C: Really force inline, and make it optional

With older kernel versions tail calls and BPF to BPF calls weren't
supported together, so it was common to force clang to inline all
functions.

But we didn't do this forcibly enough, sometimes clang doesn't inline
them. Add the right magic incantation to really force inlining.

More recent kernel versions support mixing tail calls and BPF to BPF
calls, make inlining optional for callers that want to use BPF to BPF
calls.

c.go

@@ -12,7 +12,9 @@ import (
 
 const funcTemplate = `
 // True if packet matches, false otherwise
-static inline
+{{- if not .NoInline}}
+__attribute__((__always_inline__)) static inline
+{{- end}}
 uint32_t {{.Name}}(const uint8_t *const data, const uint8_t *const data_end) {
 	__attribute__((unused))
 	uint32_t a, x, m[16];
@@ -29,8 +31,9 @@ __attribute__((unused));
 }`
 
 type cFunction struct {
-	Name   string
-	Blocks []cBlock
+	Name     string
+	NoInline bool
+	Blocks   []cBlock
 }
 
 // cBPF reg to C symbol
@@ -78,6 +81,11 @@ type COpts struct {
 	// FunctionName is the symbol to use as the generated C function. Must match regex:
 	//     [A-Za-z_][0-9A-Za-z_]*
 	FunctionName string
+
+	// NoInline doesn't force the generated function to be inlined, allowing clang to emit
+	// a BPF to BPF call.
+	// Requires at least kernel 5.10 (for x86, later for other architectures) if used with tail-calls.
+	NoInline bool
 }
 
 // ToC compiles a cBPF filter to a C function with a signature of:

c_example_test.go

@@ -75,7 +75,7 @@ func ExampleToC() {
 		bpf.RetConstant{Val: 1},
 	}
 
-	elf, err := buildC(filter, "example")
+	elf, err := buildC(filter, "example", COpts{FunctionName: "example_filter"})
 	if err != nil {
 		panic(err)
 	}
@@ -89,13 +89,9 @@ func ExampleToC() {
 // and compiles the resulting C program to eBPF / XDP using clang.
 // The XDP program XDP_DROP's incoming packets that match the filter.
 // Returns the compiled ELF
-func buildC(filter []bpf.Instruction, programName string) ([]byte, error) {
-	filterName := programName + "_filter"
-
+func buildC(filter []bpf.Instruction, programName string, opts COpts) ([]byte, error) {
 	// convert filter to C
-	ebpfFilter, err := ToC(filter, COpts{
-		FunctionName: filterName,
-	})
+	ebpfFilter, err := ToC(filter, opts)
 	if err != nil {
 		return nil, errors.Wrap(err, "converting filter to C")
 	}
@@ -104,7 +100,7 @@ func buildC(filter []bpf.Instruction, programName string) ([]byte, error) {
 	c := bytes.Buffer{}
 	err = testTemplate.Execute(&c, testTemplateOpts{
 		Filter:      ebpfFilter,
-		FilterName:  filterName,
+		FilterName:  opts.FunctionName,
 		ProgramName: programName,
 	})
 	if err != nil {

c_test.go

@@ -30,11 +30,32 @@ func TestFunctionName(t *testing.T) {
 	checkName(t, "a2", true)
 }
 
+func TestNoInline(t *testing.T) {
+	elf, err := buildC([]bpf.Instruction{
+		bpf.RetConstant{Val: 1},
+	}, entryPoint, COpts{
+		FunctionName: "filter",
+		NoInline:     true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	spec, err := ebpf.LoadCollectionSpecFromReader(bytes.NewReader(elf))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if res := testProg(t, spec.Programs[entryPoint], []byte{1}); res != match {
+		t.Fatalf("expected match, got %v", res)
+	}
+}
+
 const entryPoint = "xdp_filter"
 
 // cBackend compiles classic BPF to C, which is compiled with clang
 func cBackend(tb testing.TB, insns []bpf.Instruction, in []byte) result {
-	elf, err := buildC(insns, entryPoint)
+	elf, err := buildC(insns, entryPoint, COpts{FunctionName: "filter"})
 	if err != nil {
 		tb.Fatal(err)
 	}