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Message-ID: <CA+55aFxo8tU42+hMjRgs1dUrqn1=rzDnvikRHHWV9HwDVeYkuQ@mail.gmail.com>
Date: Thu, 23 Jun 2016 23:17:32 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Oleg Nesterov <oleg@...hat.com>, Andy Lutomirski <luto@...capital.net>,
Andy Lutomirski <luto@...nel.org>, "the arch/x86 maintainers" <x86@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>, Borislav Petkov <bp@...en8.de>,
Nadav Amit <nadav.amit@...il.com>, Kees Cook <keescook@...omium.org>,
Brian Gerst <brgerst@...il.com>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Josh Poimboeuf <jpoimboe@...hat.com>,
Jann Horn <jann@...jh.net>, Heiko Carstens <heiko.carstens@...ibm.com>
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
<torvalds@...ux-foundation.org> 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).
Linus
View attachment "patch.diff" of type "text/plain" (17448 bytes)
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