58e185a0dc
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@oracle.com> Signed-off-by: Andrii Nakryiko <andrii@kernel.org> Acked-by: Eduard Zingerman <eddyz87@gmail.com> Link: https://lore.kernel.org/bpf/20240613095014.357981-2-alan.maguire@oracle.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.