linux-kernel - Re: [REGRESSION PATCH] vsprintf: increase sizeof precision in printf

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Message-ID: <20100414150007.GD5142@nowhere>
Date:	Wed, 14 Apr 2010 17:00:10 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Joe Perches <joe@...ches.com>
Cc:	Eric Paris <eparis@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [REGRESSION PATCH] vsprintf: increase sizeof precision in
	printf_spec

On Tue, Apr 13, 2010 at 08:01:42PM -0700, Joe Perches wrote:
> On Tue, 2010-04-13 at 22:44 -0400, Eric Paris wrote:
> > On Tue, 2010-04-13 at 18:33 -0700, Joe Perches wrote:
> > > On Tue, 2010-04-13 at 21:13 -0400, Eric Paris wrote:
> > > > Patch ef0658f3de484bf9b173639cd47544584e01efa5 changed the precision field
> > > > from and int to an s8.  Problem is that we have code which uses a much larger
> > > > precision in the kernel.  An example would in the audit code where we have:
> > > > 
> > > > vsnprintf(...,..., " msg='%.1024s'", (char *)data);
> > > > 
> > > > which causes precision to be too large and end up truncating to nothing.
> > > > Raising the size of the precision fixes the audit system issue.  It also does
> > > > not affect the alignment of the struct according to pahole and is still
> > > > approprietely packed.
> > > 
> > > I don't see how it could be appropriately packed.
> > 
> > I was just saying there was no padding inside the struct, although you
> > are right about it now being longer than 64.
> 
> Which is bad.
> 
> > But what does __attribute__((packed)) buy us?
> 
> It could force the size to be 64 bits on more platforms.
> 
> > I'll gladly resend with u8 type and s16 precision if that's the best
> > solution.
> 
> Reordering struct members to keep width and precision
> together seems appropriate.  The attribute may not be.
> 
> struct printf_spec {
> 	u8 type;
> 	u8 flags;		/* flags to number() */
> 	u8 base;
> 	u8 qualifier;
> 	s16 field_width;	/* width of output field */
> 	s16 precision;		/* # of digits/chars */
> };


Yeah, we should avoid the attribute. Packing struct should be
done for pretty special cases, not to fix padding holes, because
the hole problem would be turned into an alignment access unefficiency.

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