Re: Race between "mount" uevent and /proc/mounts?

[Sergey Vlasov <vsu@altlinux.ru>]

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On Tue, Nov 01, 2005 at 04:58:16AM +0100, Kay Sievers wrote:
> On Tue, Nov 01, 2005 at 01:28:46AM +0100, Kay Sievers wrote:
> > Ok, makes sense. The attached seems to work for me. If we can get
> > something like this, we can remove the superblock claim/release events
> > completely and just read the events from the /proc/mounts file itself.

No, we need both events.  When you need to tell the user when it is
safe to disconnect the storage device, the event from detach_mnt() is
useless - it happens too early.  In fact, even the current way of
sending the event from kill_block_super() is broken, because the event
is generated before generic_shutdown_super() and sync_blockdev(), and
writing out cached data may take some time.

We could try to emit busy/free events from bd_claim() and
bd_release(); this would be triggered by most "interesting" users
(even opens with O_EXCL), but not by things like volume_id.

> New patch. Missed a check for namespace == NULL in detach_mnt().
[skip]
> +static unsigned int mounts_poll(struct file *file, poll_table *wait)
> +{
> +	struct task_struct *task = proc_task(file->f_dentry->d_inode);
> +	struct namespace *namespace;
> +	int ret = 0;
> +
> +	task_lock(task);
> +	namespace = task->namespace;
> +	if (namespace)
> +		get_namespace(namespace);
> +	task_unlock(task);
> +
> +	if (!namespace)
> +		return -EINVAL;
> +
> +	poll_wait(file, &mounts_wait, wait);
> +	if (namespace->mounts_poll_pending) {
> +		namespace->mounts_poll_pending = 0;
> +		ret = POLLIN | POLLRDNORM;
> +	}

This assumes that there will be only one process per namespace which
will call poll() on /proc/mounts.  Even though someone may argue that
it is the right approach (have a single process which watches
/proc/mounts and broadcasts updates to other interested processes,
e.g., over dbus), with the above implementation any unprivileged user
can call poll() and interfere with the operation of that designated
process.

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