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 distilled base 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 section, and when the kernel loads such modules it will use that .BTF.base to map references from split BTF to the equivalent current vmlinux base BTF types. Once this relocation process has succeeded, the module BTF available in /sys/kernel/btf will look exactly as if it was built with the current vmlinux; references to base types will be fixed up etc. A module builder - using this series along with the pahole changes - can then build a module with distilled base BTF via an out-of-tree module build, i.e. make -C . M=path/2/module The module will have a .BTF section (the split BTF) and a .BTF.base section. The latter is small in size - distilled base BTF does not need full struct/union/enum information for named types for example. For 2667 modules built with distilled base BTF, the average size observed was 1556 bytes (stddev 1563). The overall size added to this 2667 modules was 5.3Mb. 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 distilled base 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 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 section - this ensures the .BTF.ids match those used in the split/base BTF. Finally the series provides the mechanism for relocating split BTF with a new base; the distilled base BTF is used to map the references to base BTF in the split BTF to the new base. For the kernel, this relocation process happens at module load time, and we relocate split BTF references to point at types in the current vmlinux BTF. As part of this, .BTF.ids references need to be mapped 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 section as base types; the latter 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 distilled base BTF for example. - when the module is loaded, the split BTF is relocated 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 distilled base FWD sk_buff, replacing them with references 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 section, and also to display the relocated form via the "-R path_to_base_btf". 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 relocation 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 relocate split and current vmlinux BTF. To generate distilled .BTF.base sections the associated dwarves patch (to be applied on the "next" branch there) is needed. Without it, things will still work but modules will not be built with a .BTF.base section. Changes since v3[3]: - distill now checks for duplicate-named struct/unions and records them as a sized struct/union to help identify which of the multiple base BTF structs/unions it refers to (Eduard, patch 1) - added test support for multiple name handling (Eduard, patch 2) - simplified the string mapping when updating split BTF to use base BTF instead of distilled base. Since the only string references split BTF can make to base BTF are the names of the base types, create a string map from distilled string offset -> base BTF string offset and update string offsets by visiting all strings in split BTF; this saves having to do costly searches of base BTF (Eduard, patch 7,10) - fixed bpftool manpage and indentation issues (Quentin, patch 11) Also explored Eduard's suggestion of doing an implicit fallback to checking for .BTF.base section in btf__parse() when it is called to get base BTF. However while it is doable, it turned out to be difficult operationally. Since fallback is implicit we do not know the source of the BTF - was it from .BTF or .BTF.base? In bpftool, we want to try first standalone BTF, then split, then split with distilled base. Having a way to explicitly request .BTF.base via btf__parse_opts() fits that model better. Changes since v2[4]: - submitted patch to use --btf_features in Makefile.btf for pahole v1.26 and later separately (Andrii). That has landed in bpf-next now. - distilled base now encodes ENUM64 as fwd ENUM (size 8), eliminating the need for support for ENUM64 in btf__add_fwd (patch 1, Andrii) - moved to distilling only named types, augmenting split BTF with associated reference types; this simplifies greatly the distilled base BTF and the mapping operation between distilled and base BTF when relocating (most of the series changes, Andrii) - relocation now iterates over base BTF, looking for matches based on name in distilled BTF. Distilled BTF is pre-sorted by name (Andrii, patch 8) - removed most redundant compabitiliby checks aside from struct size for base types/embedded structs and kind compatibility (since we only match on name) (Andrii, patch 8) - btf__parse_opts() now replaces btf_parse() internally in libbpf (Eduard, patch 3) Changes since RFC [5]: - updated terminology; we replace clunky "base reference" BTF with distilling base BTF into a .BTF.base section. Similarly BTF reconcilation becomes BTF relocation (Andrii, most patches) - add distilled base BTF by default for out-of-tree modules (Alexei, patch 8) - distill algorithm updated to record size of embedded struct/union by recording it as a 0-vlen STRUCT/UNION with size preserved (Andrii, patch 2) - verify size match on relocation for such STRUCT/UNIONs (Andrii, patch 9) - with embedded STRUCT/UNION recording size, we can have bpftool dump a header representation using .BTF.base + .BTF sections rather than special-casing and refusing to use "format c" for that case (patch 5) - match enum with enum64 and vice versa (Andrii, patch 9) - ensure that resolve_btfids works with BTF without .BTF.base section (patch 7) - update tests to cover embedded types, arrays and function prototypes (patches 3, 12) [1] https://lore.kernel.org/bpf/20231112124834.388735-14-alan.maguire@xxxxxxxxxx/ [2] https://lore.kernel.org/bpf/20240501175035.2476830-1-alan.maguire@xxxxxxxxxx/ [3] https://lore.kernel.org/bpf/20240510103052.850012-1-alan.maguire@xxxxxxxxxx/ [4] https://lore.kernel.org/bpf/20240424154806.3417662-1-alan.maguire@xxxxxxxxxx/ [5] https://lore.kernel.org/bpf/20240322102455.98558-1-alan.maguire@xxxxxxxxxx/ Alan Maguire (11): libbpf: add btf__distill_base() creating split BTF with distilled base BTF selftests/bpf: test distilled base, split BTF generation libbpf: add btf__parse_opts() API for flexible BTF parsing bpftool: support displaying raw split BTF using base BTF section as base resolve_btfids: use .BTF.base ELF section as base BTF if -B option is used kbuild, bpf: add module-specific pahole/resolve_btfids flags for distilled base BTF libbpf: split BTF relocation selftests/bpf: extend distilled BTF tests to cover BTF relocation module, bpf: store BTF base pointer in struct module libbpf,bpf: share BTF relocate-related code with kernel bpftool: support displaying relocated-with-base split BTF include/linux/btf.h | 45 ++ include/linux/module.h | 2 + kernel/bpf/Makefile | 8 + kernel/bpf/btf.c | 166 +++-- kernel/module/main.c | 5 +- scripts/Makefile.btf | 7 + scripts/Makefile.modfinal | 4 +- .../bpf/bpftool/Documentation/bpftool-btf.rst | 15 +- tools/bpf/bpftool/bash-completion/bpftool | 7 +- tools/bpf/bpftool/btf.c | 19 +- tools/bpf/bpftool/main.c | 14 +- tools/bpf/bpftool/main.h | 2 + tools/bpf/resolve_btfids/main.c | 28 +- tools/lib/bpf/Build | 2 +- tools/lib/bpf/btf.c | 605 +++++++++++++----- tools/lib/bpf/btf.h | 59 ++ tools/lib/bpf/btf_common.c | 143 +++++ tools/lib/bpf/btf_relocate.c | 341 ++++++++++ tools/lib/bpf/libbpf.map | 3 + tools/lib/bpf/libbpf_internal.h | 3 + .../selftests/bpf/prog_tests/btf_distill.c | 346 ++++++++++ 21 files changed, 1612 insertions(+), 212 deletions(-) create mode 100644 tools/lib/bpf/btf_common.c create mode 100644 tools/lib/bpf/btf_relocate.c create mode 100644 tools/testing/selftests/bpf/prog_tests/btf_distill.c -- 2.31.1
