Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <CAHk-=wjc-ktbOr7ZHMY8gfAmHxUK+aMdDsQjeh+BvmQwnQfN_g@mail.gmail.com>
 <20200701184131.GI2786714@ZenIV.linux.org.uk> <CAHk-=wj_2v9m+yZioE4vOLGW1mc9SBa5+++LdeJ86aEeB5OXcw@mail.gmail.com>
 <20200701195914.GK2786714@ZenIV.linux.org.uk> <CAHk-=wj-CYhKZR8ZKQgi=VTx=o7n6dtwPXikvgkJ3SdiqRPd8A@mail.gmail.com>
 <87lfk26nx4.fsf@mpe.ellerman.id.au>
In-Reply-To: <87lfk26nx4.fsf@mpe.ellerman.id.au>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 2 Jul 2020 12:52:27 -0700
Message-ID: <CAHk-=wicOPQwuDUzFyDTBgr4UvQJHPdCX7_6BLaK6cve6CqBSg@mail.gmail.com>
Subject: Re: objtool clac/stac handling change..
To:     Michael Ellerman <mpe@ellerman.id.au>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Jul 2, 2020 at 6:32 AM Michael Ellerman <mpe@ellerman.id.au> wrote:
>
> Probably the simplest option for us is to just handle it in our
> unsafe_op_wrap(). I'll try and come up with something tomorrow.

IMy suggestion was to basically just always handle it in all exception cases.

And note that IU don't mean the fault handler: obviously page faults
(or unaligned faults or whatever) can happen while in a user access
region.

But I mean any time fixup_exception() triggers.

For x86, this is in fact particularly natural: it involves just always
clearing the AC bit in the "struct pt_regs" that fixup_exception()
gets anyway. We can do it without even bothering with checking for
CLAC/STAC support, since without it, AC is meaningless in kernel mode
anyway, but also because doing "user_access_end()" in the exception
would be pointless: AC is restored by the exception routine, so on x86
you *have* to do it by just modifying the return state.

             Linus