On Mon, Nov 29, 2021 at 12:56 PM Catalin Marinas
[off-list ref] wrote:

For arm64 at least __put_user() does the access_ok() check. I thought
only unsafe_put_user() should skip the checks. If __put_user() can write
arbitrary memory, we may have a bigger problem.

That's literally be the historical difference between __put_user() and
put_user() - the access check.

I think that would be useful, though it doesn't solve the potential
livelock with sub-page faults.

I was assuming we'd just do the sub-page faults.

In fact, I was assuming we'd basically just replace all the PAGE_ALIGN
and PAGE_SIZE with SUBPAGE_{ALIGN,SIZE}, together with something like

        if (size > PAGE_SIZE)
                size = PAGE_SIZE;

to limit that size thing (or possibly make that "min size" be a
parameter, so that people who have things like that "I need at least
this initial structure to be copied" issue can document their minimum
size needs).

            Linus