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From: "H. Peter Anvin" <hpa@zytor.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
	Denys Vlasenko <dvlasenk@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Oleg Nesterov <oleg@redhat.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Alexei Starovoitov <ast@plumgrid.com>,
	Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)
Date: Mon, 09 Mar 2015 10:42:40 -0700	[thread overview]
Message-ID: <54FDDB90.9010706@zytor.com> (raw)
In-Reply-To: <CA+55aFzck8nuV06AZhhFXgUQEsujZCtufw0jY4iswJUggh6ruA@mail.gmail.com>

On 03/09/2015 09:08 AM, Linus Torvalds wrote:
> 
> 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.
> 

Maybe:

	testb $(X86_EFLAGS_VM-16), PT_EFLAGS+2(%esp)

> 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.

The question is if avoiding looking at a field on the stack matters at all.

	-hpa



  parent reply	other threads:[~2015-03-09 17:43 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-09 14:05 Denys Vlasenko
2015-03-09 14:18 ` Andy Lutomirski
2015-03-09 15:00   ` Denys Vlasenko
2015-03-09 15:09     ` Andy Lutomirski
2015-03-09 19:31       ` Denys Vlasenko
2015-03-09 15:13     ` Ingo Molnar
2015-03-09 15:18       ` Andy Lutomirski
2015-03-09 15:47       ` Steven Rostedt
2015-03-09 15:54         ` Ingo Molnar
2015-03-09 16:08 ` Linus Torvalds
2015-03-09 16:28   ` Denys Vlasenko
2015-03-09 16:44     ` Linus Torvalds
2015-03-09 17:44       ` H. Peter Anvin
2015-03-09 19:13         ` Andy Lutomirski
2015-03-09 19:26           ` H. Peter Anvin
2015-03-09 19:51             ` Andy Lutomirski
2015-03-09 17:42   ` H. Peter Anvin [this message]
2015-03-09 17:45     ` Andy Lutomirski
2015-03-09 17:59       ` Linus Torvalds
2015-03-09 18:04         ` Andy Lutomirski
2015-03-09 18:16           ` Linus Torvalds
2015-03-09 18:32             ` Denys Vlasenko
2015-03-09 18:36             ` Andy Lutomirski
2015-03-10  6:25               ` Ingo Molnar

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