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Date: Thu, 23 Jun 2016 23:17:32 -0700
From: Linus Torvalds <>
To: Peter Zijlstra <>
Cc: Oleg Nesterov <>, Andy Lutomirski <>, 
	Andy Lutomirski <>, "the arch/x86 maintainers" <>, 
	Linux Kernel Mailing List <>, 
	"" <>, Borislav Petkov <>, 
	Nadav Amit <>, Kees Cook <>, 
	Brian Gerst <>, 
	"" <>, Josh Poimboeuf <>, 
	Jann Horn <>, Heiko Carstens <>
Subject: Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

On Thu, Jun 23, 2016 at 12:17 PM, Linus Torvalds
<> wrote:
> With the goal being that I'm hoping that we can then actually get rid
> of this (at least on x86-64, even if we leave it in some other
> architectures) in 4.8.

The context here was that we could almost get rid of thread-info
entirely, at least for x86-64, by moving it into struct task_struct.

It turns out that we're not *that* far off after the obvious cleanups
I already committed, but I couldn't get things quite to work.

I'm attaching a patch that I wrote today that doesn't boot, but "looks
right". The reason I'm attaching it is because I'm hoping somebody
wants to take a look and maybe see what else I missed, but mostly
because I think the patch is interesting in a couple of cases where we
just do incredibly ugly things.

First off, some code that Andy wrote when he re-organized the entry path.

Oh Gods, Andy. That pt_regs_to_thread_info() thing made me want to do
unspeakable acts on a poor innocent wax figure that looked _exactly_
like you.

I just got rid of pt_regs_to_thread_info() entirely, and just replaced
it with current_thread_info().  I'm not at all convinced that trying
to be that clever was really a good idea.

Secondly, the x86-64 ret_from_fork calling convention was documented
wrongly. It says %rdi contains the previous task pointer. Yes it does,
but it doesn't mention that %r8 is supposed to contain the new
thread_info. That was fun to find.

And thirdly, the stack size games that asm/kprobes.h plays are just
disgusting. I stared at that code for much too long. I may in fact be
going blind as a result.

The rest was fairly straightforward, although since the end result
doesn't actually work, that "straightforward" may be broken too. But
the basic approach _looks_ sane.

Comments? Anybody want to play with this and see where I went wrong?

(Note - this patch was written on top of the two thread-info removal
patches I committed in

   da01e18a37a5 x86: avoid avoid passing around 'thread_info' in stack
dumping code
   6720a305df74 locking: avoid passing around 'thread_info' in mutex
debugging code

and depends on them, since "ti->task" no longer exists with
CONFIG_THREAD_INFO_IN_TASK. "ti" and "task" will have the same value).


View attachment "patch.diff" of type "text/plain" (17448 bytes)

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