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Message-ID: <1271213041.3008.35.camel@dhcp235-240.rdu.redhat.com>
Date: Tue, 13 Apr 2010 22:44:01 -0400
From: Eric Paris <eparis@...hat.com>
To: Joe Perches <joe@...ches.com>
Cc: linux-kernel@...r.kernel.org, fweisbec@...il.com
Subject: Re: [REGRESSION PATCH] vsprintf: increase sizeof precision in
printf_spec
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.
> type isn't currently required to be u16.
> It could be u8.
>
> Perhaps this is better.
>
> 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 */
> } __attribute__((packed));
If we don't need the size in type I'm happy with this.
But what does __attribute__((packed)) buy us? I always thought that was
frowned upon because it could put us in a position where if it does
affect the layout it destroys performance on systems where unaligned
access matters. I don't think it is going to affect the alignment of
this struct, so I don't understand why we should add it. But maybe I'm
misinformed....
I'll gladly resend with u8 type and s16 precision if that's the best
solution.
-Eric
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