bpf2go,examples: remove deprecated +build tags from generated Go sources

To follow suit with the new //go:build convention of build tags, this commit
converts bpf2go to no longer emit the deprecated +build lines.

The bpf2go command line interface remains unchanged to avoid breaking users.
In the future we can introduce a new -constraints flag and deprecate -tags.

Signed-off-by: Timo Beckers <timo@isovalent.com>
Co-developed-by: Lorenz Bauer <oss@lmb.io>

cmd/bpf2go/flags.go

@@ -0,0 +1,57 @@
+package main
+
+import (
+	"flag"
+	"go/build/constraint"
+)
+
+// buildTags is a comma-separated list of build tags.
+//
+// This follows the pre-Go 1.17 syntax and is kept for compatibility reasons.
+type buildTags struct {
+	Expr constraint.Expr
+}
+
+var _ flag.Value = (*buildTags)(nil)
+
+func (bt *buildTags) String() string {
+	if bt.Expr == nil {
+		return ""
+	}
+
+	return (bt.Expr).String()
+}
+
+func (bt *buildTags) Set(value string) error {
+	ct, err := constraint.Parse("// +build " + value)
+	if err != nil {
+		return err
+	}
+
+	bt.Expr = ct
+	return nil
+}
+
+func andConstraints(x, y constraint.Expr) constraint.Expr {
+	if x == nil {
+		return y
+	}
+
+	if y == nil {
+		return x
+	}
+
+	return &constraint.AndExpr{X: x, Y: y}
+}
+
+func orConstraints(x, y constraint.Expr) constraint.Expr {
+	if x == nil {
+		return y
+	}
+
+	if y == nil {
+		return x
+	}
+
+	return &constraint.OrExpr{X: x, Y: y}
+}

cmd/bpf2go/main.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"go/build/constraint"
 	"go/token"
 	"io"
 	"os"
@@ -76,7 +77,7 @@ func run(stdout io.Writer, pkg, outputDir string, args []string) (err error) {
 	fs.StringVar(&b2g.strip, "strip", "", "`binary` used to strip DWARF from compiled BPF (default \"llvm-strip\")")
 	fs.BoolVar(&b2g.disableStripping, "no-strip", false, "disable stripping of DWARF")
 	flagCFlags := fs.String("cflags", "", "flags passed to the compiler, may contain quoted arguments")
-	fs.StringVar(&b2g.tags, "tags", "", "list of Go build tags to include in generated files")
+	fs.Var(&b2g.tags, "tags", "Comma-separated list of Go build tags to include in generated files")
 	flagTarget := fs.String("target", "bpfel,bpfeb", "clang target(s) to compile for (comma separated)")
 	fs.StringVar(&b2g.makeBase, "makebase", "", "write make compatible depinfo files relative to `directory`")
 	fs.Var(&b2g.cTypes, "type", "`Name` of a type to generate a Go declaration for, may be repeated")
@@ -157,10 +158,6 @@ func run(stdout io.Writer, pkg, outputDir string, args []string) (err error) {
 		return fmt.Errorf("-output-stem %q must not contain path separation characters", b2g.outputStem)
 	}
 
-	if strings.ContainsRune(b2g.tags, '\n') {
-		return fmt.Errorf("-tags mustn't contain new line characters")
-	}
-
 	targetArches := strings.Split(*flagTarget, ",")
 	if len(targetArches) == 0 {
 		return fmt.Errorf("no targets specified")
@@ -261,8 +258,8 @@ type bpf2go struct {
 	skipGlobalTypes bool
 	// C types to include in the generatd output.
 	cTypes cTypes
-	// Go tags included in the .go
-	tags string
+	// Build tags to be included in the output.
+	tags buildTags
 	// Base directory of the Makefile. Enables outputting make-style dependencies
 	// in .d files.
 	makeBase string
@@ -292,14 +289,14 @@ func (b2g *bpf2go) convert(tgt target, arches []string) (err error) {
 		return err
 	}
 
-	var tags []string
-	if len(arches) > 0 {
-		tags = append(tags, strings.Join(arches, " "))
-	}
-	if b2g.tags != "" {
-		tags = append(tags, b2g.tags)
+	var archConstraint constraint.Expr
+	for _, arch := range arches {
+		tag := &constraint.TagExpr{Tag: arch}
+		archConstraint = orConstraints(archConstraint, tag)
 	}
 
+	constraints := andConstraints(archConstraint, b2g.tags.Expr)
+
 	cFlags := make([]string, len(b2g.cFlags))
 	copy(cFlags, b2g.cFlags)
 	if tgt.linux != "" {
@@ -342,7 +339,7 @@ func (b2g *bpf2go) convert(tgt target, arches []string) (err error) {
 		ident:           b2g.ident,
 		cTypes:          b2g.cTypes,
 		skipGlobalTypes: b2g.skipGlobalTypes,
-		tags:            tags,
+		constraints:     constraints,
 		obj:             objFileName,
 		out:             goFile,
 	})

cmd/bpf2go/output.go

@@ -3,6 +3,7 @@ package main
 import (
 	"bytes"
 	"fmt"
+	"go/build/constraint"
 	"go/token"
 	"io"
 	"os"
@@ -19,9 +20,7 @@ import (
 const ebpfModule = "github.com/cilium/ebpf"
 
 const commonRaw = `// Code generated by bpf2go; DO NOT EDIT.
-{{- range .Tags }}
-// +build {{ . }}
-{{- end }}
+{{ with .Constraints }}//go:build {{ . }}{{ end }}
 
 package {{ .Package }}
 
@@ -161,7 +160,7 @@ var {{ .Name.Bytes }} []byte
 `
 
 var (
-	tplFuncs = map[string]interface{}{
+	tplFuncs = template.FuncMap{
 		"tag": tag,
 	}
 	commonTemplate = template.Must(template.New("common").Funcs(tplFuncs).Parse(commonRaw))
@@ -220,7 +219,7 @@ func (n templateName) CloseHelper() string {
 type outputArgs struct {
 	pkg             string
 	ident           string
-	tags            []string
+	constraints     constraint.Expr
 	cTypes          []string
 	skipGlobalTypes bool
 	obj             string
@@ -297,20 +296,20 @@ func output(args outputArgs) error {
 
 	ctx := struct {
 		*btf.GoFormatter
-		Module    string
-		Package   string
-		Tags      []string
-		Name      templateName
-		Maps      map[string]string
-		Programs  map[string]string
-		Types     []btf.Type
-		TypeNames map[btf.Type]string
-		File      string
+		Module      string
+		Package     string
+		Constraints constraint.Expr
+		Name        templateName
+		Maps        map[string]string
+		Programs    map[string]string
+		Types       []btf.Type
+		TypeNames   map[btf.Type]string
+		File        string
 	}{
 		gf,
 		ebpfModule,
 		args.pkg,
-		args.tags,
+		args.constraints,
 		templateName(args.ident),
 		maps,
 		programs,

examples/tcprtt_sockops/main.go

@@ -1,5 +1,4 @@
 //go:build linux
-// +build linux
 
 // This program demonstrates attaching an eBPF program to
 // a cgroupv2 path and using sockops to process TCP socket events.