Merge branch 'install libbpf headers when using the library'
Quentin Monnet says:
====================
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.
v4:
- Make the "libbpf_hdrs" dependency an order-only dependency in
kernel/bpf/preload/Makefile, samples/bpf/Makefile, and
tools/bpf/runqslower/Makefile. This is to avoid to unconditionally
recompile the targets.
- samples/bpf/.gitignore: prefix objects with a "/" to mark that we
ignore them when at the root of the samples/bpf/ directory.
- libbpf: add a commit to make "install_headers" depend on the header
files, to avoid exporting again if the sources are older than the
targets. This ensures that applications relying on those headers are
not rebuilt unnecessarily.
- bpftool: uncouple the copy of nlattr.h from libbpf target, to have it
depend on the source header itself. By avoiding to reinstall this
header every time, we avoid unnecessary builds of bpftool.
- samples/bpf: Add a new commit to remove the FORCE dependency for
libbpf, and replace it with a "$(wildcard ...)" on the .c/.h files in
libbpf's directory. This is to avoid always recompiling libbpf/bpftool.
- Adjust prefixes in commit subjects.
v3:
- 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).
====================
Signed-off-by: 
Andrii Nakryiko <andrii@kernel.org>
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