linux-kernel - RE: [PATCH 0/5] add printk specifier %px, unique identifier

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Date:   Tue, 28 Nov 2017 10:12:09 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Eric W. Biederman'" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     "Tobin C. Harding" <me@...in.cc>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Theodore Ts'o <tytso@....edu>,
        Kees Cook <keescook@...omium.org>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        Tycho Andersen <tycho@...ho.ws>,
        "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>,
        Radim Krcmár <rkrcmar@...hat.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: RE: [PATCH 0/5] add printk specifier %px, unique identifier

From: Eric W. Biederman
> Sent: 28 November 2017 06:27
> Linus Torvalds <torvalds@...ux-foundation.org> writes:
> 
> > On Mon, Nov 27, 2017 at 4:03 PM, Linus Torvalds
> > <torvalds@...ux-foundation.org> wrote:
> >>
> >> So the big remaining ones for me are the /proc/<pid>/stack (stack
> >> pointers) and the /proc/net/* ones.
> >>
> >> I'm a bit disappointed that those haven't been fixed already and
> >> aren't even in this series..
> >
> > Oh well, I just did /proc/<pid>/stack by making it just print 0
> > unconditionally rather than the hex number.
> 
> Patch?
> 
> I know I have used /proc/<pid>/stack manually many times when looking
> at a system where something is hung/weird and I needed to see what is
> going on.  The backtrace inside the kernel can be invaluable.

Ditto - after I spotted it.
Also the similar tracebacks from echo t >/proc/sysrq-trigger
although they are less useful unless you've a big kernel message buffer.
Although they can be requested from a keyboard if everything except the
keyboard interrupt is borked.

> At the same time I don't know if we actually need the hex address.
> But please don't break that interface it is very useful.

Definitely need to know which addresses are zero (or near zero).
I will have tied the addresses there to ones available elsewhere.
(In private trace that won't be affected by whatever kernel printf
does with %p.)

If you want to hide addresses, then maybe use a write-only sysctl.

	David