On Mon, Nov 16, 2020 at 04:52:54PM +0900, Johannes Thumshirn wrote:

Syzbot reported a possible use-after-free when printing a duplicate device
warning device_list_add().

At this point it can happen that a btrfs_device::fs_info is not correctly
setup yet, so we're accessing stale data, when printing the warning
message using the btrfs_printk() wrappers.

Instead of printing possibly uninitialized or already freed memory in
btrfs_printk(), explicitly pass in a NULL fs_info so the printing of the
device name will be skipped altogether.

This would be good to add some info about the evolution of the fix, also
a comment saying why we can't use fs_info.