add bpf2go support and wrappers for ebpf.Variable generation

This commit introduces support and wrappers around ebpf.Variable.
An ebpf.Variable does not define per-se wrappers method to automatically
infer the underlying data type and handle typed data in Go. In fact, it
relies on parameters passed to the `Variable.Get` and `Variable.Set` APIs.
This commit allows `bpf2go` to emit `VariableSpec` and `Variable` in the
generated code. In addition, a new set of wrapper methods are created
to provide support for typed method around variables. To do so, this
commit enables emitting `Dataspec`, which so far was not emitted. However,
it modifies the current implementation so that for each variable in `Dataspec`
the needed support is provided.
Supposing to have an ebpf program that defines the following two global variables:

```c
__u64 pkt_count			= 0;
const __u32 ro_variable	= 1;
char var_msg[]			= "Hello World!";
```

The steps performed during the Go code generation are as follows:

1. An alias with the variable name is generated in the Go file:

```go
type PktCountType 	= uint64
type RoVariable 	= uint32
type VarMsgType 	= [16]int8
```

2. A new type definition (non-alias, need new methods) for the variables is performed:

```go
type PktCount	ebpf.Variable
type RoVariable	ebpf.Variable
type VarMsg		ebpf.Variable
```

3. The `.Get` methods for all types are generated:

```go
func (v *PktCount) Get() (PktCountType, error) {
	var ret PktCountType
	return ret, (*ebpf.Variable)(v).Get(&ret)
}

func (v *RoVariable) Get() (RoVariableType, error) {
	var ret RoVariableType
	return ret, (*ebpf.Variable)(v).Get(&ret)
}

func (v *VarMsg) Get() (VarMsgType, error) {
	var ret VarMsgType
	return ret, (*ebpf.Variable)(v).Get(&ret)
}
```

4. The `.Set` methods for the non read-only variables are generated:

```go
func (v *PktCount) Set(val PktCountType) error {
	return (*ebpf.Variable)(v).Set(val)
}

func (v *VarMsg) set(val VarMsgType) error {
	return (*ebpf.Variable)(v).Set(ret)
}
```

4. In case the type alias is supported by the atomic package, and then
    also supported by `ebpf.Variable` (int32, uint32, int64, uint64), then
    an additional `AtomicRef` method is created for the non read-only
	variables to get an reference to the underlying data.

```go
func (v *PktCount) AtomicRef() (*ebpf.Uint64, error) {
	ret, _ := (*ebpf.Variable)(v).AtomicUint64()
	return ret
}
```

From the user perspective, ignoring the error catching, the following
operations can be performed:

```go
...
objs := bpfObjects{}
err := loadBpfObjects(&objs, nil)
...

// ref kept and can be used multiple times
aPktCount := objs.PktCount.AtomicRef()
err := aPktCount.Load()
aPktCount.Store(rand.Uint64())
...

vRoVariable, _ := objs.RoVariable.Get()
...

varMsg, _ := objs.VarMsg.Get()
varMsg[0] = 'B'
err := objs.VarMsg.Set(varMsg)
```

Signed-off-by: Simone Magnani <[email protected]>

Author: smagnani96

Diff for: btf/format.go

@@ -4,10 +4,45 @@ import (
 	"errors"
 	"fmt"
 	"strings"
+	"text/template"
+
+	"golang.org/x/text/cases"
+	"golang.org/x/text/language"
 )
 
 var errNestedTooDeep = errors.New("nested too deep")
 
+var tplVars = `
+
+type {{ .Name }} ebpf.Variable
+
+func(v *{{ .Name }}) Get() ({{ .Type }}, error) {
+	var ret  {{ .Type }}
+	return ret, (*ebpf.Variable)(v).Get(&ret)
+}
+
+{{- if not .ReadOnly }}
+func(v *{{ .Name }}) Set(val {{ .Type }}) error {
+	return (*ebpf.Variable)(v).Set(val)
+}
+
+{{ if .CanAtomic }}
+func(v *{{ .Name }}) AtomicRef() *ebpf.{{ .CapitalizedType }} {
+	ret, _ := (*ebpf.Variable)(v).Atomic{{ .CapitalizedType }}()
+	return ret
+}
+{{ end }}
+{{- end }}
+`
+
+type TplVarsData struct {
+	Name            string
+	Type            string
+	CapitalizedType string
+	ReadOnly        bool
+	CanAtomic       bool
+}
+
 // GoFormatter converts a Type to Go syntax.
 //
 // A zero GoFormatter is valid to use.
@@ -64,7 +99,12 @@ func (gf *GoFormatter) writeTypeDecl(name string, typ Type) error {
 	}
 
