Re: [PATCH bpf-next v3 00/10] install libbpf headers when using the library

On Sun, Oct 3, 2021 at 12:22 PM Quentin Monnet [off-list ref] wrote:

Libbpf is used at several locations in the repository. Most of the time,
the tools relying on it build the library in its own directory, and include
the headers from there. This works, but this is not the cleanest approach.
It generates objects outside of the directory of the tool which is being
built, and it also increases the risk that developers include a header file
internal to libbpf, which is not supposed to be exposed to user
applications.

This set adjusts all involved Makefiles to make sure that libbpf is built
locally (with respect to the tool's directory or provided build directory),
and by ensuring that "make install_headers" is run from libbpf's Makefile
to export user headers properly.

This comes at a cost: given that the libbpf was so far mostly compiled in
its own directory by the different components using it, compiling it once
would be enough for all those components. With the new approach, each
component compiles its own version. To mitigate this cost, efforts were
made to reuse the compiled library when possible:

- Make the bpftool version in samples/bpf reuse the library previously
  compiled for the selftests.
- Make the bpftool version in BPF selftests reuse the library previously
  compiled for the selftests.
- Similarly, make resolve_btfids in BPF selftests reuse the same compiled
  library.
- Similarly, make runqslower in BPF selftests reuse the same compiled
  library; and make it rely on the bpftool version also compiled from the
  selftests (instead of compiling its own version).
- runqslower, when compiled independently, needs its own version of
  bpftool: make them share the same compiled libbpf.

As a result:

- Compiling the samples/bpf should compile libbpf just once.
- Compiling the BPF selftests should compile libbpf just once.
- Compiling the kernel (with BTF support) should now lead to compiling
  libbpf twice: one for resolve_btfids, one for kernel/bpf/preload.
- Compiling runqslower individually should compile libbpf just once. Same
  thing for bpftool, resolve_btfids, and kernel/bpf/preload/iterators.

(Not accounting for the boostrap version of libbpf required by bpftool,
which was already placed under a dedicated .../boostrap/libbpf/ directory,
and for which the count remains unchanged.)

A few commits in the series also contain drive-by clean-up changes for
bpftool includes, samples/bpf/.gitignore, or test_bpftool_build.sh. Please
refer to individual commit logs for details.

v3:

Please see few problems with libbpf_hdrs phony targets. Seems like
they all can be order-only dependencies and not causing unnecessary
rebuilds.
Can you please also normalize your patch prefixes for bpftool and
other tools? We've been using a short and simple "bpftool: " prefix
for bpftool-related changes, and for other tools it would be just
"tools/runqslower" or "tools/resolve_btfids". Please update
accordingly. Thanks!

  - Remove order-only dependencies on $(LIBBPF_INCLUDE) (or equivalent)
    directories, given that they are created by libbpf's Makefile.
  - Add libbpf as a dependency for bpftool/resolve_btfids/runqslower when
    they are supposed to reuse a libbpf compiled previously. This is to
    avoid having several libbpf versions being compiled simultaneously in
    the same directory with parallel builds. Even if this didn't show up
    during tests, let's remain on the safe side.
  - kernel/bpf/preload/Makefile: Rename libbpf-hdrs (dash) dependency as
    libbpf_hdrs.
  - samples/bpf/.gitignore: Add bpftool/
  - samples/bpf/Makefile: Change "/bin/rm -rf" to "$(RM) -r".
  - samples/bpf/Makefile: Add missing slashes for $(LIBBPF_OUTPUT) and
    $(LIBBPF_DESTDIR) when buildling bpftool
  - samples/bpf/Makefile: Add a dependency to libbpf's headers for
    $(TRACE_HELPERS).
  - bpftool's Makefile: Use $(LIBBPF) instead of equivalent (but longer)
    $(LIBBPF_OUTPUT)libbpf.a
  - BPF iterators' Makefile: build bpftool in .output/bpftool (instead of
    .output/), add and clean up variables.
  - runqslower's Makefile: Add an explicit dependency on libbpf's headers
    to several objects. The dependency is not required (libbpf should have
    been compiled and so the headers exported through other dependencies
    for those targets), but they better mark the logical dependency and
    should help if exporting the headers changed in the future.
  - New commit to add an "install-bin" target to bpftool, to avoid
    installing bash completion when buildling BPF iterators and selftests.

v2: Declare an additional dependency on libbpf's headers for
    iterators/iterators.o in kernel/preload/Makefile to make sure that
    these headers are exported before we compile the object file (and not
    just before we link it).

Quentin Monnet (10):
  tools: bpftool: remove unused includes to <bpf/bpf_gen_internal.h>
  tools: bpftool: install libbpf headers instead of including the dir
  tools: resolve_btfids: install libbpf headers when building
  tools: runqslower: install libbpf headers when building
  bpf: preload: install libbpf headers when building
  bpf: iterators: install libbpf headers when building
  samples/bpf: install libbpf headers when building
  samples/bpf: update .gitignore
  selftests/bpf: better clean up for runqslower in test_bpftool_build.sh
  tools: bpftool: add install-bin target to install binary only

 kernel/bpf/preload/Makefile                   | 25 ++++++++---
 kernel/bpf/preload/iterators/Makefile         | 39 ++++++++++++------
 samples/bpf/.gitignore                        |  4 ++
 samples/bpf/Makefile                          | 41 ++++++++++++++-----
 tools/bpf/bpftool/Makefile                    | 32 +++++++++------
 tools/bpf/bpftool/gen.c                       |  1 -
 tools/bpf/bpftool/prog.c                      |  1 -
 tools/bpf/resolve_btfids/Makefile             | 17 +++++---
 tools/bpf/resolve_btfids/main.c               |  4 +-
 tools/bpf/runqslower/Makefile                 | 22 ++++++----
 tools/testing/selftests/bpf/Makefile          | 26 ++++++++----
 .../selftests/bpf/test_bpftool_build.sh       |  4 ++
 12 files changed, 148 insertions(+), 68 deletions(-)

--
2.30.2