Re: [PATCH mm-unstable v1 20/20] mm: rename FOLL_FORCE to FOLL_PTRACE

David Hildenbrand
2022-11-16T18:56:46Z

On 16.11.22 19:16, Linus Torvalds wrote:
> On Wed, Nov 16, 2022 at 2:30 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> Let's make it clearer that functionality provided by FOLL_FORCE is
>> really only for ptrace access.
>
> I'm not super-happy about this one.
>
> I do understand the "let's rename the bit so that no new user shows up".
>
> And it's true that the main traditional use is ptrace.
>
> But from the patch itself it becomes obvious that no, it's not *just*
> ptrace. At least not yet.
>
> It's used for get_arg_page(), which uses it to basically look up (and
> install) pages in the newly created VM.
>
> Now, I'm not entirely sure why it even uses FOLL_FORCE, - I think it
> might be historical, because the target should always be the new stack
> vma.
>
> Following the history of it is a big of a mess, because there's a
> number of renamings and re-organizations, but it seems to go back to
> 2007 and commit b6a2fea39318 ("mm: variable length argument support").
>

Right.

> Before that commit, we kept our own array of "this is the set of pages
> that I will install in the new VM". That commit basically just inserts
> the pages directly into the VM instead, getting rid of the array size
> limitation.
>
> So at a minimum, I think that FOLL_FORCE would need to be removed
> before any renaming to FOLL_PTRACE, because that's not some kind of
> small random case.
>
> It *might* be as simple as just removing it, but maybe there's some
> reason for having it that I don't immediately see.

Right, I have the same feeling. It might just be a copy-and-paste legacy
leftover.

>
> There _are_ also small random cases too, like get_cmdline(). Maybe
> that counts as ptrace, but the execve() case most definitely does not.

I agree. I'd suggest moving forward without this (last) patch for now
and figuring out how to further cleanup FOLL_FORCE usage on top.

@Andrew, if you intend to put this into mm-unstable, please drop the
last patch for now.

--
Thanks,

David / dhildenb