What if the system were to use the unencrypted swap (since there's a
valid header for it) and the encrypted swap (since an encrypted device
with offset was created for it) at the same time? The two swaps would
overlap and overwrite each others memory, and the system goes *ka-boom*.

Now, that's an unrealistic scenario because it's unlikely to ever
happen, and there's even a check in the kernel that prevents overlapping
devices from being accepted as valid swap devices. So an explicit
'swapon /dev/sda3' currently fails with an invalid device message.

Still, this seems a bit like a damocles sword to me.

As a lower risk, the system may end up using unencrypted swap since the
header is there and looks valid.

Swap partitions also have a size recorded in the header; if it only
serves as an UUID provider, maybe it should be set to the smallest
possible size, so it won't overlap with the encrypted side of things and
nothing terribly bad could happen even if both were somehow to be used
at the same time.

The minimum size seems to be 40 so you could prepare the partition with
(mkswap --uuid="$uuid" "$dev" 40) or something like that (assuming the
unencrypted swap is not in use at this stage).

On a side note, the offset should probably be a multiple of MiB (2048),
to retain MiB alignment on the partition/block layer which seems to be
the standard nowadays (regardless of what the underlying filesystem/swap
makes of it).

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https://bugs.launchpad.net/bugs/953875

Title:
  Encrypted swap no longer mounted at bootup

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