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

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Message-ID: <87oaegcu1o.fsf@rasmusvillemoes.dk>
Date:	Thu, 26 Nov 2015 22:47:15 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Andy Shevchenko <andy.shevchenko@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Maurizio Lombardi <mlombard@...hat.com>,
	Tejun Heo <tj@...nel.org>, Joe Perches <joe@...ches.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/14] lib/vsprintf.c: expand field_width to 24 bits

On Tue, Nov 24 2015, Andy Shevchenko <andy.shevchenko@...il.com> wrote:

> On Mon, Nov 23, 2015 at 11:29 PM, Rasmus Villemoes
> <linux@...musvillemoes.dk> 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.
>
> And any objections to put it into vsnprintf() ?

I'd like to keep it close to the type definition. And I was hoping
someone would come forward and say "yeah, that's been bugging me too,
here's a patch I've been sitting on to fix that". Almost every compiler
released this decade has _Static_assert, it's about time we start using
that instead of the current mess of homegrown workarounds...

Rasmus
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