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Date: Tue, 6 Mar 2018 20:07:20 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Daniel Micay <danielmicay@...il.com>, Ingo Molnar <mingo@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Alexander Popov <alex.popov@...ux.com>,
	Kernel Hardening <kernel-hardening@...ts.openwall.com>,
	PaX Team <pageexec@...email.hu>,
	Brad Spengler <spender@...ecurity.net>,
	Andy Lutomirski <luto@...nel.org>, Tycho Andersen <tycho@...ho.ws>,
	Laura Abbott <labbott@...hat.com>,
	Mark Rutland <mark.rutland@....com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Borislav Petkov <bp@...en8.de>,
	Richard Sandiford <richard.sandiford@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H . Peter Anvin" <hpa@...or.com>,
	"Dmitry V . Levin" <ldv@...linux.org>,
	Emese Revfy <re.emese@...il.com>, Jonathan Corbet <corbet@....net>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
	Thomas Garnier <thgarnie@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexei Starovoitov <ast@...nel.org>, Josef Bacik <jbacik@...com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Nicholas Piggin <npiggin@...il.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	"David S . Miller" <davem@...emloft.net>,
	Ding Tianhong <dingtianhong@...wei.com>,
	David Woodhouse <dwmw@...zon.co.uk>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Dominik Brodowski <linux@...inikbrodowski.net>,
	Juergen Gross <jgross@...e.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Mathias Krause <minipli@...glemail.com>,
	Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
	Kyle Huey <me@...ehuey.com>,
	Dmitry Safonov <dsafonov@...tuozzo.com>,
	Will Deacon <will.deacon@....com>, Arnd Bergmann <arnd@...db.de>,
	X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v9 4/7] x86/entry: Erase kernel stack in
 syscall_trace_enter()

On Tue, Mar 06, 2018 at 10:56:58AM -0800, Linus Torvalds wrote:
> On Tue, Mar 6, 2018 at 7:16 AM, Daniel Micay <danielmicay@...il.com> wrote:
> >
> > Zeroing locals isn't very expensive because the compiler has a chance
> > to optimize it out when it can figure out it isn't needed and they're
> > almost always used.
> 
> I thought I saw Kees with some stats, and it wasn't even noticeable.
> But maybe I mis-remember.
> 
> I definitely like the "zero local variables", and would actually
> prefer something even stronger than GCC_PLUGIN_STRUCTLEAK_BYREF_ALL
> (although I can't actually read gcc plugins, so I'm just going by
> name).
> 
> It's not necessarily just struct, it can very much be arrays and
> regular automatic variables too.
> 
> And yes, we might want to have some override switch for individual
> variables ("I really know what I'm doing and I promise I fill it
> myself, but the compiler doesn't understand me, look here:").

Right, I have one case in perf where we very carefully pass a partially
initialized structure around for performance raisins. So I would
definitely like to have that attribute that allows us to over-ride that
BYREF_ALL thing.

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