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

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Date:	Tue, 13 Apr 2010 20:01:42 -0700
From:	Joe Perches <joe@...ches.com>
To:	Eric Paris <eparis@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [REGRESSION PATCH] vsprintf: increase sizeof precision in
 printf_spec

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 */
};


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