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From:   Axel Rasmussen <axelrasmussen@google.com>
Date:   Tue, 26 Apr 2022 14:33:27 -0700
Message-ID: <CAJHvVch=8znnY-u1PbPb1qUeczn+9AR_eZOP5z9kD6PgtPQjrQ@mail.gmail.com>
Subject: Re: [PATCH v2 2/6] userfaultfd: add /dev/userfaultfd for fine grained
 access control
To:     Peter Xu <peterx@redhat.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Charan Teja Reddy <charante@codeaurora.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Dmitry V . Levin" <ldv@altlinux.org>,
        Gleb Fotengauer-Malinovskiy <glebfm@altlinux.org>,
        Hugh Dickins <hughd@google.com>, Jan Kara <jack@suse.cz>,
        Jonathan Corbet <corbet@lwn.net>,
        Mel Gorman <mgorman@techsingularity.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>, Nadav Amit <namit@vmware.com>,
        Shuah Khan <shuah@kernel.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        zhangyi <yi.zhang@huawei.com>, linux-doc@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Linuxkselftest <linux-kselftest@vger.kernel.org>
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On Tue, Apr 26, 2022 at 1:33 PM Peter Xu <peterx@redhat.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
>