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Message-ID: <20171025231116.GC12341@eros>
Date: Thu, 26 Oct 2017 10:11:16 +1100
From: "Tobin C. Harding" <me@...in.cc>
To: "Jason A. Donenfeld" <Jason@...c4.com>
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 v7] printk: hash addresses printed with %p
On Thu, Oct 26, 2017 at 12:59:08AM +0200, Jason A. Donenfeld wrote:
> On Thu, Oct 26, 2017 at 12:27 AM, Tobin C. Harding <me@...in.cc> wrote:
> > How good is unlikely()?
>
> It places that branch way at the bottom of the function so that it's
> less likely to pollute the icache.
>
> > It doesn't _feel_ right adding a check on every call to printk just to
> > check for a condition that was only true for the briefest time when the
> > kernel booted. But if unlikely() is good then I guess it doesn't hurt.
> >
> > I'm leaning towards the option 1, but then all those text books I read
> > are telling me to implement the simplest solution first then if we need
> > to go faster implement the more complex solution.
> >
> > This is a pretty airy fairy discussion now, but if you have an opinion
> > I'd love to hear it.
>
> I don't think adding a single branch there really matters that much,
> considering how many random other branches there are all over the
> printk code. However, if you really want to optimize on the little
> bits, and sensibly don't want to go with the overcomplex
> workqueue-to-statickey thing, you could consider using a plain vanilla
> `bool has_gotten_random_ptr_secret` instead of using the atomic_t. The
> reason is that there's only ever one single writer, changing from a 0
> to a 1. Basically the only thing doing the atomic_t got you was a
> cache flush surrounding the read (and the write) so that assigning
> has_gotten_random_ptr_secret=true would take effect _immediately_.
> However, since you might not necessarily about that, going with a bool
> instead will save you an expensive cache flush, while potentially
> being a microsecond out of date the first time it's used. Seems like
> an okay trade off to me. (That kind of cache latency, also, is a few
> orders of magnitude better than using a work queue for the statickey
> stuff.)
Awesome. Patch to follow.
thanks,
Tobin.
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