Date:   Fri, 3 Jul 2020 02:33:28 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Christophe Leroy <christophe.leroy@....fr>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: objtool clac/stac handling change..

On Thu, Jul 02, 2020 at 02:55:19PM -0700, Linus Torvalds wrote:

> And while XSTATE_OP() is still disgusting, it's
> 
>  (a) slightly less disgusting than it used to be
> 
>  (b) now easily fixable if we do the "exceptions clear AC" thing.
> 
> so it's an improvement all around.
> 
> If it works, that is. As mentioned: IT HAS NO TESTING.

What about load_unaligned_zeropad()?  Normally the caller doesn't
want to know about the exception on crossing into an unmapped
page.  Blanket "clear #AC of fixup, don't go through user_access_end()
in case of exception" would complicate the code that calls that sucker.