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

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Date:	Tue, 14 Jul 2009 16:22:15 +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 16:07 +0530, Jaswinder Singh Rajput wrote:
> 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));
> 

In above case I tested overlapping on both side : left and right.

but result is same so (x || x) = x, so in simplified version we can
write :

	return a1 <= b2 && b1 >= a2;

Thanks,
--
JSR

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