linux-kernel - Re: [PATCH] dma-debug: Fix the overlap() function to be correct and readable

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Date:	Tue, 14 Jul 2009 16:07:02 +0530
From:	Jaswinder Singh Rajput <jaswinder@...nel.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Joerg Roedel <joerg.roedel@....com>
Subject: Re: [PATCH] dma-debug: Fix the overlap() function to be correct
 and readable

On Tue, 2009-07-14 at 15:45 +0530, Jaswinder Singh Rajput wrote:
> > >From 35c89da82e969a2fd157478940e7ecde1e19ccc4 Mon Sep 17 00:00:00 2001
> > From: Ingo Molnar <mingo@...e.hu>
> > Date: Fri, 10 Jul 2009 21:38:02 +0200
> > Subject: [PATCH] dma-debug: Fix the overlap() function to be correct and readable
> > 
> > Linus noticed how unclean and buggy the overlap() function is:
> > 
> >  - It uses convoluted (and bug-causing) positive checks for
> >    range overlap - instead of using a more natural negative
> >    check.
> > 
> >  - Even the positive checks are buggy: a positive intersection
> >    check has four natural cases while we checked only for three,
> >    missing the (addr < start && addr2 == end) case for example.
> > 
> >  - The variables are mis-named, making it non-obvious how the
> >    check was done.
> > 
> >  - It needlessly uses u64 instead of unsigned long. Since these
> >    are kernel memory pointers and we explicitly exclude highmem
> >    ranges anyway we cannot ever overflow 32 bits, even if we
> >    could. (and on 64-bit it doesnt matter anyway)
> > 
> > All in one, this function needs a total revamp. I used Linus's
> > suggestions minus the paranoid checks (we cannot overflow really
> > because if we get totally bad DMA ranges passed far more things
> > break in the systems than just DMA debugging). I also fixed a
> > few other small details i noticed.
> > 
> > Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
> > Cc: Joerg Roedel <joerg.roedel@....com>
> > Signed-off-by: Ingo Molnar <mingo@...e.hu>
> > ---
> >  lib/dma-debug.c |   24 ++++++++++++------------
> >  1 files changed, 12 insertions(+), 12 deletions(-)
> > 
> > diff --git a/lib/dma-debug.c b/lib/dma-debug.c
> > index c9187fe..02fed52 100644
> > --- a/lib/dma-debug.c
> > +++ b/lib/dma-debug.c
> > @@ -856,22 +856,21 @@ static void check_for_stack(struct device *dev, void *addr)
> >  				"stack [addr=%p]\n", addr);
> >  }
> >  
> > -static inline bool overlap(void *addr, u64 size, void *start, void *end)
> > +static inline bool overlap(void *addr, unsigned long len, void *start, void *end)
> >  {
> > -	void *addr2 = (char *)addr + size;
> > +	unsigned long a1 = (unsigned long)addr;
> > +	unsigned long b1 = a1 + len;
> > +	unsigned long a2 = (unsigned long)start;
> > +	unsigned long b2 = (unsigned long)end;
> >  
> > -	return ((addr >= start && addr < end) ||
> > -		(addr2 >= start && addr2 < end) ||
> > -		((addr < start) && (addr2 > end)));
> > +	return !(b1 <= a2 || a1 >= b2);
> >  }
> >  
> 
> If b1 = a2 (overlap) then this function will say 0
> If a1 = b2 (overlap) then this function will say 0
> 
> if b1 > (a2 + infinite) which is not overlap this function will say 1
> 
> I think we need to test both edges.
> 
> So it should be :
> 
> 	return ((a2 <= b1 && b2 >= a1) || (a1 <= b2 && a2 <= b1));
> 

We can make it more beautiful like :

	return ((a2 <= b1 && b2 >= a1) || (a1 <= b2 && b1 >= a2));

--
JSR

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