[v3,bpf-next,00/17] Add kind layout, CRCs to BTF

Message

Alan Maguire Nov. 10, 2023, 11:02 a.m. UTC

Update struct btf_header to add a new "kind_layout" section containing
a description of how to parse the BTF kinds known about at BTF
encoding time.  This provides the opportunity for tools that might
not know all of these kinds - as is the case when older tools run
on more newly-generated BTF - to still parse the BTF provided,
even if it cannot all be used.

Also add CRCs for the BTF and base BTF (if needed) from which it was
created.  CRCs provide a few useful features:

- the base CRC allows us to explicitly identify when the split and
  base BTF are not matched
- absence of a base BTF CRC can indicate that BTF is standalone;
  i.e. not defined relative to base BTF

The former case can be used to explicitly reject mismatched
module/kernel BTF rather than assuming it is matched until an
unexpected type is encountered.

The latter case is useful for modules that are not built as
frequently as the kernel; in such cases, the module can be built
standalone by specifying an empty BTF base:

 make BTF_BASE= M=path/2/module

If CRCs are not present (as will be the case for pahole versions
prior to the proposed v1.26 which will support CRC generation),
standalone BTF can still be identified by a slower fallback
method of examining BTF type ids to ensure that BTF is
self-referential only.

To ensure existing tooling can handle standalone BTF for kernel
modules,  we remap the type ids to start after the vmlinux
BTF ids, to make it appear to be split BTF.  This allows tools
that assume split BTF for modules to operate normally.

Also add support to bpftool to dump metadata about BTF; its size,
header information and kind layout section.

The ideas here were discussed at [1], with further discussion
at [2].

Apologies for the long hiatus between v2 and this version;
I've attempted to flesh out the use of the features added so
some of the practical benefits can be demonstrated.

A followup patch for pahole will enable the CRC/kind layout
addition to BTF generation, but the kernel can still be built
and tests added will still pass; it will just be the case that
we fall back to the slowpath of standalone module identification
in the absence of CRCs in the standalone module.

Note that for additional context I will be discussing this work
along with some other issues around evolving BTF at Linux
Plumbers next week; see [4] where slides will be added shortly.

Changes since v2 [3]:

- drop "optional" kind flag (Andrii, patch 1)
- allocate "struct btf_header" for struct btf to ensure
  we can always access new fields (Andrii, patch 2)
- use an internal BTF kind array in btf.c to simplify 
  kind encoding (Andrii, patch 2)
- drop use of kind layout information for in-kernel parsing,
  since the kernel needs to be strict in what it accepts
  (Andrii, patch 6)
- added CRC verification for BTF objects and for matching
  with base object (Alexei, patches 7,8)
- fixed bpftool json output (Quentin, patch 10)
- added standalone module BTF support, tests (patches 13-17)

Changes since RFC

- Terminology change from meta -> kind_layout
  (Alexei and Andrii)
- Simplify representation, removing meta header
  and just having kind layout section (Alexei)
- Fixed bpftool to have JSON support, support
  prefix match, documented changes (Quentin)
- Separated metadata opts into add_kind_layout
  and add_crc
- Added additional positive/negative tests
  to cover basic unknown kind, one with an
  info_sz object following it and one with
  N elem_sz elements following it.
- Updated pahole-flags to use help output
  rather than version to see if features
  are present

[1] https://lore.kernel.org/bpf/CAEf4BzYjWHRdNNw4B=eOXOs_ONrDwrgX4bn=[e-mail]/
[2] https://lore.kernel.org/bpf/[e-mail]/
[3] https://lore.kernel.org/bpf/[e-mail]/
[4] https://lpc.events/event/17/contributions/1576/

