linux-kernel - Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT

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Date:	Mon, 9 Mar 2015 09:08:11 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)

On Mon, Mar 9, 2015 at 7:05 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
>     New:
>      1e6:   0f ba 64 24 38 11       btl    $0x11,0x38(%esp)

btl? Really?

Why isn't that just

    testb $2,0x3a(%esp)

which is both smaller and quite a bit faster on older machines.

Sure, the btl is easier to explain in the source code, but instead of this:

> +       btl     $X86_EFLAGS_VM_BIT,PT_EFLAGS(%esp)

you'd have to add a comment, like

    testb $2, PT_EFLAGS+2(%esp)  # X86_EFLAGS_VM_BIT

or something.

Or just at least *partially* do what we used to do, and make it all be

    movb  PT_EFLAGS+2(%esp),%al
    andb $2,%al
    orb PT_CS(%esp),%al
    testb $3,%al
    je restore_nocheck
    testb $SEGMENT_TI_MASK,PT_OLDSS(%esp)
    jne ldt_ss

which still avoids looking at SS unless needed, and is smaller and
faster than the btl, afaik.

                       Linus
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