Date: Sun, 30 Apr 2017 19:15:27 +0200
From: Borislav Petkov <bp@suse.de>
To: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Brian Gerst <brgerst@gmail.com>, Chris Metcalf <cmetcalf@mellanox.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Masami Hiramatsu <mhiramat@kernel.org>, Huang Rui <ray.huang@amd.com>,
        Jiri Slaby <jslaby@suse.cz>, Jonathan Corbet <corbet@lwn.net>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Vlastimil Babka <vbabka@suse.cz>, Chen Yucong <slaoub@gmail.com>,
        Alexandre Julliard <julliard@winehq.org>, Stas Sergeev <stsp@list.ru>,
        Fenghua Yu <fenghua.yu@intel.com>,
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        Shuah Khan <shuah@kernel.org>, linux-kernel@vger.kernel.org,
        x86@kernel.org, linux-msdos@vger.kernel.org, wine-devel@winehq.org,
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        Kees Cook <keescook@chromium.org>,
        Thomas Garnier <thgarnie@google.com>,
        Dmitry Vyukov <dvyukov@google.com>
Subject: Re: [v6 PATCH 06/21] x86/insn-eval: Add utility functions to get
 segment selector
Message-ID: <20170430171527.seipp3xnvhs2hlr7@pd.tnic>
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On Wed, Apr 26, 2017 at 01:44:43PM -0700, Ricardo Neri wrote:
> I regard that the role of this function is to obtain the the segment
> selector from either of the prefixes or inferred from the operands. It
> is the role of caller to determine if the segment selector should be
> ignored.

No, this is wrong. The function is called resolve_seg_selector() and it
gives you the segment selector. CS, DS, ES, and SS in 64-bit mode are
treated as null segments and your function should return/signal exactly
that, i.e, saying that those should be ignored in that case.

> I double-checked the latest version of the Intel Software Development
> manual [2], in the table 3-5 in section 3.7.4 mentions that DS is
> default segment for all data references, except string destinations. I
> tested this code with the UMIP-protected instructions and whenever I use
> %edi the default segment is %ds.

Yes, all correct. Except that we're adding a more-or-less generic x86
insn decoder so we should make it so...

> Is this example valid? The documentation of MOVS specifies that it
> always moves DS:(E)SI to ES:(E)DI.

... that the decoder should do exactly that:

	if (MOVS and rDI)
		return SEG_ES;

And you're handing in struct insn * so you can easily check which insn
you're looking at.

Thanks.

-- 
Regards/Gruss,
    Boris.

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