Alan Maguire (17):
  btf: add kind layout encoding, crcs to UAPI
  libbpf: support kind layout section handling in BTF
  libbpf: use kind layout to compute an unknown kind size
  libbpf: add kind layout encoding, crc support
  libbpf: BTF validation can use kind layout for unknown kinds
  btf: support kernel parsing of BTF with kind layout
  bpf: add BTF CRC verification where present
  bpf: verify base BTF CRC to ensure it matches module BTF
  bpf: switch to --btf_features, add crc,kind_layout features
  bpftool: add BTF dump "format meta" to dump header/metadata
  bpftool: update doc to describe bpftool btf dump .. format meta
  selftests/bpf: test kind encoding/decoding
  bpf: support standalone BTF in modules
  bpf: allow opt-out from using split BTF for modules
  selftests/bpf: generalize module load to support specifying a module
    name
  selftests/bpf: build separate bpf_testmod module with standalone BTF
  selftests/bpf: update btf_module test to ensure standalone BTF works

 include/uapi/linux/btf.h                      |  18 +
 kernel/bpf/btf.c                              | 435 +++++++++++++-
 scripts/Makefile.modfinal                     |   6 +-
 scripts/pahole-flags.sh                       |   3 +
 .../bpf/bpftool/Documentation/bpftool-btf.rst |  30 +-
 tools/bpf/bpftool/bash-completion/bpftool     |   2 +-
 tools/bpf/bpftool/btf.c                       |  91 ++-
 tools/include/uapi/linux/btf.h                |  18 +
 tools/lib/bpf/btf.c                           | 323 ++++++++--
 tools/lib/bpf/btf.h                           |  11 +
 tools/lib/bpf/libbpf.map                      |   1 +
 tools/testing/selftests/bpf/Makefile          |   8 +-
 .../selftests/bpf/bpf_testmod/Makefile        |  16 +-
 .../bpf_testmod_standalone-events.h           |  57 ++
 .../bpf/bpf_testmod/bpf_testmod_standalone.c  | 551 ++++++++++++++++++
 .../bpf/bpf_testmod/bpf_testmod_standalone.h  |  31 +
 .../bpf_testmod_standalone_kfunc.h            | 109 ++++
 .../selftests/bpf/prog_tests/bpf_mod_race.c   |   8 +-
 .../selftests/bpf/prog_tests/btf_kind.c       | 176 ++++++
 .../selftests/bpf/prog_tests/btf_module.c     |  19 +-
 .../selftests/bpf/prog_tests/module_attach.c  |   6 +-
 tools/testing/selftests/bpf/test_progs.c      |   6 +-
 tools/testing/selftests/bpf/test_verifier.c   |   6 +-
 tools/testing/selftests/bpf/testing_helpers.c |  24 +-
 tools/testing/selftests/bpf/testing_helpers.h |   4 +-
 25 files changed, 1828 insertions(+), 131 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/bpf_testmod/bpf_testmod_standalone-events.h
 create mode 100644 tools/testing/selftests/bpf/bpf_testmod/bpf_testmod_standalone.c
 create mode 100644 tools/testing/selftests/bpf/bpf_testmod/bpf_testmod_standalone.h
 create mode 100644 tools/testing/selftests/bpf/bpf_testmod/bpf_testmod_standalone_kfunc.h
 create mode 100644 tools/testing/selftests/bpf/prog_tests/btf_kind.c

Comments

Alan Maguire Nov. 10, 2023, 4:45 p.m. UTC | #1

On 10/11/2023 11:02, Alan Maguire wrote:
> Update struct btf_header to add a new "kind_layout" section containing
> a description of how to parse the BTF kinds known about at BTF
> encoding time.  This provides the opportunity for tools that might
> not know all of these kinds - as is the case when older tools run
> on more newly-generated BTF - to still parse the BTF provided,
> even if it cannot all be used.
> 

apologies; this series collided with the Makefile.btf changes that
eliminated scripts/pahole-flags.sh, so patch 9 triggers a mismerge.
I'll send an updated version once I've respun and tested. Again sorry
for the noise!

Alan