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Message-ID: <3BE32E24.E89E83CD@didntduck.org>
Date: Fri, 02 Nov 2001 18:37:08 -0500
From: Brian Gerst <bgerst@didntduck.org>
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To: Ken Ashcraft <kash@Stanford.EDU>
CC: linux-kernel@vger.kernel.org
Subject: Re: null pointer questions
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Ken Ashcraft wrote:
> 
> > > 2. What happens if I pass a null pointer as the destination parameter
> > > to copy_from_user?  Does copy_from_user handle it safely or will the
> > > kernel seg fault?
> >
> > The kernel won't crash, but it might fail (depending on whether 0 is a
> > valid user space address or not).
> 
> Why does it matter if 0 is a valid user space or not?  If I make the call
> 
> copy_from_user(0, user_ptr, 4);
> 
> the null pointer is the kernel address, not the user address.  Can you
> clarify please?

copy_from_user uses the string move instruction on the x86, so the
exception code would assume the source faulted not the dest.  It would
return -EFAULT instead of causing an oops.

-- 

						Brian Gerst
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