linux-kernel - Re: [PATCH 01/29] Revert "userfaultfd: don't fail on unrecognized features"

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Message-ID: <ZCYMu5P2BJy/2z5t@x1n>
Date:   Thu, 30 Mar 2023 18:27:07 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Leonardo Bras Soares Passos <lsoaresp@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        linux-stable <stable@...r.kernel.org>
Subject: Re: [PATCH 01/29] Revert "userfaultfd: don't fail on unrecognized
 features"

On Thu, Mar 30, 2023 at 12:04:09PM -0700, Axel Rasmussen wrote:
> On Thu, Mar 30, 2023 at 8:57 AM Peter Xu <peterx@...hat.com> wrote:
> >
> > This is a proposal to revert commit 914eedcb9ba0ff53c33808.
> >
> > I found this when writting a simple UFFDIO_API test to be the first unit
> > test in this set.  Two things breaks with the commit:
> >
> >   - UFFDIO_API check was lost and missing.  According to man page, the
> >   kernel should reject ioctl(UFFDIO_API) if uffdio_api.api != 0xaa.  This
> >   check is needed if the api version will be extended in the future, or
> >   user app won't be able to identify which is a new kernel.
> 
> 100% agreed, this was a mistake.
> 
> >
> >   - Feature flags checks were removed, which means UFFDIO_API with a
> >   feature that does not exist will also succeed.  According to the man
> >   page, we should (and it makes sense) to reject ioctl(UFFDIO_API) if
> >   unknown features passed in.
> 
> I still strongly disagree with reverting this part, my feeling is
> still that doing so makes things more complicated for no reason.
> 
> Re: David's point, it's clearly wrong to change semantics so a thing
> that used to work now fails. But this instead makes it more permissive
> - existing userspace programs continue to work as-is, but *also* one
> can achieve the same thing more simply (combine probing +
> configuration into one step). I don't see any problem with that,
> generally.
> 
> But, if David and others don't find my argument convincing, it isn't
> the end of the world. It just means I have to go update my userspace
> code to be a bit more complicated. :)

I'd say it's fine if it's your own program that you can in full control
easily. :) Sorry again for not noticing that earlier.

There's one reason that we may consider keeping the behavior.  IMHO it is
when there're major softwares that uses the "wrong" ABI (let's say so;
because it's not following the man pages).  If you're aware any such major
softwares (especially open sourced) will break due to this revert patch,
please shoot.

> 
> I also still think the man page is incorrect or at least incomplete no
> matter what we do here; we should be sure to update it as a follow-up.

So far it looks still fine if with this revert.  Maybe I overlooked
somewhere?

I'll add this into my todo, but with low priority.  If you have suggestion
already on how to improve the man page please do so before me!

-- 
Peter Xu