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Date:   Fri, 3 Jul 2020 22:59:22 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Michael Ellerman <mpe@ellerman.id.au>,
        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>
Subject: Re: objtool clac/stac handling change..
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On Fri, Jul 03, 2020 at 02:10:08PM -0700, Linus Torvalds wrote:
> On Fri, Jul 3, 2020 at 2:02 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > Actually, for more serious problem consider arch/x86/lib/copy_user_64.S
> 
> What? No.
> 
> > In case of an unhandled fault on attempt to read an (unaligned) word,
> > the damn thing falls back to this:
> > SYM_CODE_START_LOCAL(.Lcopy_user_handle_tail)
> >         movl %edx,%ecx
> > 1:      rep movsb
> > 2:      mov %ecx,%eax
> >         ASM_CLAC
> >         ret
> >
> >         _ASM_EXTABLE_UA(1b, 2b)
> > SYM_CODE_END(.Lcopy_user_handle_tail)
> 
> In the case of "we did an unaligned word at the end of a page, we took
> a fault, and now we have to start all over", the _least_ of our
> problems is that part of "starting over" would now imply doing a
> "stac" again.

What do you mean, start over?  It's picking a few remaining bytes out
of that word, *not* redoing the entire thing.