Split BPF Type Format (BTF) provides huge advantages in that kernel modules only have to provide type information for types that they do not share with the core kernel; for core kernel types, split BTF refers to core kernel BTF type ids. So for a STRUCT sk_buff, a module that uses that structure (or a pointer to it) simply needs to refer to the core kernel type id, saving the need to define the structure and its many dependents. This cuts down on duplication and makes BTF as compact as possible. However, there is a downside. This scheme requires the references from split BTF to base BTF to be valid not just at encoding time, but at use time (when the module is loaded). Even a small change in kernel types can perturb the type ids in core kernel BTF, and due to pahole's parallel processing of compilation units, even an unchanged kernel can have different type ids if BTF is re-generated. So we have a robustness problem for split BTF for cases where a module is not always compiled at the same time as the kernel. This problem is particularly acute for distros which generally want module builders to be able to compile a module for the lifetime of a Linux stable-based release, and have it continue to be valid over the lifetime of that release, even as changes in data structures (and hence BTF types) accrue. Today it's not possible to generate BTF for modules that works beyond the initial kernel it is compiled against - kernel bugfixes etc invalidate the split BTF references to vmlinux BTF, and BTF is no longer usable for the module. The goal of this series is to provide options to provide additional context for cases like this. That context comes in the form of "base reference" BTF; it stands in for the base BTF, and contains information about the types referenced from split BTF, but not their full descriptions. The modified split BTF will refer to type ids in this .BTF.base_ref section, and when the kernel loads such modules it will use the base reference BTF to map references from split BTF to the current vmlinux BTF - a process of reconciling split BTF with the currently-running kernel's vmlinux base BTF. A module builder - using this series along with the pahole changes - can then build a module with base reference BTF via BTF_BASE_REF=1 make -C . M=path/2/module For this to work, pahole will have to be built with libbpf based on this series and with the patch titled [RFC dwarves] btf_encoder: add base_ref BTF feature to generate split BTF with base refs The module will have a .BTF section (the split BTF) and a .BTF.base_ref section. The latter is small in size - base reference BTF does not need full struct/union/enum information for named types for example. For 2667 modules built with base reference BTF, the average size observed was 1556 bytes (stddev 1563). Note that for the in-tree modules, this approach is not needed as split and base BTF in the case of in-tree modules are always built and re-built together. The series first focuses on generating split BTF with base reference BTF, and provides btf__parse_opts() which allows specification of the section name from which to read BTF data, since we now have both .BTF and .BTF.base_ref sections that can contain such data. Then we add support to resolve_btfids for generating the .BTF.ids section with reference to the .BTF.base_ref section - this ensures the .BTF.ids match those used in the split/base reference BTF. Finally the series provides the mechanism for reconciling split BTF with a new base; the base reference BTF is used to map the references to base BTF in the split BTF to the new base. For the kernel, this reconciliation process happens at module load time, and we reconcile split BTF references to base BTF with the current vmlinux BTF. .BTF.ids need to be reconciled also. So concretely, what happens is - we generate split BTF in the .BTF section of a module that refers to types in the .BTF.base_ref section as base types; these are not full type descriptions but provide information about the base type. So a STRUCT sk_buff would be represented as a FWD struct sk_buff in base reference BTF for example. - when the module is loaded, the split BTF is reconciled with vmlinux BTF; in the case of the FWD struct sk_buff, we find the STRUCT sk_buff in vmlinux BTF and map all split BTF references to the base reference FWD sk_buff to the vmlinux BTF STRUCT sk_buff. Support is also added to bpftool to be able to display split BTF relative to its .BTF.base_ref section and display the reconciled form. A previous approach to this problem [1] utilized standalone BTF for such cases - where the BTF is not defined relative to base BTF so there is no reconciliation required. The problem with that approach is that from the verifier perspective, some types are special, and having a custom representation of a core kernel type that did not necessarily match the current representation is not tenable. So the approach taken here was to preserve the split BTF model while minimizing the representation of the context needed to reconcile split and current vmlinux BTF. [1] https://lore.kernel.org/bpf/20231112124834.388735-14-alan.maguire@xxxxxxxxxx/ Alan Maguire (13): libbpf: add support to btf__add_fwd() for ENUM64 libbpf: add btf__new_split_base_ref() creating split BTF with reference base BTF selftests/bpf: test split base reference BTF generation libbpf: add btf__parse_opts() API for flexible BTF parsing bpftool: support displaying raw split BTF using base reference BTF as base kbuild,bpf: switch to using --btf_features for pahole v1.26 and later resolve_btfids: use .BTF.base_ref BTF as base BTF if -r option is used kbuild, bpf: add module-specific pahole/resolve_btfids flags for base reference BTF libbpf: split BTF reconciliation module, bpf: store BTF base reference pointer in struct module libbpf,bpf: share BTF reconcile-related code with kernel selftests/bpf: extend base reference tests cover BTF reconciliation bpftool: support displaying reconciled-with-base split BTF include/linux/btf.h | 29 + include/linux/module.h | 2 + kernel/bpf/Makefile | 8 + kernel/bpf/btf.c | 197 +++++- kernel/module/main.c | 2 + scripts/Makefile.btf | 12 +- scripts/Makefile.modfinal | 4 +- .../bpf/bpftool/Documentation/bpftool-btf.rst | 17 + tools/bpf/bpftool/btf.c | 33 +- tools/bpf/bpftool/main.c | 14 +- tools/bpf/bpftool/main.h | 2 + tools/bpf/resolve_btfids/main.c | 17 +- tools/lib/bpf/Build | 2 +- tools/lib/bpf/btf.c | 434 +++++++++---- tools/lib/bpf/btf.h | 56 ++ tools/lib/bpf/btf_common.c | 146 +++++ tools/lib/bpf/btf_reconcile.c | 614 ++++++++++++++++++ tools/lib/bpf/libbpf.map | 3 + tools/lib/bpf/libbpf_internal.h | 2 + .../bpf/prog_tests/btf_split_base_ref.c | 254 ++++++++ 20 files changed, 1668 insertions(+), 180 deletions(-) create mode 100644 tools/lib/bpf/btf_common.c create mode 100644 tools/lib/bpf/btf_reconcile.c create mode 100644 tools/testing/selftests/bpf/prog_tests/btf_split_base_ref.c -- 2.39.3
