From 7dbbe9b4b3efc8539a7be23082e5ee6fbb4751da Mon Sep 17 00:00:00 2001
From: Michael Stapelberg <michael@stapelberg.de>
Date: Sun, 17 Apr 2022 15:23:09 +0200
Subject: [PATCH] fix subsystem invocation: send exit code afterwards
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This fixes scp(1) with OpenSSH ≥ 9.
---
 ssh.go | 36 ++++++++++++++++++++++--------------
 1 file changed, 22 insertions(+), 14 deletions(-)

diff --git a/ssh.go b/ssh.go
index 59b23ec..551e12c 100644
--- a/ssh.go
+++ b/ssh.go
@@ -2,7 +2,6 @@ package main
 
 import (
 	"context"
-	"encoding/binary"
 	"fmt"
 	"io"
 	"log"
@@ -210,6 +209,11 @@ type subsystem struct {
 	SubsystemName string
 }
 
+// exitStatus is a message for returning exit status as specified in RFC4254, Section 6.10
+type exitStatus struct {
+	Status uint32
+}
+
 func findShell() string {
 	if path, err := exec.LookPath("sh"); err == nil {
 		return path
@@ -270,23 +274,27 @@ func (s *session) request(ctx context.Context, req *ssh.Request) error {
 
 		log.Printf("starting SFTP subsystem")
 
+		req.Reply(true, nil)
+
 		srv, err := sftp.NewServer(s.channel, sftp.WithDebug(os.Stderr))
 		if err != nil {
 			return err
 		}
 
-		go func() {
-			err := srv.Serve()
-			if err != nil {
-				log.Printf("(sftp.Server).Serve(): %v", err)
-				if err == io.EOF {
-					srv.Close()
-					log.Printf("sftp client exited session")
-				}
+		exitCode := uint32(0)
+		if err := srv.Serve(); err != nil {
+			log.Printf("(sftp.Server).Serve(): %v", err)
+			if err == io.EOF {
+				defer srv.Close()
+				log.Printf("sftp client exited session")
+			} else {
+				exitCode = 1
 			}
-		}()
+		}
 
-		req.Reply(true, nil)
+		// See https://tools.ietf.org/html/rfc4254#section-6.10
+		_, err = s.channel.SendRequest("exit-status", false /* wantReply */, ssh.Marshal(exitStatus{exitCode}))
+		return err
 
 	case "shell":
 		req.Payload = []byte("\x00\x00\x00\x02sh")
@@ -353,13 +361,13 @@ func (s *session) request(ctx context.Context, req *ssh.Request) error {
 				if err := cmd.Wait(); err != nil {
 					log.Printf("err: %v", err)
 				}
-				status := make([]byte, 4)
+				var status exitStatus
 				if ws, ok := cmd.ProcessState.Sys().(syscall.WaitStatus); ok {
-					binary.BigEndian.PutUint32(status, uint32(ws.ExitStatus()))
+					status.Status = uint32(ws.ExitStatus())
 				}
 
 				// See https://tools.ietf.org/html/rfc4254#section-6.10
-				if _, err := s.channel.SendRequest("exit-status", false /* wantReply */, status); err != nil {
+				if _, err := s.channel.SendRequest("exit-status", false /* wantReply */, ssh.Marshal(status)); err != nil {
 					log.Printf("err2: %v", err)
 				}
 				s.channel.Close()