Re: [PATCH v2 0/2] kdump: simplify code

[Matthew Wilcox]

On Sat, Dec 11, 2021 at 05:53:46PM +0000, David Laight wrote:
> From: Tiezhu Yang
> > Sent: 11 December 2021 03:33
> > 
> > v2:
> >   -- add copy_to_user_or_kernel() in lib/usercopy.c
> >   -- define userbuf as bool type
> 
> Instead of having a flag to indicate whether the buffer is user or kernel,
> would it be better to have two separate buffer pointers.
> One for a user space buffer, the other for a kernel space buffer.
> Exactly one of the buffers should always be NULL.

No.  You should be using an iov_iter instead.  See
https://lore.kernel.org/all/Ya4bdB0UBJCZhUSo@xxxxxxxxxxxxxxxxxxxx/
for a start on this.