linux-kernel - Re: [RFC/INCOMPLETE 00/13] x86: Rewrite exit-to-userspace code

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Message-ID: <20150618101433.GA6149@gmail.com>
Date:	Thu, 18 Jun 2015 12:14:34 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Borislav Petkov <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Brian Gerst <brgerst@...il.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [RFC/INCOMPLETE 00/13] x86: Rewrite exit-to-userspace code


* Andy Lutomirski <luto@...capital.net> wrote:

> > So assuming you fix the UML build I'm inclined to go for it, even in this 
> > incomplete form, to increase testing coverage.
> >
> > Doing that will also decrease ongoing merge friction between your work and 
> > other entry code cleanups ...
> 
> Sounds good to me.  I'm not convinced this is 4.2 material, though. Would it go 
> in a separate branch for now?

Please send it out as a separate series, on top of -tip, I'll probably stick it 
into a separate branch for the time being.

> On a related note, do you have any idea what work_notifysig_v86 in entry_32.S is 
> for?  It seems unnecessary and wrong to me.  Unnecessary because we have 
> return_to_32bit.  Wrong because I don't see how we can reliably enter vm86 mode 
> if we have exit work enabled -- one of the giant turds in vm86_32.c literally 
> jumps from C code to resume_userspace on vm86 entry, and resume_userspace 
> promptly checks for work and might land in work_notifysig_v86 before we ever 
> make it to v8086/user mode.
> 
> I think it may actually be impossible to use vm86 under ptrace.  ISTR I had some 
> trouble when trying to strace my test case...

Should be tested really, I'm all for removing it if simple vm86 mode games 
continue to work ;-)

Thanks,

	Ingo
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