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

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Date:	Wed, 9 Sep 2015 12:36:43 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Joe Perches <joe@...ches.com>
Cc:	Maurizio Lombardi <mlombard@...hat.com>, akpm@...ux-foundation.org,
	linux@...musvillemoes.dk, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/vsprintf.c: increase the size of the field_width
 variable

Hello,

On Wed, Sep 09, 2015 at 09:33:52AM -0700, Joe Perches wrote:
> On Wed, 2015-09-09 at 12:13 +0200, Maurizio Lombardi wrote:
> > When printing a bitmap using the "%*pb[l]" printk format
> > a 16 bit variable (field_width) is used to store the size of the bitmap.
> > In some cases 16 bits are not sufficient, the variable overflows and
> > printk does not work as expected.
> 
> If more than 16 bits are necessary, it couldn't work
> as a single printk is limited to 1024 bytes.

It's weird to fail silently tho.  What we can do is just capping it at
16bit max and append something to indicate that the bitmap has been
truncated if truncation actually happened.

> > 3. Bitmap should be set, but still empty
> > # cat /sys/bus/pseudo/drivers/scsi_debug/map
> []
> > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> []
> > @@ -384,8 +384,8 @@ struct printf_spec {
> >  	u8	flags;		/* flags to number() */
> >  	u8	base;		/* number base, 8, 10 or 16 only */
> >  	u8	qualifier;	/* number qualifier, one of 'hHlLtzZ' */
> > -	s16	field_width;	/* width of output field */
> >  	s16	precision;	/* # of digits/chars */
> > +	s32	field_width;	/* width of output field */
> >  };
> >  
> >  static noinline_for_stack
> 
> And this makes the sizeof struct printf_spec more than
> 8 bytes which isn't desireable on x86-32.

Ah, right, we pass this around on stack.

Thanks.

-- 
tejun
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