linux-kernel - Re: [PATCH V8 1/2] printk: remove tabular output for NULL pointer

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Date:   Thu, 26 Oct 2017 20:37:28 +1100
From:   "Tobin C. Harding" <me@...in.cc>
To:     Joe Perches <joe@...ches.com>
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 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.

> 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()
> 
> > > > Remove the attempt to print NULL pointers with a correct width.
> > > 
> > > the correct width for a %p is the default width.
> > 
> > It is the default width if we are printing addresses. Once we hash 64
> > bit address to a 32 bit identifier then we don't have a default width.
> 
> Perhaps that 32 bit identifier should use leading 0's for
> the default width.

That's a fair comment.

> aside:
> 
> Why hash 64 bits to 32?
> Why shouldn't the hash width be 64 bits on 64 bit systems?

Quoted from Linus in an earlier thread discussing this change

	Date: Thu, 12 Oct 2017 11:37:22 -0700 Linus Torvalds wrote:

	In fact, I'd prefer mapping the pointer to a 32-bit value, even on
	64-bit architectures. When people use these things for debugging and
	for identifying which device node or socket or whatever they are
	tracking, we're generally talking a (small) handful of different
	devices or whatever.


Hope this helps,
Tobin.