bpf: enable -nostdinc and other stricter compilation options

We want to have our code as close to usual kernel conventions as
possible to ease review, so add a few more switches for clang
where we emit useful warnings in our code if it's not the case,
that is, -std=gnu89 and -Wdeclaration-after-statement.

We also use quite a number of shadow declarations where they are
really useless; I'm not sure whether clang ends up allocating new
stack space for them; in any case lets add -Wshadow given the
current locations are fixed now.

Also, given our headers are all self-contained, add -nostdinc to
fully /guarantee/ that clang does not search the standard include
paths anymore.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

bpf/Makefile.bpf

@@ -3,11 +3,14 @@
 
 FLAGS := -I$(ROOT_DIR)/bpf/include -I$(ROOT_DIR)/bpf -D__NR_CPUS__=$(shell nproc) -O2 -g
 
-CLANG_FLAGS := ${FLAGS} -target bpf -emit-llvm
+CLANG_FLAGS := ${FLAGS} -target bpf -std=gnu89 -nostdinc -emit-llvm
 # eBPF verifier enforces unaligned access checks where necessary, so don't
 # let clang complain too early.
-CLANG_FLAGS += -Wall -Wextra -Werror
-CLANG_FLAGS += -Wno-address-of-packed-member -Wno-unknown-warning-option -Wno-gnu-variable-sized-type-not-at-end
+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 += -Wdeclaration-after-statement
 LLC_FLAGS   := -march=bpf -mcpu=probe -mattr=dwarfris
 
 # BUILD_PERMUTATIONS_DEP is a dummy file dependency that ensures all targets

bpf/init.sh

@@ -268,11 +268,12 @@ function bpf_compile()
 	TYPE=$3
 	EXTRA_OPTS=$4
 
-	clang -O2 -target bpf -emit-llvm				\
-	      -Wall -Wextra -Werror					\
+	clang -O2 -target bpf -std=gnu89 -nostdinc -emit-llvm		\
+	      -Wall -Wextra -Werror -Wshadow				\
 	      -Wno-address-of-packed-member				\
 	      -Wno-unknown-warning-option				\
 	      -Wno-gnu-variable-sized-type-not-at-end			\
+	      -Wdeclaration-after-statement				\
 	      -I. -I$DIR -I$LIB -I$LIB/include				\
 	      -D__NR_CPUS__=$(nproc)					\
 	      -DENABLE_ARP_RESPONDER=1					\

pkg/datapath/loader/compile.go

@@ -82,12 +82,13 @@ type directoryInfo struct {
 }
 
 var (
-	standardCFlags = []string{"-O2", "-target", "bpf",
-		fmt.Sprintf("-D__NR_CPUS__=%d", runtime.NumCPU()),
-		"-Wall", "-Wextra", "-Werror",
+	standardCFlags = []string{"-O2", "-target", "bpf", "-std=gnu89",
+		"-nostdinc", fmt.Sprintf("-D__NR_CPUS__=%d", runtime.NumCPU()),
+		"-Wall", "-Wextra", "-Werror", "-Wshadow",
 		"-Wno-address-of-packed-member",
 		"-Wno-unknown-warning-option",
-		"-Wno-gnu-variable-sized-type-not-at-end"}
+		"-Wno-gnu-variable-sized-type-not-at-end",
+		"-Wdeclaration-after-statement"}
 	standardLDFlags = []string{"-march=bpf"}
 
 	// testIncludes allows the unit tests to inject additional include

test/bpf/Makefile

@@ -4,9 +4,13 @@
 include ../../Makefile.defs
 
 FLAGS := -I../../bpf/ -I../../bpf/include -I. -D__NR_CPUS__=$(shell nproc) -O2
-FLAGS_CLANG := -Wno-gnu-variable-sized-type-not-at-end
+FLAGS_CLANG := -Wall -Wextra -Werror -Wshadow -Wno-unused-parameter
+FLAGS_CLANG += -Wno-address-of-packed-member
+FLAGS_CLANG += -Wno-unknown-warning-option
+FLAGS_CLANG += -Wno-gnu-variable-sized-type-not-at-end
+FLAGS_CLANG += -Wdeclaration-after-statement
 
-BPF_CC_FLAGS := ${FLAGS} -target bpf -emit-llvm
+BPF_CC_FLAGS := ${FLAGS} -target bpf -std=gnu89 -nostdinc -emit-llvm
 BPF_LLC_FLAGS := -march=bpf -mcpu=probe -filetype=obj
 
 LIB := $(shell find ../../bpf/ -name '*.h')