Add automatic support for p11-kit

This commit adds support to automatically use the p11-kit-proxy module
on the PKCS#11 KMS if no other module has been specified.

Fixes #259

kms/pkcs11/pkcs11.go

@@ -12,11 +12,17 @@ import (
 	"encoding/hex"
 	"fmt"
 	"math/big"
+	"os"
+	"os/exec"
+	"runtime"
 	"strconv"
+	"strings"
 	"sync"
+	"time"
 
 	"github.com/ThalesIgnite/crypto11"
 	"github.com/pkg/errors"
+
 	"go.step.sm/crypto/kms/apiv1"
 	"go.step.sm/crypto/kms/uri"
 )
@@ -60,7 +66,6 @@ func New(ctx context.Context, opts apiv1.Options) (*PKCS11, error) {
 		}
 
 		config.Pin = u.Pin()
-		config.Path = u.Get("module-path")
 		config.TokenLabel = u.Get("token")
 		config.TokenSerial = u.Get("serial")
 		if v := u.Get("slot-id"); v != "" {
@@ -70,6 +75,10 @@ func New(ctx context.Context, opts apiv1.Options) (*PKCS11, error) {
 			}
 			config.SlotNumber = &n
 		}
+		// Get module or default to use p11-kit-proxy.so
+		if config.Path = u.Get("module-path"); config.Path == "" {
+			config.Path = findP11KitProxy(ctx)
+		}
 	}
 	if config.Pin == "" && opts.Pin != "" {
 		config.Pin = opts.Pin
@@ -402,4 +411,34 @@ func findCertificate(ctx P11, rawuri string) (*x509.Certificate, error) {
 	return cert, nil
 }
 
+// findP11KitProxy uses pkg-config to locate p11-kit-proxy.so
+func findP11KitProxy(ctx context.Context) string {
+	var out strings.Builder
+
+	// It should be more than enough even in constraint VMs
+	ctx, cancel := context.WithTimeout(ctx, 5*time.Second)
+	defer cancel()
+
+	cmd := exec.CommandContext(ctx, "pkg-config", "--variable=proxy_module", "p11-kit-1")
+	cmd.Stdout = &out
+	if err := cmd.Run(); err != nil {
+		return ""
+	}
+
+	path := strings.TrimSpace(out.String())
+	if _, err := os.Stat(path); err != nil {
+		if runtime.GOOS != "darwin" {
+			return ""
+		}
+
+		// pkg-config might return an .so file instead of a .dylib on macOs.
+		path = strings.Replace(path, ".so", ".dylib", 1)
+		if _, err := os.Stat(path); err != nil {
+			return ""
+		}
+	}
+
+	return path
+}
+
 var _ apiv1.CertificateManager = (*PKCS11)(nil)

kms/pkcs11/pkcs11_test.go

@@ -41,6 +41,18 @@ func TestNew(t *testing.T) {
 		return k.p11, nil
 	}
 
+	var (
+		wantMissingModule    *PKCS11
+		wantErrMissingModule = true
+	)
+	if findP11KitProxy(context.Background()) != "" {
+		wantMissingModule = k
+		wantErrMissingModule = false
+	}
+
+	canceledContext, cancel := context.WithCancel(context.Background())
+	cancel()
+
 	type args struct {
 		ctx  context.Context
 		opts apiv1.Options
@@ -68,10 +80,14 @@ func TestNew(t *testing.T) {
 			URI:  "pkcs11:module-path=/usr/local/lib/softhsm/libsofthsm2.so;token=pkcs11-test",
 			Pin:  "passowrd",
 		}}, k, false},
-		{"fail missing module", args{context.Background(), apiv1.Options{
+		{"perhaps with missing module", args{context.Background(), apiv1.Options{
 			Type: "pkcs11",
 			URI:  "pkcs11:token=pkcs11-test",
 			Pin:  "passowrd",
+		}}, wantMissingModule, wantErrMissingModule},
+		{"fail findP11KitProxy", args{canceledContext, apiv1.Options{
+			Type: "pkcs11",
+			URI:  "pkcs11:token=pkcs11-test?pin-value=password",
 		}}, nil, true},
 		{"fail missing pin", args{context.Background(), apiv1.Options{
 			Type: "pkcs11",