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Date:   Tue, 4 Feb 2020 19:21:10 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     Ross Zwisler <zwisler@chromium.org>
Cc:     Raul Rangel <rrangel@google.com>,
        David Howells <dhowells@redhat.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Mattias Nissler <mnissler@chromium.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org,
        Benjamin Gordon <bmgordon@google.com>,
        Micah Morton <mortonm@google.com>,
        Dmitry Torokhov <dtor@google.com>, Jan Kara <jack@suse.cz>,
        Aleksa Sarai <cyphar@cyphar.com>
Subject: Re: [PATCH v5] Add a "nosymfollow" mount option.
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On Tue, Feb 04, 2020 at 04:49:48PM -0700, Ross Zwisler wrote:
> On Tue, Feb 4, 2020 at 3:11 PM Ross Zwisler <zwisler@chromium.org> wrote:
> > On Tue, Feb 4, 2020 at 2:53 PM Raul Rangel <rrangel@google.com> wrote:
> > > > --- a/include/uapi/linux/mount.h
> > > > +++ b/include/uapi/linux/mount.h
> > > > @@ -34,6 +34,7 @@
> > > >  #define MS_I_VERSION   (1<<23) /* Update inode I_version field */
> > > >  #define MS_STRICTATIME (1<<24) /* Always perform atime updates */
> > > >  #define MS_LAZYTIME    (1<<25) /* Update the on-disk [acm]times lazily */
> > > > +#define MS_NOSYMFOLLOW (1<<26) /* Do not follow symlinks */
> > > Doesn't this conflict with MS_SUBMOUNT below?
> > > >
> > > >  /* These sb flags are internal to the kernel */
> > > >  #define MS_SUBMOUNT     (1<<26)
> >
> > Yep.  Thanks for the catch, v6 on it's way.
> 
> It actually looks like most of the flags which are internal to the
> kernel are actually unused (MS_SUBMOUNT, MS_NOREMOTELOCK, MS_NOSEC,
> MS_BORN and MS_ACTIVE).  Several are unused completely, and the rest
> are just part of the AA_MS_IGNORE_MASK which masks them off in the
> apparmor LSM, but I'm pretty sure they couldn't have been set anyway.
> 
> I'll just take over (1<<26) for MS_NOSYMFOLLOW, and remove the rest in
> a second patch.
> 
> If someone thinks these flags are actually used by something and I'm
> just missing it, please let me know.

Afraid you did miss it ...

/*
 * sb->s_flags.  Note that these mirror the equivalent MS_* flags where
 * represented in both.
 */
...
#define SB_SUBMOUNT     (1<<26)

It's not entirely clear to me why they need to be the same, but I haven't
been paying close attention to the separation of superblock and mount
flags, so someone else can probably explain the why of it.