linux-ext4 - Re: [PATCH 05/34] fs: introduce MOUNT_ATTR

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Message-ID: <20201101144544.GC23378@infradead.org>
Date:   Sun, 1 Nov 2020 14:45:44 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org,
        John Johansen <john.johansen@...onical.com>,
        James Morris <jmorris@...ei.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Geoffrey Thomas <geofft@...reload.com>,
        Mrunal Patel <mpatel@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Andy Lutomirski <luto@...nel.org>,
        Amir Goldstein <amir73il@...il.com>,
        Miklos Szeredi <miklos@...redi.hu>,
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        Aleksa Sarai <cyphar@...har.com>,
        Lennart Poettering <lennart@...ttering.net>,
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        containers@...ts.linux-foundation.org,
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        selinux@...r.kernel.org
Subject: Re: [PATCH 05/34] fs: introduce MOUNT_ATTR_IDMAP

On Thu, Oct 29, 2020 at 01:32:23AM +0100, Christian Brauner wrote:
> Introduce a new mount bind mount property to allow idmapping mounts. The
> MOUNT_ATTR_IDMAP flag can be set via the new mount_setattr() syscall
> together with a file descriptor referring to a user namespace.

Shouldn't this go to the end of the series once all the infrastructure
is in place?

> +config IDMAP_MOUNTS
> +	bool "Support id mappings per mount"
> +	default n

n is the default default.

But why do we need a config option here anyway?

> +#ifdef CONFIG_IDMAP_MOUNTS
> +		if (kattr->attr_set & MNT_IDMAPPED) {
> +			struct user_namespace *user_ns;
> +			struct vfsmount *vmnt;

All the code here looks like it should go into a helper.

> +				struct user_namespace *user_ns = READ_ONCE(m->mnt.mnt_user_ns);
> +				WRITE_ONCE(m->mnt.mnt_user_ns, get_user_ns(kattr->userns));

More unreadable long lines.

> +	if (attr->attr_set & MOUNT_ATTR_IDMAP) {
> +		struct ns_common *ns;
> +		struct user_namespace *user_ns;
> +		struct file *file;
> +
> +		file = fget(attr->userns);

The code here looks like another candidate for a self contained helper.

> +
> +static inline struct user_namespace *mnt_user_ns(const struct vfsmount *mnt)
> +{
> +#ifdef CONFIG_IDMAP_MOUNTS
> +	return READ_ONCE(mnt->mnt_user_ns);
> +#else
> +	return &init_user_ns;
> +#endif

How is the READ_ONCE on a pointer going to work?