package/bpftrace: fix build with per-package directories

Buildroot commit f7b8508e4f in 2021.08-rc1
added the libbpf package which contains the usage of the
'install_uapi_headers' make target to update toolchain headers older
than kernel version 5.9.  This does not work with per-package
directories.

Building this defconfig that includes all packages depending on libbpf:

  BR2_x86_64=y
  BR2_x86_rocketlake=y
  BR2_KERNEL_HEADERS_VERSION=y
  BR2_DEFAULT_KERNEL_VERSION="5.8.18"
  BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_5_8=y
  BR2_TOOLCHAIN_BUILDROOT_CXX=y
  BR2_PER_PACKAGE_DIRECTORIES=y
  BR2_PACKAGE_BPFTRACE=y
  BR2_PACKAGE_DPDK=y
  BR2_PACKAGE_IPROUTE2=y
  BR2_PACKAGE_HOST_PAHOLE=y

on the 2026.02.x branch as of 0be927b89a
is broken:

output/build/bpftrace-0.24.2/src/attached_probe.cpp: In static member
 function 'static bpftrace::Result<std::unique_ptr<bpftrace::AttachedIterProbe> >
 bpftrace::AttachedIterProbe::make(bpftrace::Probe&,
 const bpftrace::BpfProgram&, std::optional<int>)':
output/build/bpftrace-0.24.2/src/attached_probe.cpp:1435:30: error:
 aggregate 'bpf_iter_link_info linfo' has incomplete type and cannot be
 defined
 1435 |     union bpf_iter_link_info linfo;

libbpf overwrites kernel header files installed in $STAGING_DIR when
they are too old by adding 'install_uapi_headers' as make target. In
the case of per-package directories these header files are stored only
in output/per-package/libbpf/.

When output/per-package/bpftrace/ is populated, the content of
output/per-package/ from other packages overwrites the header files
installed by libbpf because the list of rsync'ed packages is sorted
alphabetically:
https://gitlab.com/buildroot.org/buildroot/-/blob/2026.02.1/package/pkg-generic.mk#L796

To prevent this problem let libbpf install the updated kernel headers in
a new directory.

Other packages need to find this directory. This mostly happens through
pkg-config, but bpftrace doesn't use that:
https://github.com/bpftrace/bpftrace/blob/v0.25.1/cmake/FindLibBpf.cmake#L9
Therefore, use a configure option in bpftrace to set the correct path.

Please note that libbpf will add '/linux' to the path provided in
$UAPIDIR:
https://github.com/libbpf/libbpf/blob/v1.7.0/src/Makefile#L169
so the headers end up in the previously non-existing directory
$(STAGING_DIR)/usr/include/bpf/linux/:

$ find output/per-package/libbpf/host/x86_64-buildroot-linux-gnu/sysroot/ -iname bpf.h -exec ls -lao {} \;
-rw-r--r-- 1 bernd 26434 25. Apr 13:15 output/per-package/libbpf/host/x86_64-buildroot-linux-gnu/sysroot/usr/include/bpf/bpf.h
-rw-r--r-- 1 bernd 283012 25. Apr 13:15 output/per-package/libbpf/host/x86_64-buildroot-linux-gnu/sysroot/usr/include/bpf/linux/bpf.h
-rw-r--r-- 12 bernd 160166 25. Apr 13:08 output/per-package/libbpf/host/x86_64-buildroot-linux-gnu/sysroot/usr/include/linux/bpf.h

bpf.h with 283012 bytes is the update-to-date version needed for bpftrace:

$ grep -m1 bpf_iter_link_info output/per-package/libbpf/host/x86_64-buildroot-linux-gnu/sysroot/usr/include/bpf/linux/bpf.h
union bpf_iter_link_info {
$

$ grep -m1 bpf_iter_link_info output/per-package/libbpf/host/x86_64-buildroot-linux-gnu/sysroot/usr/include/linux/bpf.h
$

By adding '-I$(STAGING_DIR)/usr/include/bpf' to the CXXFLAGS of bpftrace
its include finds the correct header:
https://github.com/bpftrace/bpftrace/blob/v0.25.1/src/bpfprogram.cpp#L3

  #include <linux/bpf.h>

The dpdk package uses pkg-config` to detect libbpf and will use the updated
.pc file:
https://github.com/DPDK/dpdk/blob/v25.11/drivers/net/tap/bpf/meson.build#L7

The iproute2 package is not affected by the problem because it contains
"matching sanitized kernel headers":
https://git.kernel.org/pub/scm/network/iproute2/iproute2.git/tree/README?h=v6.17.0#n44

As a host package pahole is also not affected.

Fixes:
https://autobuild.buildroot.net/results/d8b/d8b416e40bf7bd019cd93e6f2d42c98fc12d4160/

Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
Signed-off-by: Arnout Vandecappelle <arnout@rnout.be>

package/bpftrace/bpftrace.mk

@@ -28,6 +28,7 @@ endif
 BPFTRACE_CONF_OPTS += \
 	-DBUILD_SHARED_LIBS:BOOL=OFF \
 	-DBUILD_TESTING:BOOL=OFF \
+	-DCMAKE_CXX_FLAGS="$(TARGET_CXXFLAGS) -I$(STAGING_DIR)/usr/include/bpf" \
 	-DENABLE_MAN:BOOL=OFF
 
 $(eval $(cmake-package))

package/libbpf/libbpf.mk

@@ -25,7 +25,12 @@ endef
 # toolchain.
 # https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a5cbe05a6673b85bed2a63ffcfea6a96c6410cff
 ifeq ($(BR2_TOOLCHAIN_HEADERS_AT_LEAST_5_9),)
-LIBBPF_UPDATE_UAPI_HEADERS = install_uapi_headers
+LIBBPF_UPDATE_UAPI_HEADERS = install_uapi_headers UAPIDIR=/usr/include/bpf
+
+define LIBBPF_FIX_STAGING_PC
+	$(SED) 's/-I$${includedir}/-I$${includedir}\/bpf -I$${includedir}/' $(STAGING_DIR)/usr/lib/pkgconfig/libbpf.pc
+endef
+LIBBPF_POST_INSTALL_STAGING_HOOKS += LIBBPF_FIX_STAGING_PC
 endif
 
 define LIBBPF_INSTALL_STAGING_CMDS