Re: [PATCH 1/1] liveupdate: luo_file: Add internal APIs for file preservation

[Pratyush Yadav]

Hi Sami,

On Sat, Jun 13 2026, Samiullah Khawaja wrote:

> From: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
>
> Live update orchestrator file handlers depend on the preservation of
> other files. To make sure that the dependency is preserved, the file
> handlers needs to fetch the preservation token of the preserved
> dependency. Similarly during restore, a file handler wants to fetch the
> restored file of the dependency.
>
> Add APIs that allows fetching token of dependency during preservation,
> and fetching the restored file dependency during restore.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> Signed-off-by: Samiullah Khawaja <skhawaja@xxxxxxxxxx>

We discussed this once already on a call, but I'll write my argument out
here for everyone else to get a say as well.

While it isn't obvious, this patch implicitly defines a part of the uAPI
for live update. This patch says to VMMs (or other live update users)
that "you can restore dependent files in any order". That is, VMMs
don't have to restore the files in a topological sort order or
dependencies, they can do so in any order and the kernel will manage the
dependencies on its own.

But on the preservation side, VMMs still do need to follow the
topological order of dependencies. Because if they don't, the
liveupdate_get_token_outgoing() call will fail and preservation can't
proceed.

In simple words, if file type A depends on file type B, VMMs always need
to preserve B before A, because A's preservation will try to find B's
token, and if B is not preserved that will fail. On the _restore_ side
though, liveupdate_get_file_incoming() implicitly retrieves the file so
the VMM can restore then in any order.

I don't like this for a couple reasons. First, this makes the API
asymmetric. If the VMM needs to manage dependency order during
preservation anyway, why not do it on retrieve as well?

Second, the API is easier to misuse. The VMM can restore A but not B,
and then close the session. It will go on its merry way never knowing it
did something wrong. For example, guest_memfd depends on its VM FD. With
this patch, LUO will allow restoring guest_memfd without restoring the
VM FD. This makes the guest_memfd practically useless. Yes, it is a bug
in the VMM anyway, but if guest_memfd restore was denied, then it would
be easier to catch.

The kernel will keep itself safe in either case, but it will make the
API harder to misuse. And you can always _relax_ the ordering
requirement if there is a need in the future, but you can't go the other
way round.

So that's my question: do we enforce restore ordering? The code change
should be relatively simple. You just need to fail if the file is not
already restored in liveupdate_get_file_incoming().

In either case, please at least add a piece in the documentation about
this ordering. We should not leave it implicit.

[...]

-- 
Regards,
Pratyush Yadav