linux-kernel - Re: [PATCH v2 2/6] userfaultfd: add /dev/userfaultfd for fine grained access control

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Date:   Tue, 26 Apr 2022 14:33:27 -0700
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Charan Teja Reddy <charante@...eaurora.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Dmitry V . Levin" <ldv@...linux.org>,
        Gleb Fotengauer-Malinovskiy <glebfm@...linux.org>,
        Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
        Jonathan Corbet <corbet@....net>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
        Shuah Khan <shuah@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        zhangyi <yi.zhang@...wei.com>, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Linuxkselftest <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v2 2/6] userfaultfd: add /dev/userfaultfd for fine grained
 access control

On Tue, Apr 26, 2022 at 1:33 PM Peter Xu <peterx@...hat.com> wrote:
>
> Axel,
>
> On Fri, Apr 22, 2022 at 02:29:41PM -0700, Axel Rasmussen wrote:
> > @@ -65,6 +66,8 @@ struct userfaultfd_ctx {
> >       unsigned int flags;
> >       /* features requested from the userspace */
> >       unsigned int features;
> > +     /* whether or not to handle kernel faults */
> > +     bool handle_kernel_faults;
>
> Could you help explain why we need this bool?  I failed to figure out
> myself on the difference against "!(ctx->flags & UFFD_USER_MODE_ONLY)".

Ah, yeah you're right, we can get rid of it and just rely on
UFFD_USER_MODE_ONLY.

Just to add context, in a previous version I never sent out, I had:

ctx->handle_kernel_faults = userfaultfd_allowed(...);

That's wrong for other reasons, but if we were going to do that we'd
have to store the result, since it's a function not just of the flags,
but also of the method used to create the userfaultfd. I changed this
without also dropping the boolean, which can now be cleaned up. I'll
include this change in a v3.

>
> Thanks,
>
> --
> Peter Xu
>