linux-kernel - Re: [PATCH v3 04/14] lib/vsprintf.c: expand field

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Message-ID: <1449178138.15393.161.camel@linux.intel.com>
Date:	Thu, 03 Dec 2015 23:28:58 +0200
From:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:	Joe Perches <joe@...ches.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Martin Kletzander <mkletzan@...hat.com>
Cc:	Andy Shevchenko <andy.shevchenko@...il.com>,
	Maurizio Lombardi <mlombard@...hat.com>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits

On Thu, 2015-12-03 at 12:54 -0800, Joe Perches wrote:
> On Thu, 2015-12-03 at 21:51 +0100, Rasmus Villemoes wrote:
> > Maurizio Lombardi reported a problem [1] with the %pb extension: It
> > doesn't work for sufficiently large bitmaps, since the size is
> > stashed
> > in the field_width field of the struct printf_spec, which is
> > currently
> > an s16. Concretely, this manifested itself in
> > /sys/bus/pseudo/drivers/scsi_debug/map being empty, since the
> > bitmap
> > printer got a size of 0, which is the 16 bit truncation of the
> > actual
> > bitmap size.
> > 
> > We do want to keep struct printf_spec at 8 bytes so that it can
> > cheaply be passed by value. The qualifier field is only used for
> > internal bookkeeping in format_decode, so we might as well use a
> > local
> > variable for that. This gives us an additional 8 bits, which we can
> > then use for the field width.
> > 
> > To stay in 8 bytes, we need to do a little rearranging and make the
> > type member a bitfield as well. For consistency, change all the
> > members to bit fields. gcc doesn't generate much worse code with
> > these
> > changes (in fact, bloat-o-meter says we save 300 bytes - which I
> > think
> > is a little surprising).
> > 
> > I didn't find a BUILD_BUG/compiletime_assertion/... which would
> > work
> > outside function context, so for now I just open-coded it.
> > 
> > [1] http://thread.gmane.org/gmane.linux.kernel/2034835
> 
> Thanks for keeping at this Rasmus.
> This seems quite reasonable.

I like most of the stuff here, though, Joe, can we avoid open-coded
BUILD_BUG_ON()?


-- 
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy

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