Re: [PATCH v2] btrfs: validate device maj:min during open

[Boris Burkov <boris@bur.io>]

On Fri, Mar 08, 2024 at 09:42:56AM -0800, Christoph Hellwig wrote:
> On Fri, Mar 08, 2024 at 09:32:54AM -0800, Boris Burkov wrote:
> > You remove/add the device in a way that results in a new bd_dev while
> > the filesystem is unmounted but btrfs is still caching the struct
> > btrfs_device. When we unmount a multi-device fs, we don't clear the
> > device cache, since we need it to remount with just one device name
> > later.
> > 
> > The mechanism I used for getting a different bd_dev was partitioning two
> > different devices in two different orders.
> 
> Ok, so we have a btrfs_device without a bdev around, which seems a bit
> dangerous.  Also relying on the dev_t for any kind of device identify
> seems very dangerous.  Aren't there per-device UUIDs or similar
> identifiers that are actually reliabe and can be used instead of the
> dev_t?
> 

I was led to believe this wasn't possible while still actually
implementing temp_fsid. But now that I think of it again, I am less sure.
You could imagine them having identical images except a device uuid and the
code being smart enough to handle that.

Maybe Anand can explain why that wouldn't work :)