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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