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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Date:   Wed, 26 Apr 2017 10:05:42 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
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 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.

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.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--