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

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Date:   Tue, 28 Nov 2017 17:44:54 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
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: Linus Torvalds
> Sent: 28 November 2017 17:33
> 
> On Mon, Nov 27, 2017 at 10:26 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
> >>
> >> Oh well, I just did /proc/<pid>/stack by making it just print 0
> >> unconditionally rather than the hex number.
> >
> > Patch?
> 
> Oh, apparently I never pushed out yesterday.
> 
> The patch literally just affects the (useless) hex number. So:
> 
>     cat /proc/self/stack
> 
> now prints out
> 
>     [<0>] proc_pid_stack+0xaa/0x100
>     [<0>] proc_single_show+0x48/0x80
>     [<0>] seq_read+0xd2/0x410
>     ...
> 
> instead of putting some randomized kernel address there.

Not sure I've done it on Linux - getting a hexdump of the stack is hard.
But I know I've used the absolute return addresses to help hand-decode
the stack.
Usually needed to work out which stack frame is which - especially when the
stack decode doesn't actually (obviously) contain the addresses of each frame.

I don't know how these new stack traceback methods work, but the best one
I've seen in the past disassembled forwards remembering the stack offset
and unprocessed branch targets until it found a return address.
It only had to track %sp and %bp.

	David