features: automatically wrap all feature probe errors

Instead of relying on repeated calls to fmt.Errorf("...: %w", err) to ensure
we never return naked sentinels, add error wrappers at a few key locations.

This abstracts and centralizes the wrapping logic to make the probe logic
itself a bit leaner.

Signed-off-by: Timo Beckers <timo@isovalent.com>

features/errors.go

@@ -0,0 +1,38 @@
+package features
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/cilium/ebpf"
+)
+
+// wrapProbeErrors wraps err to prevent callers from directly comparing
+// it to exported sentinels. Error rewriting in Go can be implemented by
+// deferring a closure over a named error return variable. This gives the
+// closure access to the stack space where the return value will be written,
+// allowing it to intercept and rewrite all returns, regardless of whether
+// or not the return statements use the named return variable.
+//
+//  func foo() (err error) {
+//    defer func() {
+//      err = wrapProbeErrors(err)
+//    }
+//    return errors.New("this error will be wrapped")
+//  }
+func wrapProbeErrors(err error) error {
+	if err == nil {
+		return nil
+	}
+
+	// Wrap all errors to prevent them from being compared directly
+	// to exported sentinels by the caller.
+	errStr := "%w"
+
+	if !errors.Is(err, ebpf.ErrNotSupported) {
+		// Wrap unexpected errors with an appropriate error string.
+		errStr = "unexpected error during feature probe: %w"
+	}
+
+	return fmt.Errorf(errStr, err)
+}

features/map.go

@@ -2,7 +2,6 @@ package features
 
 import (
 	"errors"
-	"fmt"
 	"os"
 	"sync"
 	"unsafe"
@@ -98,7 +97,12 @@ func createMapTypeAttr(mt ebpf.MapType) *sys.MapCreateAttr {
 // HaveMapType probes the running kernel for the availability of the specified map type.
 //
 // See the package documentation for the meaning of the error return value.
-func HaveMapType(mt ebpf.MapType) error {
+func HaveMapType(mt ebpf.MapType) (err error) {
+	defer func() {
+		// This closure modifies a named return variable.
+		err = wrapProbeErrors(err)
+	}()
+
 	if err := validateMaptype(mt); err != nil {
 		return err
 	}
@@ -108,7 +112,7 @@ func HaveMapType(mt ebpf.MapType) error {
 
 func validateMaptype(mt ebpf.MapType) error {
 	if mt > mt.Max() {
-		return fmt.Errorf("%w", os.ErrInvalid)
+		return os.ErrInvalid
 	}
 
 	if mt == ebpf.StructOpsMap {
@@ -146,11 +150,7 @@ func haveMapType(mt ebpf.MapType) error {
 	// of the struct known by the running kernel, meaning the kernel is too old
 	// to support the given map type.
 	case errors.Is(err, unix.EINVAL), errors.Is(err, unix.E2BIG):
-		err = fmt.Errorf("%w", ebpf.ErrNotSupported)
-
-	// Wrap unexpected errors.
-	case err != nil:
-		err = fmt.Errorf("unexpected error during feature probe: %w", err)
+		err = ebpf.ErrNotSupported
 	}
 
 	mc.mapTypes[mt] = err

features/misc.go

@@ -78,7 +78,12 @@ func HaveV3ISA() error {
 
 // probeMisc checks the kernel for a given supported misc by creating
 // a specialized program probe and loading it.
-func probeMisc(mt miscType) error {
+func probeMisc(mt miscType) (err error) {
+	defer func() {
+		// This closure modifies a named return variable.
+		err = wrapProbeErrors(err)
+	}()
+
 	mc.Lock()
 	defer mc.Unlock()
 	err, ok := miscs.miscTypes[mt]
@@ -102,11 +107,7 @@ func probeMisc(mt miscType) error {
 	// of the struct known by the running kernel, meaning the kernel is too old
 	// to support the given map type.
 	case errors.Is(err, unix.EINVAL), errors.Is(err, unix.E2BIG):
-		err = fmt.Errorf("%w", ebpf.ErrNotSupported)
-
-	// Wrap unexpected errors.
-	case err != nil:
-		err = fmt.Errorf("unexpected error during feature probe: %w", err)
+		err = ebpf.ErrNotSupported
 	}
 
 	miscs.miscTypes[mt] = err

features/prog.go

@@ -101,7 +101,12 @@ var HaveProgType = HaveProgramType
 // HaveProgramType probes the running kernel for the availability of the specified program type.
 //
 // See the package documentation for the meaning of the error return value.
-func HaveProgramType(pt ebpf.ProgramType) error {
+func HaveProgramType(pt ebpf.ProgramType) (err error) {
+	defer func() {
+		// This closure modifies a named return variable.
+		err = wrapProbeErrors(err)
+	}()
+
 	if err := validateProgramType(pt); err != nil {
 		return err
 	}
@@ -112,7 +117,7 @@ func HaveProgramType(pt ebpf.ProgramType) error {
 
 func validateProgramType(pt ebpf.ProgramType) error {
 	if pt > pt.Max() {
-		return fmt.Errorf("%w", os.ErrInvalid)
+		return os.ErrInvalid
 	}
 
 	if progLoadProbeNotImplemented(pt) {
@@ -148,11 +153,7 @@ func haveProgramType(pt ebpf.ProgramType) error {
 	// of the struct known by the running kernel, meaning the kernel is too old
 	// to support the given prog type.
 	case errors.Is(err, unix.EINVAL), errors.Is(err, unix.E2BIG):
-		err = fmt.Errorf("%w", ebpf.ErrNotSupported)
-
-	// Wrap unexpected errors.
-	case err != nil:
-		err = fmt.Errorf("unexpected error during feature probe: %w", err)
+		err = ebpf.ErrNotSupported
 	}
 
 	pc.types[pt] = err
@@ -173,7 +174,12 @@ func haveProgramType(pt ebpf.ProgramType) error {
 // Only `nil` and `ebpf.ErrNotSupported` are conclusive.
 //
 // Probe results are cached and persist throughout any process capability changes.
-func HaveProgramHelper(pt ebpf.ProgramType, helper asm.BuiltinFunc) error {
+func HaveProgramHelper(pt ebpf.ProgramType, helper asm.BuiltinFunc) (err error) {
+	defer func() {
+		// This closure modifies a named return variable.
+		err = wrapProbeErrors(err)
+	}()
+
 	if err := validateProgramType(pt); err != nil {
 		return err
 	}
@@ -187,7 +193,7 @@ func HaveProgramHelper(pt ebpf.ProgramType, helper asm.BuiltinFunc) error {
 
 func validateProgramHelper(helper asm.BuiltinFunc) error {
 	if helper > helper.Max() {
-		return fmt.Errorf("%w", os.ErrInvalid)
+		return os.ErrInvalid
 	}
 
 	return nil
@@ -225,11 +231,7 @@ func haveProgramHelper(pt ebpf.ProgramType, helper asm.BuiltinFunc) error {
 	// to support the given prog type.
 	case errors.Is(err, unix.EINVAL), errors.Is(err, unix.E2BIG):
 		// TODO: possibly we need to check verifier output here to be sure
-		err = fmt.Errorf("%w", ebpf.ErrNotSupported)
-
-	// Wrap unexpected errors.
-	case err != nil:
-		err = fmt.Errorf("unexpected error during feature probe: %w", err)
+		err = ebpf.ErrNotSupported
 	}
 
 	pc.helpers[pt][helper] = err