 	typ = skipQualifiers(typ)
-	fmt.Fprintf(&gf.w, "type %s ", name)
+	// custom handling Datasec types in writeDatasecLit
+	_, ok := typ.(*Datasec)
+	if !ok {
+		fmt.Fprintf(&gf.w, "type %s ", name)
+	}
+
 	if err := gf.writeTypeLit(typ, 0); err != nil {
 		return err
 	}
@@ -295,41 +335,82 @@ func (gf *GoFormatter) writeStructField(m Member, depth int) error {
 }
 
 func (gf *GoFormatter) writeDatasecLit(ds *Datasec, depth int) error {
-	gf.w.WriteString("struct { ")
+	tmpl, err := template.New("varsHelpers").Parse(tplVars)
+	if err != nil {
+		return fmt.Errorf("failed to parse template: %w", err)
+	}
 
-	prevOffset := uint32(0)
 	for i, vsi := range ds.Vars {
 		v, ok := vsi.Type.(*Var)
 		if !ok {
 			return fmt.Errorf("can't format %s as part of data section", vsi.Type)
 		}
 
-		if v.Linkage != GlobalVar {
-			// Ignore static, extern, etc. for now.
-			continue
-		}
-
-		if v.Name == "" {
-			return fmt.Errorf("variable %d: empty name", i)
+		id := gf.identifier(v.Name)
+		va := getSuppAtomicType(v.Type)
+		tplArgs := TplVarsData{
+			Name:            id,
+			Type:            id + "Type",
+			CapitalizedType: cases.Title(language.Und, cases.NoLower).String(va),
+			ReadOnly:        strings.HasPrefix(ds.Name, ".ro"),
+			CanAtomic:       va != "",
 		}
 
-		gf.writePadding(vsi.Offset - prevOffset)
-		prevOffset = vsi.Offset + vsi.Size
-
-		fmt.Fprintf(&gf.w, "%s ", gf.identifier(v.Name))
+		fmt.Fprintf(&gf.w, "type %s =", tplArgs.Type)
 
 		if err := gf.writeType(v.Type, depth); err != nil {
 			return fmt.Errorf("variable %d: %w", i, err)
 		}
 
-		gf.w.WriteString("; ")
+		if err := tmpl.Execute(&gf.w, tplArgs); err != nil {
+			return fmt.Errorf("failed to execute template for variable %s: %w", tplArgs.Name, err)
+		}
 	}
 
-	gf.writePadding(ds.Size - prevOffset)
-	gf.w.WriteString("}")
 	return nil
 }
 
+// getSuppAtomicType returns the corresponding Go type for the
+// provided argument if it supports package atomic primitives.
+// Current support for int32, uint32, int64, uint64.
+func getSuppAtomicType(t Type) string {
+	checkInt := func(t *Int) string {
+		ret := ""
+		switch t.Size {
+		// uint32/int32 and uint64/int64
+		case 4:
+			ret = "int32"
+		case 8:
+			ret = "int64"
+		default:
+			return ""
+		}
+		if t.Encoding == Unsigned {
+			ret = "u" + ret
+		}
+		return ret
+	}
+
+	switch v := skipQualifiers(t).(type) {
+	case *Int:
+		return checkInt(v)
+	case *Typedef:
+		if vv, ok := v.Type.(*Int); ok {
+			return checkInt(vv)
+		}
+	case *Enum:
+		i := &Int{
+			Name:     v.Name,
+			Size:     v.Size,
+			Encoding: Unsigned,
+		}
+		if v.Signed {
+			i.Encoding = Signed
+		}
+		return checkInt(i)
+	}
+	return ""
+}
 func (gf *GoFormatter) writePadding(bytes uint32) {
 	if bytes > 0 {
 		fmt.Fprintf(&gf.w, "_ [%d]byte; ", bytes)

Diff for: cmd/bpf2go/gen/output.go

@@ -49,6 +49,10 @@ func (n templateName) MapSpecs() string {
 	return string(n) + "MapSpecs"
 }
 
+func (n templateName) VariableSpecs() string {
+	return string(n) + "VariableSpecs"
+}
+
 func (n templateName) Load() string {
 	return n.maybeExport("load" + toUpperFirst(string(n)))
 }
@@ -65,6 +69,10 @@ func (n templateName) Maps() string {
 	return string(n) + "Maps"
 }
 
