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Date: Tue, 6 Mar 2018 11:16:51 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Ard Biesheuvel <ard.biesheuvel@...aro.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>,
Borislav Petkov <bp@...en8.de>, Richard Sandiford <richard.sandiford@....com>,
Thomas Gleixner <tglx@...utronix.de>, "H . Peter Anvin" <hpa@...or.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>, "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>,
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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 6, 2018 at 11:07 AM, Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
>
> The compiler usually does a pretty good job of detecting which scalar
> variables are never initialized by regular assignment.
Sure, but "usually" is not really the same as always. Sometimes scalar
types are initialized by passing a reference to them too.
> We could easily extend this to scalar and array types, but we'd first
> need to see what the performance impact is, because I don't think it
> will be negligible.
For scalar types, I suspect it will be entirely unnoticeable, because
they are not only small, but it's rare that this kind of "initialize
by passing a reference" happens in the first place.
For arrays, I agree. We very well may have arrays that we really want
to do magic things about. But even then I'd rather have a "don't
initialize this" flag for critical stuff that really *does* get
initialized some other way. Then we can grep for those things and be
more careful.
If somebody has big arrays on the stack, that's often a problem
anyway. It may be common in non-kernel code, but kernel code is very
special.
Linus
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