linux-kernel - Re: [tip:x86/asm] x86/asm/entry: Add user_mode_ignore

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Date:	Sun, 29 Mar 2015 09:08:16 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	Brad Spengler <spender@...ecurity.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Lutomirski <luto@...nel.org>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/asm] x86/asm/entry: Add user_mode_ignore_vm86()


* Denys Vlasenko <dvlasenk@...hat.com> wrote:

> On 03/24/2015 08:46 PM, Andy Lutomirski wrote:
> > On Tue, Mar 24, 2015 at 12:44 PM, Ingo Molnar <mingo@...nel.org> wrote:
> >>
> >> * Andy Lutomirski <luto@...capital.net> wrote:
> >>
> >>> On Mon, Mar 23, 2015 at 5:26 AM, tip-bot for Andy Lutomirski
> >>> <tipbot@...or.com> wrote:
> >>>> Commit-ID:  a67e7277d01ccfd39b0db5a198c2643cc19dd79c
> >>>> Gitweb:     http://git.kernel.org/tip/a67e7277d01ccfd39b0db5a198c2643cc19dd79c
> >>>> Author:     Andy Lutomirski <luto@...nel.org>
> >>>> AuthorDate: Wed, 18 Mar 2015 18:33:29 -0700
> >>>> Committer:  Ingo Molnar <mingo@...nel.org>
> >>>> CommitDate: Mon, 23 Mar 2015 11:13:36 +0100
> >>>>
> >>>> x86/asm/entry: Add user_mode_ignore_vm86()
> >>>>
> >>>> user_mode() is dangerous and user_mode_vm() has a confusing name.
> >>>>
> >>>> Add user_mode_ignore_vm86() (equivalent to current user_mode()).
> >>>> We'll change the small number of legitimate users of user_mode()
> >>>> to user_mode_ignore_vm86().
> >>>>
> >>>> Inspired by grsec, although this works rather differently.
> >>>
> >>> Ingo, does this mean that you changed your mind or do you still want
> >>> a patch to delete user_mode_ignore_vm86 and just use user_mode
> >>> everywhere instead?
> >>
> >> Would be still nice to have it as an add on patch, if you agree with
> >> my arguments.
> > 
> > Given that there are only a very small number of callers left and
> > they're all Obviously Correct (tm), I'm not too worried about it.
> > Maybe if we kill off __copy_to_user, I'll be inspired to kill off
> > user_mode_ignore_vm86 as well :)
> 
> 
> I was looking at the code involving this function and it looks
> like a much better name for user_mode_ignore_vm86() would be
> user_mode_cs().
> 
> Every time we use it, we check vm8086 mode just before it:
> 
> perf_event.c
> 
>         if (regs->flags & X86_VM_MASK)
>                 return 0x10 * regs->cs;
> 
>         if (user_mode_ignore_vm86(regs) && regs->cs != __USER_CS)
>                 return get_segment_base(regs->cs);
> 
> 
> traps.c (three similar instances):
> 
>         if (v8086_mode(regs)) {
> ...
>                 goto exit;
>         }
>         if (user_mode_ignore_vm86(regs))...
> 
> 
> "_ignore_vm86" part doesn't quite work as an explanation.
> user_mode_cs() would immediately tell me "do we have a user's cs?"

So what the function name wanted to express is something like this:

	if (user_mode_vm86_mode_already_checked_so_this_is_marginally_faster_but_dont_use_it_otherwise_because_that_would_be_a_roothole()) 
	{
		...
	}

but that name was considered somewhat long.

Thanks,

	Ingo
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