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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.
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