linux-kernel - Re: [v6 PATCH 03/21] x86/mpx: Do not use R/EBP as base in the SIB byte with Mod = 0

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Message-ID: <1493333398.74182.10.camel@ranerica-desktop>
Date:   Thu, 27 Apr 2017 15:49:58 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Borislav Petkov <bp@...e.de>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        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>,
        Nathan Howard <liverlint@...il.com>,
        Adan Hawthorn <adanhawthorn@...il.com>,
        Joe Perches <joe@...ches.com>
Subject: Re: [v6 PATCH 03/21] x86/mpx: Do not use R/EBP as base in the SIB
 byte with Mod = 0

On Wed, 2017-04-26 at 10:05 +0200, Borislav Petkov wrote:
> On Tue, Apr 25, 2017 at 07:04:20PM -0700, Ricardo Neri wrote:
> > For the specific case of ModRM.mod being 0, I feel I need to clarify
> > that REX.B is not decoded and if SIB.base is %r13 the base is also 0.
> 
> Well, that all doesn't matter. The rule is this:
> 
> ModRM.mod == 00b and ModRM.r/m == 101b -> effective address: disp32
> 
> See Table 2-2. "32-Bit Addressing Forms with the ModR/M Byte" in the SDM.

You are right. This summarizes the rule. Then I will shorten the
comment.
> 
> So the base register is not used. How that base register is specified
> then doesn't matter (undecoded REX bits or not).
> 
> > This comment adds clarity because REX.X is decoded when determining
> > SIB.index.
> 
> Well, that's a different thing. The REX bits participating in the SIB
> fields don't matter about this particular case. We only want to say that
> we're returning a disp32 without a base register and the comment should
> keep it simple without extraneous information.
> 
> I know, you want to mention what Table 2-5. "Special Cases of REX
> Encodings" says but we should avoid unnecessary content in the comment.
> People who want details can stare at the manuals - the comment should
> only document what that particular case is.
> 
> Btw, you could write it even better:
> 
> 	if (!X86_MODRM_MOD(insn->modrm.value) && X86_MODRM_RM(insn->modrm.value) == 5)
> 
> and then it is basically a 1:1 copy of the rule from Table 2-2.

It is!

Thanks and BR,
Ricardo