From 905b9f9785263297611a61facc8b6bdf4c3e257f Mon Sep 17 00:00:00 2001
From: Ophestra <cat@gensokyo.uk>
Date: Fri, 29 Aug 2025 02:56:56 +0900
Subject: [PATCH] container/initoverlay: invalid argument type

This eliminates generic WrapErr from overlay.

Signed-off-by: Ophestra <cat@gensokyo.uk>
---
 container/initoverlay.go      | 40 +++++++++++++++++++++++++++++++----
 container/initoverlay_test.go | 34 +++++++++++++++++++++++++----
 2 files changed, 66 insertions(+), 8 deletions(-)

diff --git a/container/initoverlay.go b/container/initoverlay.go
index f9daaaa..c3f1c59 100644
--- a/container/initoverlay.go
+++ b/container/initoverlay.go
@@ -3,7 +3,6 @@ package container
 import (
 	"encoding/gob"
 	"fmt"
-	"io/fs"
 	"slices"
 	"strings"
 )
@@ -19,6 +18,39 @@ const (
 
 func init() { gob.Register(new(MountOverlayOp)) }
 
+const (
+	// OverlayEphemeralUnexpectedUpper is set when [MountOverlayOp.Work] is nil
+	// and [MountOverlayOp.Upper] holds an unexpected value.
+	OverlayEphemeralUnexpectedUpper = iota
+	// OverlayReadonlyLower is set when [MountOverlayOp.Lower] contains less than
+	// two entries when mounting readonly.
+	OverlayReadonlyLower
+	// OverlayEmptyLower is set when [MountOverlayOp.Lower] has length of zero.
+	OverlayEmptyLower
+)
+
+// OverlayArgumentError is returned for [MountOverlayOp] supplied with invalid argument.
+type OverlayArgumentError struct {
+	Type  uintptr
+	Value string
+}
+
+func (e *OverlayArgumentError) Error() string {
+	switch e.Type {
+	case OverlayEphemeralUnexpectedUpper:
+		return fmt.Sprintf("upperdir has unexpected value %q", e.Value)
+
+	case OverlayReadonlyLower:
+		return "readonly overlay requires at least two lowerdir"
+
+	case OverlayEmptyLower:
+		return "overlay requires at least one lowerdir"
+
+	default:
+		return fmt.Sprintf("invalid overlay argument error %#x", e.Type)
+	}
+}
+
 // Overlay appends an [Op] that mounts the overlay pseudo filesystem on [MountOverlayOp.Target].
 func (f *Ops) Overlay(target, state, work *Absolute, layers ...*Absolute) *Ops {
 	*f = append(*f, &MountOverlayOp{
@@ -89,7 +121,7 @@ func (o *MountOverlayOp) early(_ *setupState, k syscallDispatcher) error {
 			o.ephemeral = true // intermediate root not yet available
 
 		default:
-			return msg.WrapErr(fs.ErrInvalid, fmt.Sprintf("upperdir has unexpected value %q", o.Upper))
+			return &OverlayArgumentError{OverlayEphemeralUnexpectedUpper, o.Upper.String()}
 		}
 	}
 	// readonly handled in apply
@@ -152,12 +184,12 @@ func (o *MountOverlayOp) apply(state *setupState, k syscallDispatcher) error {
 
 	if o.upper == zeroString && o.work == zeroString { // readonly
 		if len(o.Lower) < 2 {
-			return msg.WrapErr(fs.ErrInvalid, "readonly overlay requires at least two lowerdir")
+			return &OverlayArgumentError{OverlayReadonlyLower, zeroString}
 		}
 		// "upperdir=" and "workdir=" may be omitted. In that case the overlay will be read-only
 	} else {
 		if len(o.Lower) == 0 {
-			return msg.WrapErr(fs.ErrInvalid, "overlay requires at least one lowerdir")
+			return &OverlayArgumentError{OverlayEmptyLower, zeroString}
 		}
 		options = append(options,
 			OptionOverlayUpperdir+"="+o.upper,
diff --git a/container/initoverlay_test.go b/container/initoverlay_test.go
index 64b0bf3..9dae211 100644
--- a/container/initoverlay_test.go
+++ b/container/initoverlay_test.go
@@ -2,12 +2,38 @@ package container
 
 import (
 	"errors"
-	"io/fs"
 	"os"
 	"testing"
 )
 
 func TestMountOverlayOp(t *testing.T) {
+	t.Run("argument error", func(t *testing.T) {
+		testCases := []struct {
+			name string
+			err  *OverlayArgumentError
+			want string
+		}{
+			{"unexpected upper", &OverlayArgumentError{OverlayEphemeralUnexpectedUpper, "/proc/"},
+				`upperdir has unexpected value "/proc/"`},
+
+			{"lower ro short", &OverlayArgumentError{OverlayReadonlyLower, zeroString},
+				"readonly overlay requires at least two lowerdir"},
+
+			{"lower short", &OverlayArgumentError{OverlayEmptyLower, zeroString},
+				"overlay requires at least one lowerdir"},
+
+			{"oob", &OverlayArgumentError{0xdeadbeef, zeroString},
+				"invalid overlay argument error 0xdeadbeef"},
+		}
+		for _, tc := range testCases {
+			t.Run(tc.name, func(t *testing.T) {
+				if got := tc.err.Error(); got != tc.want {
+					t.Errorf("Error: %q, want %q", got, tc.want)
+				}
+			})
+		}
+	})
+
 	checkOpBehaviour(t, []opBehaviourTestCase{
 		{"mkdirTemp invalid ephemeral", &Params{ParentPerm: 0705}, &MountOverlayOp{
 			Target: MustAbs("/"),
@@ -16,7 +42,7 @@ func TestMountOverlayOp(t *testing.T) {
 				MustAbs("/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052"),
 			},
 			Upper: MustAbs("/proc/"),
-		}, nil, msg.WrapErr(fs.ErrInvalid, `upperdir has unexpected value "/proc/"`), nil, nil},
+		}, nil, &OverlayArgumentError{OverlayEphemeralUnexpectedUpper, "/proc/"}, nil, nil},
 
 		{"mkdirTemp upper ephemeral", &Params{ParentPerm: 0705}, &MountOverlayOp{
 			Target: MustAbs("/"),
@@ -81,7 +107,7 @@ func TestMountOverlayOp(t *testing.T) {
 			{"evalSymlinks", expectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/.ro-store", nil},
 		}, nil, []kexpect{
 			{"mkdirAll", expectArgs{"/sysroot/nix/store", os.FileMode(0755)}, nil, nil},
-		}, msg.WrapErr(fs.ErrInvalid, "readonly overlay requires at least two lowerdir")},
+		}, &OverlayArgumentError{OverlayReadonlyLower, zeroString}},
 
 		{"success ro noPrefix", &Params{ParentPerm: 0755}, &MountOverlayOp{
 			Target: MustAbs("/nix/store"),
@@ -129,7 +155,7 @@ func TestMountOverlayOp(t *testing.T) {
 			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/work"}, "/mnt-root/nix/.rw-store/.work", nil},
 		}, nil, []kexpect{
 			{"mkdirAll", expectArgs{"/sysroot/nix/store", os.FileMode(0700)}, nil, nil},
-		}, msg.WrapErr(fs.ErrInvalid, "overlay requires at least one lowerdir")},
+		}, &OverlayArgumentError{OverlayEmptyLower, zeroString}},
 
 		{"evalSymlinks upper", &Params{ParentPerm: 0700}, &MountOverlayOp{
 			Target: MustAbs("/nix/store"),