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

Sergey Vlasov
2005-11-01T19:55:13Z

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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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