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

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Date:	Thu, 03 Dec 2015 12:54:07 -0800
From:	Joe Perches <joe@...ches.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	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 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.

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