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From:   Andy Lutomirski <luto@kernel.org>
Date:   Wed, 1 Jul 2020 17:47:48 -0700
Message-ID: <CALCETrXpKAR2A0+96b+RWUE0vUmoPwmp-mfF1u=g2270DNi96w@mail.gmail.com>
Subject: Re: objtool clac/stac handling change..
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        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>
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On Wed, Jul 1, 2020 at 1:51 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Wed, Jul 1, 2020 at 1:36 PM Andy Lutomirski <luto@amacapital.net> wrote:
> >
> > We ought to be able to do it the way I described and get decent code generation too.
>
> No, we really can't.
>
> Each access really needs to jump to an exception label. Otherwise any
> time you have multiple operations (think "strncpy()" and friends) you
> have to test in between each access.
>
> That is why *fundamnetally* the interface to "unsafe_get/put_user()"
> takes a label for the error case. There is absolutely no way to make
> any other interface work efficiently.

You inspired me to mock it up.  I don't think I did anything special
here, except that I mocked up unsafe_put_user() and a fudged it a
little bit because I'm using gcc 9.3.1 which doesn't support asm goto
outputs.  Code like this:

    if (unsafe_put_user(&a, user_a))
        goto error;
    if (unsafe_put_user(&b, user_b))
        goto error;
    if (unsafe_put_user(&c, user_c))
        goto error;
    if (unsafe_put_user(&d, user_d))
        goto error;

generates a series of movs.  The conditions are entirely omitted from
the generated assembly output because gcc is smart enough to figure
out that the return value of unsafe_put_user() indicates which way the
asm goto went.  I don't think I could generate better output by hand
than gcc generated from my test.

So I stand by my claim. :)  Each access does need to jump, but that
jump can be entirely within the exception entry, and we don't need to
generate any actual jump instructions.

--Andy

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