Re: null pointer questions

Brian Gerst (bgerst@didntduck.org)
Fri, 02 Nov 2001 18:37:08 -0500


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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/