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Date: Thu, 26 Oct 2017 17:11:03 -0700
From: Joe Perches <joe@...ches.com>
To: "Tobin C. Harding" <me@...in.cc>
Cc: kernel-hardening@...ts.openwall.com, "Jason A. Donenfeld"
 <Jason@...c4.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>, 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>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH V8 1/2] printk: remove tabular output for NULL pointer

On Fri, 2017-10-27 at 10:57 +1100, Tobin C. Harding wrote:
> On Thu, Oct 26, 2017 at 07:47:19AM -0700, Joe Perches wrote:
> > On Thu, 2017-10-26 at 20:37 +1100, Tobin C. Harding wrote:
> > > On Thu, Oct 26, 2017 at 01:05:39AM -0700, Joe Perches wrote:
> > > > On Thu, 2017-10-26 at 17:27 +1100, Tobin C. Harding wrote:
> > > > > Hi Joe,
> > > > > 
> > > > > thanks for your review.
> > > > > 
> > > > > On Wed, Oct 25, 2017 at 09:57:23PM -0700, Joe Perches wrote:
> > > > > > On Thu, 2017-10-26 at 13:53 +1100, Tobin C. Harding wrote:
> > > > > > > Currently pointer() checks for a NULL pointer argument and then if so
> > > > > > > attempts to print "(null)" with _some_ standard width. This width cannot
> > > > > > > correctly be ascertained here because many of the printk specifiers
> > > > > > > print pointers of varying widths.
> > > > > > 
> > > > > > I believe this is not a good change.
> > > > > > Only pointers without a <foo> extension call pointer()
> > > > > 
> > > > > Sorry, I don't understand what you mean here. All the %p<foo> specifier code is
> > > > > handled by pointer()?
> > > > 
> > > > Sorry, I was imprecise/wrong.
> > > > 
> > > > None of the %p<foo> extensions except %pK and %p<invalid_foo>
> > > > actually use this bit of the pointer() call.
> > > 
> > > 	if (!ptr && *fmt != 'K') {
> > > 		/*
> > > 		 * Print (null) with the same width as a pointer so it makes
> > > 		 * tabular output look nice.
> > > 		 */
> > > 		if (spec.field_width == -1)
> > > 			spec.field_width = default_width;
> > > 		return string(buf, end, "(null)", spec);
> > > 	}
> > > 
> > > Is there something I'm missing here? This code reads like its all %p<foo>
> > > (including %p and %p<invalid_foo>) except %pK that hit this block when
> > > a NULL pointer is passed in.
> > 
> > The idea for aligning is described in commit 5e0579812834a
> > 
> > $ git log --stat -p -1 --format=email 5e0579812834a
> > From 5e0579812834ab7fa072db4a15ebdff68d62e2e7 Mon Sep 17 00:00:00 2001
> > From: Joe Perches <joe@...ches.com>
> > Date: Tue, 26 Oct 2010 14:22:50 -0700
> > Subject: [PATCH] vsprintf.c: use default pointer field size for "(null)"
> >  strings
> > 
> > It might be nicer to align the output.
> > 
> > For instance, ACPI messages sometimes have "(null)" pointers.
> > 
> > $ dmesg | grep "(null)"  -A 1 -B 1
> > [    0.198733] ACPI: Dynamic OEM Table Load:
> > [    0.198745] ACPI: SSDT (null) 00239 (v02  PmRef  Cpu0Ist 00003000 INTL 20051117)
> > [    0.199294] ACPI: SSDT 7f596e10 001C7 (v02  PmRef  Cpu0Cst 00003001 INTL 20051117)
> > [    0.200708] ACPI: Dynamic OEM Table Load:
> > [    0.200721] ACPI: SSDT (null) 001C7 (v02  PmRef  Cpu0Cst 00003001 INTL 20051117)
> > [    0.201950] ACPI: SSDT 7f597f10 000D0 (v02  PmRef  Cpu1Ist 00003000 INTL 20051117)
> > [    0.203386] ACPI: Dynamic OEM Table Load:
> > [    0.203398] ACPI: SSDT (null) 000D0 (v02  PmRef  Cpu1Ist 00003000 INTL 20051117)
> > [    0.203871] ACPI: SSDT 7f595f10 00083 (v02  PmRef  Cpu1Cst 00003000 INTL 20051117)
> > [    0.205301] ACPI: Dynamic OEM Table Load:
> > [    0.205315] ACPI: SSDT (null) 00083 (v02  PmRef  Cpu1Cst 00003000 INTL 20051117)
> 
> Does this give the correct level of control? Would it not be better to
> control the output of NULL pointers in the code that prints them. The
> other side of the coin is that with padding a random single debug
> message ends up with unwanted white space, e.g
> 
>  [    0.205315] foo: This pointer (null)         some useful error message 
> 
> Just thoughts.

I'm not sure there are any of those uses.
Perhaps you could show actual examples.

> > > > All of the other valid %p<foo> extension uses do not end up
> > > > at this block being executed so it's effectively only regular
> > > > pointers being output by number()
> > 
> > Because passing NULL to any of the %p<foo> extensions
> > excluding %pK is probably a defect.
> 
> This implies that passing NULL to %p is a defect also, does it not.

No, it does not imply that.

%p and %pK just print the value of the pointer arg.
%p<foo> extensions other than %pK dereference the pointer arg.
NULL
dereferences cause an oops.

> > I'd expect there could be cases of userland parsers that
> > expect a certain width for pointer fields.
> > 
> > $ git grep -E "\bseq_.*%p\W" | wc -l
> > 112
> 
> This is a good point. Making %p now prefix with 0x could also
> potentially break things for the same reason.

I thought it was agreed not to do that.

> Perhaps your suggestion of
> having leading 0's in front of the 32 bit identifier on 64 bit machines
> solves a number of these problems (without the 0x prefix).
> 
> 1. It leaves the output layout unchanged, no userland breakages.
> 2. It still has the advantages of a 32 bit hash mentioned by Linus.
> 3. It makes explicit that something funny is going on with the address,
>    multiple addresses with 32 leading 0's will stand out.

cheers, Joe

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