+func (n templateName) Variables() string {
+	return string(n) + "Variables"
+}
+
 func (n templateName) Programs() string {
 	return string(n) + "Programs"
 }
@@ -82,6 +90,8 @@ type GenerateArgs struct {
 	Constraints constraint.Expr
 	// Maps to be emitted.
 	Maps []string
+	// Variables to be emitted.
+	Variables []string
 	// Programs to be emitted.
 	Programs []string
 	// Types to be emitted.
@@ -103,19 +113,16 @@ func Generate(args GenerateArgs) error {
 		return fmt.Errorf("file %q contains an invalid character", args.ObjectFile)
 	}
 
-	for _, typ := range args.Types {
-		if _, ok := btf.As[*btf.Datasec](typ); ok {
-			// Avoid emitting .rodata, .bss, etc. for now. We might want to
-			// name these types differently, etc.
-			return fmt.Errorf("can't output btf.Datasec: %s", typ)
-		}
-	}
-
 	maps := make(map[string]string)
 	for _, name := range args.Maps {
 		maps[name] = internal.Identifier(name)
 	}
 
+	vars := make(map[string]string)
+	for _, name := range args.Variables {
+		vars[name] = internal.Identifier(name)
+	}
+
 	programs := make(map[string]string)
 	for _, name := range args.Programs {
 		programs[name] = internal.Identifier(name)
@@ -146,6 +153,7 @@ func Generate(args GenerateArgs) error {
 		Constraints constraint.Expr
 		Name        templateName
 		Maps        map[string]string
+		Variables   map[string]string
 		Programs    map[string]string
 		Types       []btf.Type
 		TypeNames   map[btf.Type]string
@@ -157,6 +165,7 @@ func Generate(args GenerateArgs) error {
 		args.Constraints,
 		templateName(args.Stem),
 		maps,
+		vars,
 		programs,
 		types,
 		typeNames,

Diff for: cmd/bpf2go/gen/output.tpl

@@ -54,6 +54,7 @@ func {{ .Name.LoadObjects }}(obj interface{}, opts *ebpf.CollectionOptions) (err
 type {{ .Name.Specs }} struct {
 	{{ .Name.ProgramSpecs }}
 	{{ .Name.MapSpecs }}
+	{{ .Name.VariableSpecs }}
 }
 
 // {{ .Name.Specs }} contains programs before they are loaded into the kernel.
@@ -80,6 +81,7 @@ type {{ .Name.MapSpecs }} struct {
 type {{ .Name.Objects }} struct {
 	{{ .Name.Programs }}
 	{{ .Name.Maps }}
+	{{ .Name.Variables }}
 }
 
 func (o *{{ .Name.Objects }}) Close() error {
@@ -98,6 +100,24 @@ type {{ .Name.Maps }} struct {
 {{- end }}
 }
 
+// {{ .Name.VariableSpecs }} contains variables before they are loaded into the kernel.
+//
+// It can be passed ebpf.CollectionSpec.Assign.
+type {{ .Name.VariableSpecs }} struct {
+{{- range $name, $id := .Variables }}
+	{{ $id }} *ebpf.VariableSpec `ebpf:"{{ $name }}"`
+{{- end }}
+}
+
+// {{ .Name.Variables }} contains all variables after they have been loaded into the kernel.
+//
+// It can be passed to {{ .Name.LoadObjects }} or ebpf.CollectionSpec.LoadAndAssign.
+type {{ .Name.Variables }} struct {
+{{- range $name, $id := .Variables }}
+	{{ $id }} *{{ $id }} `ebpf:"{{ $name }}"`
+{{- end }}
+}
+
 func (m *{{ .Name.Maps }}) Close() error {
 	return {{ .Name.CloseHelper }}(
 {{- range $id := .Maps }}

Diff for: cmd/bpf2go/gen/types.go

@@ -12,10 +12,6 @@ func CollectGlobalTypes(spec *ebpf.CollectionSpec) []btf.Type {
 	var types []btf.Type
 	for _, typ := range collectMapTypes(spec.Maps) {
 		switch btf.UnderlyingType(typ).(type) {
-		case *btf.Datasec:
-			// Avoid emitting .rodata, .bss, etc. for now. We might want to
-			// name these types differently, etc.
-			continue
 
 		case *btf.Int:
 			// Don't emit primitive types by default.

