linux-kernel - Re: [PATCH v6 0/4] enable early printing of hashed pointers

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Date:   Wed, 6 Jun 2018 15:02:20 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Tobin C. Harding" <me@...in.cc>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Kees Cook <keescook@...omium.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Theodore Ts'o <tytso@....edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 0/4] enable early printing of hashed pointers

On Mon, 28 May 2018, Tobin C. Harding wrote:

> Currently printing pointers early in the boot sequence can result in a
> dummy string '(____ptrval____)' being printed.  While resolving this
> issue it was noticed that we can use the hw RNG if available for hashing
> pointers.
> 
> Patch one and two do the ground work to be able to use hw RNG removing
> from get_random_bytes_arch() the call to get_random_bytes() and
> returning the number of bytes of random material successfully returned. 
> 
> Patch three uses the hw RNG to get keying material if it is available.
> 
> Patch four further assists debugging early in the boot sequence for
> machines that do not have a hw RNG by adding a command line option
> 'debug_boot_weak_hash'.  If enabled, non-cryptographically secure hashing
> is used instead of siphash so we can hash at any time. 
> 
> During the versions of this set I have been totally confused about which
> patches go through which tree.  This version again puts all 4 patches
> together in the hope they will go through Andrew's tree.
> 
> 
> Steve,
> 
> Could you please take a quick squiz at the final 2 patches if you get a
> chance.  I assumed we are in preemptible context during early_init based
> on your code (and code comment) and called static_branch_disable()
> directly if hw RNG returned keying material.  It's a pretty simple
> change but I'd love to get someone else to check I've not noob'ed it.

early_initcalls() are not that early :) They run in thread context fully
preemtible so calling static_branch_disable() is perfectly fine.

Thanks,

	tglx