linux-kernel - Re: [12/24] proc: Add fs_context support to procfs [ver #7]

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Date:   Tue, 26 Jun 2018 09:57:07 +0100
From:   David Howells <dhowells@...hat.com>
To:     Andrei Vagin <avagin@...tuozzo.com>
Cc:     dhowells@...hat.com, viro@...iv.linux.org.uk,
        linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-afs@...ts.infradead.org
Subject: Re: [12/24] proc: Add fs_context support to procfs [ver #7]

Andrei Vagin <avagin@...tuozzo.com> wrote:

> > > > -	mnt = kern_mount_data(&proc_fs_type, ns, 0);
> > 
> > Here ns->user_ns and get_current_cred()->user_ns are not always equal
> 
> What do you think about the attached patch?
> ...
> -	fc = vfs_new_fs_context(&proc_fs_type, NULL, 0,
> -				FS_CONTEXT_FOR_KERNEL_MOUNT);
> +	fc = vfs_new_fs_context_userns(&proc_fs_type, NULL, 0,
> +				FS_CONTEXT_FOR_KERNEL_MOUNT, ns->user_ns);

Or you could just change fc->user_ns immediately after calling
vfs_new_fs_context().  This is what network filesystems should do with
fc->net_ns, for example.

> -struct fs_context *vfs_new_fs_context(struct file_system_type *fs_type,
> +struct fs_context *vfs_new_fs_context_userns(struct file_system_type *fs_type,
>  				      struct dentry *reference,
>  				      unsigned int sb_flags,
> -				      enum fs_context_purpose purpose)
> +				      enum fs_context_purpose purpose,
> +				      struct user_namespace *user_ns)


If you'd really rather add a new parameter, please don't rename the function
to vfs_new_fs_context_userns() - just add a new parameter.  There don't need
to be two versions of it.


This brings me to another thought:  I want to add the ability to let
namespaces be configured by userspace, for example:

	fd = fsopen("nfs");
	sprintf(buf, "ns user %d", my_user_ns_fd);
	write(fd, buf);
	sprintf(buf, "ns net %d", my_net_ns_fd);
	write(fd, buf);
	write(fd, "s fedoraproject.org:/pub");
	write(fd, "o intr");
	...

I think therefore, I might need to insert another phase between creating the
context and calling the filesystem initialiser:

	fc = vfs_new_fs_context(&afs_fs_type, mntpt, 0,
				FS_CONTEXT_FOR_SUBMOUNT);

followed by:

	vfs_sb_set_namespace(fc, THIS_IS_USER_NS, user_ns);
	vfs_sb_set_namespace(fc, THIS_IS_NET_NS, net_ns);

but then we'd need to do:

	vfs_begin_options(fc);

before continuing (unless we made this happen automatically on the receipt of
the first option):

	afs_mntpt_set_params(fc, mntpt);
	vfs_get_tree(fc);
	mnt = vfs_create_mount(fc, 0);

Alternatively, we could do the namespace setting after initialisation and let
the fs apply the changes itself.

David