linux-kernel - Re: [PATCH v5] printk: hash addresses printed with %p

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Date:   Thu, 19 Oct 2017 03:36:20 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     "Tobin C. Harding" <me@...in.cc>,
        kernel-hardening@...ts.openwall.com,
        "Theodore Ts'o" <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tycho Andersen <tycho@...ker.com>,
        "Roberts, William C" <william.c.roberts@...el.com>,
        Tejun Heo <tj@...nel.org>,
        Jordan Glover <Golden_Miller83@...tonmail.ch>,
        Greg KH <gregkh@...uxfoundation.org>,
        Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>,
        Ian Campbell <ijc@...lion.org.uk>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <wilal.deacon@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Chris Fries <cfries@...gle.com>,
        Dave Weinstein <olorin@...gle.com>,
        Daniel Micay <danielmicay@...il.com>,
        Djalal Harouni <tixxdz@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] printk: hash addresses printed with %p

On Thu, Oct 19, 2017 at 3:31 AM, Sergey Senozhatsky
<sergey.senozhatsky.work@...il.com> wrote:
> On (10/19/17 03:03), Jason A. Donenfeld wrote:
> [..]
>> 1) Go back to the spinlock yourself.
>
> so we ruled out NMI deadlocks?

Oh, right. No, I haven't thought through this enough to rule it out.
Indeed if that's an issue, the locks in the _once code will also be an
issue.

So if locking is totally impossible, then a race-free way of doing
this is with a tri-state compare and exchange. Things are either: in
state 1: no key, state 2: getting key, state 3: have key. If state 1
or 2, print the placeholder token. If state 3, do the hashing.