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

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Date:   Mon, 24 Sep 2018 01:01:51 +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>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/6] fsmount: do not use legacy MS_ flags

On Sun, Sep 23, 2018 at 11:45:17PM +0100, David Howells wrote:
> Christian Brauner <christian@...uner.io> wrote:
> 
> > Of course, I'm not sure what the reasons for all of the other arguments to
> > this function are since it's not yet implemented.
> 
> Well, dfd, path and atflags are pretty standard.  atflags conveys things like
> AT_EMPTY_PATH or AT_SYMLINK_NOFOLLOW and dfd conveys a file descriptor
> pointing to a vfs object or AT_FDCWD.
> 
> > Seems that attr_values and attr_mask could be compacted to a single
> > attr_mask maybe?
> 
> If you don't have a mask, you can't really do recursion.  Without the mask,
> you have to supply the entire set of options absolutely - and this would get
> stamped on everything in the target range.
> 
> With a mask in combination with the set of desired values, you can turn on or
> off a specific subset of the attributes without affecting the rest - without
> needing to know the rest.

Ok, understood. What about passing the different attrs as a struct?

struct mount_attr {
        unsigned int attr_cmd,
        unsigned int attr_values,
        unsigned int attr_mask,

};

mount_setattr(int dfd, const char *path, unsigned int atflags,
              struct mount_attr *attr);

I find that to be a little cleaner in all honesty.
One could also add a version argument similar to what we currently do
for vfs fcaps so that kernel and userspace can easily navigate
compabitility when a new member gets added or removed in later releases.

Christian

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