Diff for: cmd/bpf2go/main.go

@@ -370,6 +370,11 @@ func (b2g *bpf2go) convert(tgt gen.Target, goarches gen.GoArches) (err error) {
 		}
 	}
 
+	var vars []string
+	for name := range spec.Variables {
+		vars = append(vars, name)
+	}
+
 	var programs []string
 	for name := range spec.Programs {
 		programs = append(programs, name)
@@ -397,6 +402,7 @@ func (b2g *bpf2go) convert(tgt gen.Target, goarches gen.GoArches) (err error) {
 		Stem:        b2g.identStem,
 		Constraints: constraints,
 		Maps:        maps,
+		Variables:   vars,
 		Programs:    programs,
 		Types:       types,
 		ObjectFile:  filepath.Base(objFileName),

Diff for: go.mod

@@ -6,9 +6,16 @@ require (
 	github.com/go-quicktest/qt v1.101.0
 	github.com/google/go-cmp v0.6.0
 	github.com/jsimonetti/rtnetlink/v2 v2.0.1
+	github.com/stretchr/testify v1.9.0
 	golang.org/x/sys v0.20.0
 )
 
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)
+
 require (
 	github.com/josharian/native v1.1.0 // indirect
 	github.com/kr/pretty v0.3.1 // indirect
@@ -17,5 +24,6 @@ require (
 	github.com/mdlayher/socket v0.4.1 // indirect
 	github.com/rogpeppe/go-internal v1.11.0 // indirect
 	golang.org/x/net v0.23.0 // indirect
-	golang.org/x/sync v0.1.0 // indirect
+	golang.org/x/sync v0.8.0 // indirect
+	golang.org/x/text v0.18.0
 )

Diff for: go.sum

@@ -1,4 +1,6 @@
 github.com/creack/pty v1.1.9/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ33E=
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/go-quicktest/qt v1.101.0 h1:O1K29Txy5P2OK0dGo59b7b0LR6wKfIhttaAhHUyn7eI=
 github.com/go-quicktest/qt v1.101.0/go.mod h1:14Bz/f7NwaXPtdYEgzsx46kqSxVwTbzVZsDC26tQJow=
 github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
@@ -16,12 +18,22 @@ github.com/mdlayher/netlink v1.7.2/go.mod h1:xraEF7uJbxLhc5fpHL4cPe221LI2bdttWlU
 github.com/mdlayher/socket v0.4.1 h1:eM9y2/jlbs1M615oshPQOHZzj6R6wMT7bX5NPiQvn2U=
 github.com/mdlayher/socket v0.4.1/go.mod h1:cAqeGjoufqdxWkD7DkpyS+wcefOtmu5OQ8KuoJGIReA=
 github.com/pkg/diff v0.0.0-20210226163009-20ebb0f2a09e/go.mod h1:pJLUxLENpZxwdsKMEsNbx1VGcRFpLqf3715MtcvvzbA=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/rogpeppe/go-internal v1.9.0/go.mod h1:WtVeX8xhTBvf0smdhujwtBcq4Qrzq/fJaraNFVN+nFs=
 github.com/rogpeppe/go-internal v1.11.0 h1:cWPaGQEPrBb5/AsnsZesgZZ9yb1OQ+GOISoDNXVBh4M=
 github.com/rogpeppe/go-internal v1.11.0/go.mod h1:ddIwULY96R17DhadqLgMfk9H9tvdUzkipdSkR5nkCZA=
+github.com/stretchr/testify v1.9.0 h1:HtqpIVDClZ4nwg75+f6Lvsy/wHu+3BoSGCbBAcpTsTg=
+github.com/stretchr/testify v1.9.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
 golang.org/x/net v0.23.0 h1:7EYJ93RZ9vYSZAIb2x3lnuvqO5zneoD6IvWjuhfxjTs=
 golang.org/x/net v0.23.0/go.mod h1:JKghWKKOSdJwpW2GEx0Ja7fmaKnMsbu+MWVZTokSYmg=
-golang.org/x/sync v0.1.0 h1:wsuoTGHzEhffawBOhz5CYhcrV4IdKZbEyZjBMuTp12o=
-golang.org/x/sync v0.1.0/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.8.0 h1:3NFvSEYkUoMifnESzZl15y791HH1qU2xm6eCJU5ZPXQ=
+golang.org/x/sync v0.8.0/go.mod h1:Czt+wKu1gCyEFDUtn0jG5QVvpJ6rzVqr5aXyt9drQfk=
 golang.org/x/sys v0.20.0 h1:Od9JTbYCk261bKm4M/mw7AklTlFYIa0bIp9BgSm1S8Y=
 golang.org/x/sys v0.20.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
+golang.org/x/text v0.18.0 h1:XvMDiNzPAl0jr17s6W9lcaIhGUfUORdGCNsuLmPG224=
+golang.org/x/text v0.18.0/go.mod h1:BuEKDfySbSR4drPmRPG/7iBdf8hvFMuRexcpahXilzY=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=