On Tue, 2024-05-28 at 13:24 +0100, Alan Maguire wrote: > To support more robust split BTF, adding supplemental context for the > base BTF type ids that split BTF refers to is required. Without such > references, a simple shuffling of base BTF type ids (without any other > significant change) invalidates the split BTF. Here the attempt is made > to store additional context to make split BTF more robust. > > This context comes in the form of distilled base BTF providing minimal > information (name and - in some cases - size) for base INTs, FLOATs, > STRUCTs, UNIONs, ENUMs and ENUM64s along with modified split BTF that > points at that base and contains any additional types needed (such as > TYPEDEF, PTR and anonymous STRUCT/UNION declarations). This > information constitutes the minimal BTF representation needed to > disambiguate or remove split BTF references to base BTF. The rules > are as follows: > > - INT, FLOAT, FWD are recorded in full. > - if a named base BTF STRUCT or UNION is referred to from split BTF, it > will be encoded as a zero-member sized STRUCT/UNION (preserving > size for later relocation checks). Only base BTF STRUCT/UNIONs > that are either embedded in split BTF STRUCT/UNIONs or that have > multiple STRUCT/UNION instances of the same name will _need_ size > checks at relocation time, but as it is possible a different set of > types will be duplicates in the later to-be-resolved base BTF, > we preserve size information for all named STRUCT/UNIONs. > - if an ENUM[64] is named, a ENUM forward representation (an ENUM > with no values) of the same size is used. > - in all other cases, the type is added to the new split BTF. > > Avoiding struct/union/enum/enum64 expansion is important to keep the > distilled base BTF representation to a minimum size. > > When successful, new representations of the distilled base BTF and new > split BTF that refers to it are returned. Both need to be freed by the > caller. > > So to take a simple example, with split BTF with a type referring > to "struct sk_buff", we will generate distilled base BTF with a > 0-member STRUCT sk_buff of the appropriate size, and the split BTF > will refer to it instead. > > Tools like pahole can utilize such split BTF to populate the .BTF > section (split BTF) and an additional .BTF.base section. Then > when the split BTF is loaded, the distilled base BTF can be used > to relocate split BTF to reference the current (and possibly changed) > base BTF. > > So for example if "struct sk_buff" was id 502 when the split BTF was > originally generated, we can use the distilled base BTF to see that > id 502 refers to a "struct sk_buff" and replace instances of id 502 > with the current (relocated) base BTF sk_buff type id. > > Distilled base BTF is small; when building a kernel with all modules > using distilled base BTF as a test, overall module size grew by only > 5.3Mb total across ~2700 modules. > > Signed-off-by: Alan Maguire <alan.maguire@xxxxxxxxxx> > --- I think this looks good, don't see any logical inconsistencies. Acked-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
