linux-kernel - Re: [PATCH 5/6] fsmount: do not use legacy MS

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Date:   Mon, 24 Sep 2018 15:18:16 +0200
From:   Christian Brauner <christian@...uner.io>
To:     David Howells <dhowells@...hat.com>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        Al Viro <viro@...IV.linux.org.uk>, aviro@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/6] fsmount: do not use legacy MS_ flags

On Mon, Sep 24, 2018 at 01:37:45PM +0100, David Howells wrote:
> Christian Brauner <christian@...uner.io> wrote:
> 
> > I have thought a little more about splitting up the mount flags into
> > sensible sets. I think the following four sets make sense:
> >
> > enum {
> >         MOUNT_ATTR_PROPAGATION = 1,
> >         MOUNT_ATTR_SECURITY,
> >         MOUNT_ATTR_SYNC,
> >         MOUNT_ATTR_TIME,
> > };
> 
> Al (I think it was) has been against splitting them up before (I've previously
> proposed splitting the topology propagation flags from the mount attributes).

Right, that request could probably be fulfilled by the first draft for
this idea that I had but didn't send out.
Basically, having a sequential enum that only ever gets bumped when we
run out of flags in a set, i.e.

enum {
        MOUNT_ATTR_SET_1 = 1,
        MOUNT_ATTR_SET_2 = 2,
        MOUNT_ATTR_SET_3 = 3,
        .
        .
        .
};

Then we would currently only define a single set
enum {
        MOUNT_ATTR_SET_1 = 1,
};

dump all the current mount flags we would like to support in there and
call it a day until we run out of flags at which point we introduce
MOUNT_ATTR_SET_2.
But honestly, I find defining cuts by forming sets of logically related
flags to be more intuitive and transparent for kernel- and userspace.
But I'm just another muppet with an opinion. :)

> 
> > #define MOUNT_ATTR_NOATIME     (1<<1)
> > #define MOUNT_ATTR_RELATIME    (1<<3)
> > #define MOUNT_ATTR_STRICTATIME (1<<4)
> 
> These aren't independent, but are actually settings on the same dial, so I
> would suggest that they shouldn't be separate flags.  I'm not sure about
> LAZYTIME though.

So what you or Miklos suggested before, namely making them an enum too?

> 
> > enum {
> >         MOUNT_ATTR_PROPAGATION = 1,
> >         MOUNT_ATTR_SECURITY,
> >         MOUNT_ATTR_SECURITY_1 = MOUNT_ATTR_SECURITY,
> >         MOUNT_ATTR_SYNC,
> >         MOUNT_ATTR_TIME,
> >         MOUNT_ATTR_SECURITY_2,
> > };
> 
> In UAPI headers, always explicitly number your symbols, even in an enum, just
> to make sure that the numbers don't get transparently changed by accident.

+1 and thanks for the tip!

> 
> > These flags will likely become AT_* flags or be tied to a syscall
> > afaict.
> >
> > #define MS_REMOUNT      32
> > #define MS_BIND	        4096
> > #define MS_MOVE	        8192
> > #define MS_REC	        16384
> 
> MS_REMOUNT: fd = fspick(); fscommand(fd, FSCONFIG_CMD_RECONFIGURE);
> 
> MS_REMOUNT|MS_BIND: mount_setattr().
> 
> MS_BIND: fd = open_tree(..., OPEN_TREE_CLONE); move_mount(fd, "", ...);
> 
> MS_MOVE: fd = open_tree(..., 0); move_mount(fd, "", ...);
> 
> MS_REC: AT_RECURSIVE
> 
> > Internal sb flags will not be part of the new mount attr sets. (They
> > should - imho - not be exposed to userspace at all.):
> 
> Agreed.
> 
> > What remains is an odd duck that probably could be thrown into security
> > but also *shrug*
> >
> > #define MS_I_VERSION    (1<<23)
> 
> Um.  I think it would probably belong with atime settings.
> 
> David

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