linux-kernel - Re: [PATCH] lib/vsprintf.c: increase the size of the field

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Message-ID: <20150910143629.GB8114@mtj.duckdns.org>
Date:	Thu, 10 Sep 2015 10:36:29 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Joe Perches <joe@...ches.com>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Maurizio Lombardi <mlombard@...hat.com>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/vsprintf.c: increase the size of the field_width
 variable

Hello, Joe.

On Wed, Sep 09, 2015 at 12:26:39PM -0700, Joe Perches wrote:
> > > %*pb is meant for smallish bitmaps, not big ones.

I'm not so sure about that.  It's one thing to truncate printk output
cuz the actual formatted result is too long but crippling the entire
printf family of functions regardless of the actual output, which
might as well be just "7-65540", doesn't seem like a good idea.  These
are pretty basic library functions and people shouldn't need to worry
about niggles like that.

> > Yup. And that leads to my other confusion: Given that the expected
> > output is given as "0-15", does the bitmap really consist of > S16_MAX
> > bits with only the first 16 set?
> 
> No idea.  Tejun?

The use case isn't from me, but why not?

> Perhaps the code in lib/vsprintf.c that sets
> spec.field_width and spec.precision from format argument
> input should be changed to use a temporary int and
> clamped to S16_MIN -> S16_MAX.

I think it should be able to handle bitmaps >64k.  We don't want to
cripple the fundamental string formatter because we can't pass around
struct larger than 8 bytes.  That just reeks of poor decision making.
Why are we even copying the struct on invocations?  Only some
functions modify the values after all.  We might as well pass around
pointer to the struct and let the callees wihch modify them copy the
fieldsin local vars like normal functions.

Thanks.

-- 
tejun
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