[Buildroot] [RFC PATCH v1 0/1] libbpf: Remove bpf.h in linux.mk

[Buildroot] [RFC PATCH v1 0/1] libbpf: Remove bpf.h in linux.mk

[Francis Laniel]

Hi everyone.


I hope you are fine and the same for your relatives.

The goal of this contribution is to fix this a recurrent building problem which
happens in buildroot CI (sorry for only taking care of this now...) [1].
Actually, to build host-pahole, we install host-libbpf uapi headers to the host
but we remove bpf.h to avoid a conflict with this file while building the
kernel [2].
Then host-pahole will use the system uapi headers to be built.
The problem with this approach is that it works while the system uapi headers
permits building host-pahole which is not the case within buildroot CI.

So, a solution to this problem would be to keep bpf.h when installing uapi
headers and to remove it right before building the kernel to avoid the conflict.
As a consequence, we are able to build host-pahole with a good bpf.h and we can
then build the kernel.

I sadly know this approach is far from being perfect but this is the only way I
see to fix this build problem... So, if someone has a better idea I am up to
implement it!

Francis Laniel (1):
  package/libbpf: remove bpf.h in linux.mk

 linux/linux.mk           | 23 +++++++++++++++++++++++
 package/libbpf/libbpf.mk | 21 ++++-----------------
 2 files changed, 27 insertions(+), 17 deletions(-)


Best regards and thank you in advance.
---
[1] http://autobuild.buildroot.net/results/7b3f560a773791e00b8bb32f64c0d39749b809b1/build-end.log
[2] https://github.com/buildroot/buildroot/blob/f3f441aea23dbf84fec85a9cef1aaf276c4484f0/package/libbpf/libbpf.mk#L61
-- 
2.25.1

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[Buildroot] [RFC PATCH v1 1/1] package/libbpf: remove bpf.h in linux.mk

[Francis Laniel]

Removing bpf.h in host-libbpf leads to problems when building host-pahole [1].
So, we keep this file to build host-pahole and remove it before building the
kernel to avoid conflicts with kernel bpf.h.

Signed-off-by: Francis Laniel <flaniel@linux.microsoft.com>

[1] http://autobuild.buildroot.net/results/7b3f560a773791e00b8bb32f64c0d39749b809b1/build-end.log
---
 linux/linux.mk           | 23 +++++++++++++++++++++++
 package/libbpf/libbpf.mk | 21 ++++-----------------
 2 files changed, 27 insertions(+), 17 deletions(-)

diff --git a/linux/linux.mk b/linux/linux.mk
index 322ccabbd9..85a38fd3ee 100644
--- a/linux/linux.mk
+++ b/linux/linux.mk
@@ -122,6 +122,29 @@ endif
 
 ifeq ($(BR2_LINUX_KERNEL_NEEDS_HOST_PAHOLE),y)
 LINUX_DEPENDENCIES += host-pahole
+
+# To build host-pahole, we need to install host-libbpf.
+# Particularly, we need to use libbpf Makefile target install_uapi_headers to
+# install a particular header file.
+# But, this generates a conflict when building the kernel:
+# In file included from libbpf_internal.h:17:0, from strset.c:9:
+# relo_core.h:10:6: error: nested redefinition of 'enum bpf_core_relo_kind'
+# enum bpf_core_relo_kind {
+# ^~~~~~~~~~~~~~~~~~
+# relo_core.h:10:6: error: redeclaration of 'enum bpf_core_relo_kind'
+# In file included from libbpf_legacy.h:13:0,
+# 		from libbpf_internal.h:16,
+# 		from strset.c:9:
+# /home/francis/buildroot/output/host/include/linux/bpf.h:6497:6: note: originally defined here
+# enum bpf_core_relo_kind {
+# So, better to remove it now as we already built host-pahole.
+define LINUX_REMOVE_LIBBPF_HEADER
+	if [ -f $(HOST_DIR)/include/linux/bpf.h ]; then \
+		rm $(HOST_DIR)/include/linux/bpf.h; \
+	fi
+endef
+
+LINUX_PRE_BUILD_HOOKS += LINUX_REMOVE_LIBBPF_HEADER
 else
 define LINUX_FIXUP_CONFIG_PAHOLE_CHECK
 	if grep -q "^CONFIG_DEBUG_INFO_BTF=y" $(KCONFIG_DOT_CONFIG); then \
diff --git a/package/libbpf/libbpf.mk b/package/libbpf/libbpf.mk
index 987d1145f1..6fd69f2536 100644
--- a/package/libbpf/libbpf.mk
+++ b/package/libbpf/libbpf.mk
@@ -39,26 +39,13 @@ define LIBBPF_INSTALL_TARGET_CMDS
 		-C $(@D)/src install DESTDIR=$(TARGET_DIR)
 endef
 
-# We need to install_uapi_headers so we have btf.h to compile
-# host-pahole.
-# Nonetheless, this target adds bpf.h which generates a conflict when
-# building the kernel:
-# In file included from libbpf_internal.h:17:0, from strset.c:9:
-# relo_core.h:10:6: error: nested redefinition of 'enum bpf_core_relo_kind'
-# enum bpf_core_relo_kind {
-# ^~~~~~~~~~~~~~~~~~
-# relo_core.h:10:6: error: redeclaration of 'enum bpf_core_relo_kind'
-# In file included from libbpf_legacy.h:13:0,
-# 		from libbpf_internal.h:16,
-# 		from strset.c:9:
-# /home/francis/buildroot/output/host/include/linux/bpf.h:6497:6: note: originally defined here
-# enum bpf_core_relo_kind {
-# So, better to remove remove it now since we do not need it to build
-# host-pahole, the only user of host-libbpf.
+# We need to install_uapi_headers so we have btf.h to compile host-pahole.
+# Nonetheless, this target adds bpf.h which generates a conflict when building
+# the kernel.
+# So, if this file is present, it will be removed by linux.mk.
 define HOST_LIBBPF_INSTALL_CMDS
 	$(HOST_MAKE_ENV) $(HOST_CONFIGURE_OPTS) $(MAKE) \
 		-C $(@D)/src install install_uapi_headers DESTDIR=$(HOST_DIR)
-	rm $(HOST_DIR)/include/linux/bpf.h
 endef
 
 $(eval $(generic-package))
-- 
2.25.1

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