Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW

[Linus Torvalds]

I'm still reading through this, but

 STOP DOING THIS

On Mon, Aug 8, 2022 at 12:32 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> +       VM_BUG_ON(!is_cow_mapping(vma->vm_flags));

Using BUG_ON() for debugging is simply not ok.

And saying "but it's just a VM_BUG_ON()" does not change *anything*.
At least Fedora enables that unconditionally for normal people, it is
not some kind of "only VM people do this".

Really. BUG_ON() IS NOT FOR DEBUGGING.

Stop it. Now.

If you have a condition that must not happen, you either write that
condition into the code, or - if you are convinced it cannot happen -
you make it a WARN_ON_ONCE() so that people can report it to you.

The BUG_ON() will just make the machine die.

And for the facebooks and googles of the world, the WARN_ON() will be
sufficient.

               Linus