[RFC PATCH v3 2/4] liveupdate: Add feature header for fine-grained ABI compatibility

[sashiko-bot at kernel.org]

> LUO and its file handlers currently rely solely on coarse-grained
> compatibility strings (such as luo-v5 and memfd-v2) to validate
> handed-over state across kexec. Because compatibility is checked only
> at the whole-structure level, any change to a serialized structure,
> even an optional additive field, requires bumping the compatibility
> string and breaks live update between adjacent kernels.
> 
> Introduce a feature header (struct luo_ser_hdr) ahead of serialized LUO
> structures so subsystems can evolve their serialized state at
> per-feature granularity. Each subsystem groups the fields of its
> serialized structure under feature indices, starting with index 0 for the
> base fields. When a subsystem adds a new capability, it appends any new
> fields to the end of its payload structure under a new feature index.
> Retired fields are kept in place so existing field offsets never shift;
> if the type or format of a field needs to change, the existing field and
> [ ... ]
> Also, introduce the LUO_CORE_FEAT_CORE for LUO for the existing mechanism
> which is also a required feature.
> 
> Signed-off-by: Logan Odell <loganodell at google.com>
> Signed-off-by: Tarun Sahu <tarunsahu at google.com>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261026174217.603114-1-tarunsahu@google.com?part=2