bpf: unify CFLAGS handling in Makefile.clang

This commit unifies CFLAGS deduction into a common Makefile.clang
for both BPF unit tests and main 'all' target (used by lint and
verifier tests).

Signed-off-by: Marc Suñé <marc.sune@isovalent.com>

Author: msune

bpf/.gitignore

@@ -7,6 +7,9 @@ compile_commands.json
 # ignore CLANG intermediate compilation tmp files
 *.o.tmp
 
+# ignore generated CFLAGS
+**.clang_flags.mk
+
 # ignore scapy autgen and python cache
 tests/scapy/__pycache__/*
 tests/output/*

bpf/Makefile

@@ -243,9 +243,9 @@ clean:
 	$(QUIET) $(foreach TARGET,$(SUBDIRS), \
 		$(MAKE) $(SUBMAKEOPTS) -C $(TARGET) clean;)
 	$(QUIET)rm -fr *.o *.ll *.i *.s
+	$(QUIET)rm -f $(CLANG_FLAGS_FILE)
 	$(QUIET)rm -f $(TARGET)
 
-
 BEAR_CLI   = $(shell which bear 2> /dev/null)
 gen_compile_commands:
 ifeq (, $(BEAR_CLI))

bpf/Makefile.bpf

@@ -3,26 +3,13 @@
 
 FLAGS := -I$(ROOT_DIR)/bpf -I$(ROOT_DIR)/bpf/include -O2 -g
 
-CLANG_FLAGS := ${FLAGS} --target=bpf -std=gnu99 -nostdinc
-# eBPF verifier enforces unaligned access checks where necessary, so don't
-# let clang complain too early.
-CLANG_FLAGS += -ftrap-function=__undefined_trap
-CLANG_FLAGS += -Wall -Wextra -Werror -Wshadow
-CLANG_FLAGS += -Wno-address-of-packed-member
-CLANG_FLAGS += -Wno-unknown-warning-option
-CLANG_FLAGS += -Wno-gnu-variable-sized-type-not-at-end
-CLANG_FLAGS += -Wimplicit-int-conversion -Wenum-conversion
-CLANG_FLAGS += -Wimplicit-fallthrough
-CLANG_FLAGS += -mcpu=v3
+# Note, FLAGS needs to be defined before inclusion
+include $(ROOT_DIR)/bpf/Makefile.clang
 
 LIB := $(shell find $(ROOT_DIR)/bpf -name '*.h')
 BPF_C := $(patsubst %.o,%.c,$(BPF))
 BPF_ASM := $(patsubst %.o,%.s,$(BPF))
 
-CLANG      ?= clang
-HOST_CC    ?= gcc
-HOST_STRIP ?= strip
-
 ifeq ($(CROSS_ARCH),arm64)
   HOST_CC = aarch64-linux-gnu-gcc
   HOST_STRIP = aarch64-linux-gnu-strip

bpf/Makefile.clang

@@ -0,0 +1,19 @@
+CLANG      ?= clang
+HOST_CC    ?= gcc
+HOST_STRIP ?= strip
+
+# Base CFLAGS (CLANG_FLAGS) are read from pkg/datapath/loader/compile.go
+# to keep them synced with the loader
+
+CLANG_FLAGS_FILE := $(shell realpath --relative-to=$(CURDIR) $(ROOT_DIR)/bpf/.clang_flags.mk)
+CLANG_CFLAGS_GO  := $(ROOT_DIR)/pkg/datapath/loader/tools/clang_cflags.go
+LOADER_GO_FILES := $(shell find $(ROOT_DIR)/pkg/datapath/loader -type f -name '*.go')
+
+# Rebuild only when go files are modified in the loader
+$(CLANG_FLAGS_FILE): $(LOADER_GO_FILES)
+	$(QUIET) echo 'CLANG_FLAGS := $(FLAGS) '`$(GO) run $(CLANG_CFLAGS_GO)` > $@
+
+-include $(CLANG_FLAGS_FILE)
+
+# Mimics the mcpu values set by cilium-agent. See GetBPFCPU().
+CLANG_FLAGS += -mcpu=v3

bpf/tests/Makefile

@@ -5,20 +5,14 @@ include ../../Makefile.defs
 
 MAKEFLAGS += -r
 
-CLANG ?= clang
-
 FLAGS := -I$(ROOT_DIR)/bpf -I$(ROOT_DIR)/bpf/include -g
 
-# Base CFLAGS (CLANG_FLAGS) are read from pkg/datapath/loader/compile.go
-# to keep them synced with the loader
-CLANG_FLAGS := $(FLAGS) $(shell $(GO) run $(ROOT_DIR)/pkg/datapath/loader/tools/clang_cflags.go)
+# Note, FLAGS needs to be defined before inclusion
+include ../Makefile.clang
 
 # Create dependency files for each .o file.
 CLANG_FLAGS += -MD
 
-# Mimics the mcpu values set by cilium-agent. See GetBPFCPU().
-CLANG_FLAGS += -mcpu=v3
-
 # Autogenerated scapy pkts
 SCAPY_HDR := $(ROOT_DIR)/bpf/tests/output/gen_pkts.h
 SCAPY_HDR_TS := $(ROOT_DIR)/bpf/tests/output/gen_pkts.h.stamp