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From:   Axel Rasmussen <axelrasmussen@google.com>
Date:   Thu, 18 Aug 2022 10:22:27 -0700
Message-ID: <CAJHvVcjSjk15TVRTi9x+CMjrWoNeUJBiZiH1boQvQzwd-pdOtQ@mail.gmail.com>
Subject: Re: [PATCH v6 2/5] userfaultfd: add /dev/userfaultfd for fine grained
 access control
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.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>,
        Peter Xu <peterx@redhat.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 <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linuxkselftest <linux-kselftest@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        linux-security-module@vger.kernel.org,
        Mike Rapoport <rppt@linux.ibm.com>
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On Wed, Aug 17, 2022 at 11:32 PM Greg KH <gregkh@linuxfoundation.org> wrote:
>
> On Thu, Aug 18, 2022 at 08:26:38AM +0200, Greg KH wrote:
> > On Wed, Aug 17, 2022 at 02:47:25PM -0700, Axel Rasmussen wrote:
> > > +static int userfaultfd_dev_open(struct inode *inode, struct file *file)
> > > +{
> > > +   return 0;
> >
> > If your open does nothing, no need to list it here at all, right?
> >
> > > +}
> > > +
> > > +static long userfaultfd_dev_ioctl(struct file *file, unsigned int cmd, unsigned long flags)
> > > +{
> > > +   if (cmd != USERFAULTFD_IOC_NEW)
> > > +           return -EINVAL;
> > > +
> > > +   return new_userfaultfd(flags);
> > > +}
> > > +
> > > +static const struct file_operations userfaultfd_dev_fops = {
> > > +   .open = userfaultfd_dev_open,
> > > +   .unlocked_ioctl = userfaultfd_dev_ioctl,
> > > +   .compat_ioctl = userfaultfd_dev_ioctl,
> >
> > Why do you need to set compat_ioctl?  Shouldn't it just default to the
> > existing one?
> >
> > And why is this a device node at all?  Shouldn't the syscall handle all
> > of this (to be honest, I didn't read anything but the misc code, sorry.)
>
> Ah, read the documentation now.  Seems you want to make it easier for
> people to get permissions on a system.  Doesn't seem wise, but hey, it's
> not my feature...

Thanks for taking a look Greg!

WIth the syscall, the only way to get access to this feature is to
have CAP_SYS_PTRACE. Which gives you access to this, *plus* a bunch
more stuff.

My basic goal is to grant access to just this feature by itself, not
really just to make it easier to access. I think a device node is the
simplest way to achieve that (see the cover letter for considered
alternatives).

The other feedback looks like good simplification to me - I'll send
another version with those changes. I have to admit this is the first
time I've messed with misc device nodes, so apologies for being overly
explicit. :)

>
> thanks,
>
> greg k-h