sandbox: resolve inverted flags in op

Signed-off-by: Ophestra <cat@gensokyo.uk>

sandbox/path.go

@@ -2,9 +2,11 @@ package sandbox
 
 import (
 	"errors"
+	"fmt"
 	"io/fs"
 	"os"
 	"path"
+	"strconv"
 	"strings"
 	"syscall"
 )
@@ -28,14 +30,17 @@ func toHost(name string) string {
 
 func createFile(name string, perm os.FileMode, content []byte) error {
 	if err := os.MkdirAll(path.Dir(name), 0755); err != nil {
-		return err
+		return msg.WrapErr(err, err.Error())
 	}
 	f, err := os.OpenFile(name, syscall.O_CREAT|syscall.O_EXCL|syscall.O_WRONLY, perm)
 	if err != nil {
-		return err
+		return msg.WrapErr(err, err.Error())
 	}
 	if content != nil {
 		_, err = f.Write(content)
+		if err != nil {
+			err = msg.WrapErr(err, err.Error())
+		}
 	}
 	return errors.Join(f.Close(), err)
 }
@@ -50,7 +55,23 @@ func ensureFile(name string, perm os.FileMode) error {
 	}
 
 	if mode := fi.Mode(); mode&fs.ModeDir != 0 || mode&fs.ModeSymlink != 0 {
-		err = syscall.EISDIR
+		err = msg.WrapErr(syscall.EISDIR,
+			fmt.Sprintf("path %q is a directory", name))
 	}
 	return err
 }
+
+var hostProc = newProcPats(hostPath)
+
+func newProcPats(prefix string) *procPaths {
+	return &procPaths{prefix, prefix + "/self", prefix + "/self/mountinfo"}
+}
+
+type procPaths struct {
+	prefix    string
+	self      string
+	mountinfo string
+}
+
+func (p *procPaths) stdout() string   { return p.self + "/fd/1" }
+func (p *procPaths) fd(fd int) string { return p.self + "/fd/" + strconv.Itoa(fd) }