linux-kernel - Re: [PATCH v4 01/17] x86/mpx: Do not use SIB index if index points to R/ESP

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Message-ID: <CADRb1HZxCcRaCyyok3yP1aii=ePdh69K0H5XErUEFsOBaucgxw@mail.gmail.com>
Date:   Fri, 24 Feb 2017 10:00:33 -0500
From:   Adan Hawthorn <adanhawthorn@...il.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brian Gerst <brgerst@...il.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Liang Z Li <liang.z.li@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Huang Rui <ray.huang@....com>, Jiri Slaby <jslaby@...e.cz>,
        Jonathan Corbet <corbet@....net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Chen Yucong <slaoub@...il.com>,
        Alexandre Julliard <julliard@...ehq.org>,
        Stas Sergeev <stsp@...t.ru>, Fenghua Yu <fenghua.yu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-msdos@...r.kernel.org, wine-devel@...ehq.org,
        Adam Buchbinder <adam.buchbinder@...il.com>,
        Colin Ian King <colin.king@...onical.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Qiaowei Ren <qiaowei.ren@...el.com>
Subject: Re: [PATCH v4 01/17] x86/mpx: Do not use SIB index if index points to R/ESP

On Thu, Feb 23, 2017 at 9:33 PM, Joe Perches <joe@...ches.com> wrote:
> On Thu, 2017-02-23 at 14:17 -0800, Ricardo Neri wrote:
>> On Thu, 2017-02-23 at 08:24 +0100, Peter Zijlstra wrote:
>> > On Wed, Feb 22, 2017 at 10:36:50PM -0800, Ricardo Neri wrote:
>> > > +                 /*
>> > > +                  * A negative offset generally means a error, except
>> > > +                  * -EDOM, which means that the contents of the register
>> > > +                  * should not be used as index.
>> > > +                  */
>> > >                   if (indx_offset < 0)
>> > > -                         goto out_err;
>> > > +                         if (indx_offset == -EDOM)
>> > > +                                 indx = 0;
>> > > +                         else
>> > > +                                 goto out_err;
>> > > +                 else
>> > > +                         indx = regs_get_register(regs, indx_offset);
>> >
>> > Kernel coding style requires more brackets than are strictly required by
>> > C, any block longer than 1 line needs then. Also, if one leg of a
>> > conditional needs them, then they should be on both legs.
>> >
>> > Your code has many such instances, please change them all.
>>
>> Will do. Sorry for the noise. These instances escaped the checkpatch
>> script.
>
> Also, this code would read better with the inner test
> reversed or done first
>
>                 if (indx_offset < 0) {
>                         if (indx_offset != -EDOM)
>                                 goto out_err;
>                         indx = 0;
>                 } else {
>                         indx = regs_get_register(etc...)
>                 }
>
> or
>                 if (indx_offset == -EDOM)
>                         indx = 0;
>                 else if (indx_offset < 0)
>                         goto err;

Or, goto out_err;

>                 else
>                         indx = regs_get_register(etc...)
>
> The compiler should generate the same code in any
> case, but either could improve reader understanding.

Also, it may be a tweak more efficient to handle the most likely
runtime case in the conditional stack first (whichever that may be).