fix: validate volume mount source paths before spawning detached process

pkg/runner/config_builder.go

@@ -9,6 +9,8 @@ import (
 	"log/slog"
 	"maps"
 	"net/url"
+	"os"
+	"path/filepath"
 	"slices"
 	"strings"
 
@@ -1042,6 +1044,22 @@ func (b *runConfigBuilder) processVolumeMounts() error {
 			return fmt.Errorf("invalid volume format: %s (%w)", volume, err)
 		}
 
+		// Validate source path exists on the host filesystem (CLI context only).
+		// Skip for Kubernetes operator context where paths are container-relative,
+		// and for resource URIs which are not filesystem paths.
+		if b.buildContext != BuildContextOperator && !strings.HasPrefix(source, "resource://") {
+			absSource := source
+			if !filepath.IsAbs(absSource) {
+				absSource, err = filepath.Abs(absSource)
+				if err != nil {
+					return fmt.Errorf("failed to resolve volume mount source path %q: %w", source, err)
+				}
+			}
+			if _, err := os.Stat(absSource); err != nil {
+				return fmt.Errorf("volume mount source path does not exist: %s", source)
+			}
+		}
+
 		// Check for duplicate mount target
 		if existingSource, isDuplicate := existingMounts[target]; isDuplicate {
 			slog.Warn("Skipping duplicate mount target", "target", target, "existing_source", existingSource)

pkg/runner/config_builder_test.go

@@ -244,6 +244,11 @@ func TestRunConfigBuilder_Build_WithVolumeMounts(t *testing.T) {
 	// Create a mock environment variable validator
 	mockValidator := &mockEnvVarValidator{}
 
+	// Create temporary directories for volume mount source paths
+	tmpDir1 := t.TempDir()
+	tmpDir2 := t.TempDir()
+	tmpDir3 := t.TempDir()
+
 	testCases := []struct {
 		name                string
 		builderOptions      []RunConfigBuilderOption
@@ -263,7 +268,7 @@ func TestRunConfigBuilder_Build_WithVolumeMounts(t *testing.T) {
 		{
 			name: "Volumes without permission profile but with profile name",
 			builderOptions: []RunConfigBuilderOption{
-				WithVolumes([]string{"/host:/container"}),
+				WithVolumes([]string{tmpDir1 + ":/container"}),
 				WithPermissionProfileNameOrPath(permissions.ProfileNone),
 			},
 			expectError:         false,
@@ -273,7 +278,7 @@ func TestRunConfigBuilder_Build_WithVolumeMounts(t *testing.T) {
 		{
 			name: "Read-only volume with existing profile",
 			builderOptions: []RunConfigBuilderOption{
-				WithVolumes([]string{"/host:/container:ro"}),
+				WithVolumes([]string{tmpDir1 + ":/container:ro"}),
 				WithPermissionProfile(permissions.BuiltinNoneProfile()),
 			},
 			expectError:         false,
@@ -283,7 +288,7 @@ func TestRunConfigBuilder_Build_WithVolumeMounts(t *testing.T) {
 		{
 			name: "Read-write volume with existing profile",
 			builderOptions: []RunConfigBuilderOption{
-				WithVolumes([]string{"/host:/container"}),
+				WithVolumes([]string{tmpDir1 + ":/container"}),
 				WithPermissionProfile(permissions.BuiltinNoneProfile()),
 			},
 			expectError:         false,
@@ -294,9 +299,9 @@ func TestRunConfigBuilder_Build_WithVolumeMounts(t *testing.T) {
 			name: "Multiple volumes with existing profile",
 			builderOptions: []RunConfigBuilderOption{
 				WithVolumes([]string{
-					"/host1:/container1:ro",
-					"/host2:/container2",
-					"/host3:/container3:ro",
+					tmpDir1 + ":/container1:ro",
+					tmpDir2 + ":/container2",
+					tmpDir3 + ":/container3:ro",
 				}),
 				WithPermissionProfile(permissions.BuiltinNoneProfile()),
 			},
@@ -312,6 +317,14 @@ func TestRunConfigBuilder_Build_WithVolumeMounts(t *testing.T) {
 			},
 			expectError: true,
 		},
+		{
+			name: "Non-existent source path",
+			builderOptions: []RunConfigBuilderOption{
+				WithVolumes([]string{"/nonexistent/path/that/does/not/exist:/container"}),
+				WithPermissionProfile(permissions.BuiltinNoneProfile()),
+			},
+			expectError: true,
+		},
 	}
 
 	for _, tc := range testCases {