Re: null pointer questions
Brian Gerst (firstname.lastname@example.org)
Fri, 02 Nov 2001 18:39:53 -0500
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.
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