Re: null pointer questions

[Brian Gerst <bgerst@didntduck.org>]

Brian Gerst wrote:
> 
> 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.

Err, would return non-zero instead of -EFAULT.

-- 

						Brian Gerst