From 9950d1225d1284ed8f47da3f027bb1444954c0d2 Mon Sep 17 00:00:00 2001
From: Neil Alexander <neilalexander@users.noreply.github.com>
Date: Thu, 1 Aug 2024 21:53:48 +0100
Subject: [PATCH] Improve link and handshake errors

---
 src/core/link.go    | 10 ++++++----
 src/core/version.go | 23 ++++++++++++++++-------
 2 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/src/core/link.go b/src/core/link.go
index f45c2cee..ee2ad06f 100644
--- a/src/core/link.go
+++ b/src/core/link.go
@@ -140,7 +140,7 @@ const ErrLinkAlreadyConfigured = linkError("peer is already configured")
 const ErrLinkNotConfigured = linkError("peer is not configured")
 const ErrLinkPriorityInvalid = linkError("priority value is invalid")
 const ErrLinkPinnedKeyInvalid = linkError("pinned public key is invalid")
-const ErrLinkPasswordInvalid = linkError("password is invalid")
+const ErrLinkPasswordInvalid = linkError("invalid password supplied")
 const ErrLinkUnrecognisedSchema = linkError("link schema unknown")
 const ErrLinkMaxBackoffInvalid = linkError("max backoff duration invalid")
 
@@ -363,9 +363,11 @@ func (l *links) add(u *url.URL, sintf string, linkType linkType) error {
 				_ = lc.Close()
 				phony.Block(l, func() {
 					state._conn = nil
-					if state._err = err; state._err != nil {
-						state._errtime = time.Now()
+					if err == nil {
+						err = fmt.Errorf("remote side closed the connection")
 					}
+					state._err = err
+					state._errtime = time.Now()
 				})
 
 				// If the link is persistently configured, back off if needed
@@ -647,7 +649,7 @@ func (l *links) handler(linkType linkType, options linkOptions, conn net.Conn, s
 		l.core.log.Infof("Disconnected %s: %s, source %s; error: %s",
 			dir, remoteStr, localStr, err)
 	}
-	return nil
+	return err
 }
 
 func urlForLinkInfo(u url.URL) url.URL {
diff --git a/src/core/version.go b/src/core/version.go
index 28b16430..bb3b9538 100644
--- a/src/core/version.go
+++ b/src/core/version.go
@@ -8,7 +8,6 @@ import (
 	"bytes"
 	"crypto/ed25519"
 	"encoding/binary"
-	"fmt"
 	"io"
 
 	"golang.org/x/crypto/blake2b"
@@ -38,6 +37,16 @@ const (
 	metaPriority                   // uint8
 )
 
+type handshakeError string
+
+func (e handshakeError) Error() string { return string(e) }
+
+const ErrHandshakeInvalidPreamble = handshakeError("invalid handshake, remote side is not Yggdrasil")
+const ErrHandshakeInvalidLength = handshakeError("invalid handshake length, possible version mismatch")
+const ErrHandshakeInvalidPassword = handshakeError("invalid password supplied, check your config")
+const ErrHandshakeHashFailure = handshakeError("invalid hash length")
+const ErrHandshakeIncorrectPassword = handshakeError("password does not match remote side")
+
 // Gets a base metadata with no keys set, but with the correct version numbers.
 func version_getBaseMetadata() version_metadata {
 	return version_metadata{
@@ -77,7 +86,7 @@ func (m *version_metadata) encode(privateKey ed25519.PrivateKey, password []byte
 		return nil, err
 	}
 	if n != ed25519.PublicKeySize {
-		return nil, fmt.Errorf("hash writer only wrote %d bytes", n)
+		return nil, ErrHandshakeHashFailure
 	}
 	hash := hasher.Sum(nil)
 	bs = append(bs, ed25519.Sign(privateKey, hash)...)
@@ -94,11 +103,11 @@ func (m *version_metadata) decode(r io.Reader, password []byte) error {
 	}
 	meta := [4]byte{'m', 'e', 't', 'a'}
 	if !bytes.Equal(bh[:4], meta[:]) {
-		return fmt.Errorf("invalid handshake preamble")
+		return ErrHandshakeInvalidPreamble
 	}
 	hl := binary.BigEndian.Uint16(bh[4:6])
 	if hl < ed25519.SignatureSize {
-		return fmt.Errorf("invalid handshake length")
+		return ErrHandshakeInvalidLength
 	}
 	bs := make([]byte, hl)
 	if _, err := io.ReadFull(r, bs); err != nil {
@@ -132,15 +141,15 @@ func (m *version_metadata) decode(r io.Reader, password []byte) error {
 
 	hasher, err := blake2b.New512(password)
 	if err != nil {
-		return fmt.Errorf("invalid password supplied")
+		return ErrHandshakeInvalidPassword
 	}
 	n, err := hasher.Write(m.publicKey)
 	if err != nil || n != ed25519.PublicKeySize {
-		return fmt.Errorf("failed to generate hash")
+		return ErrHandshakeHashFailure
 	}
 	hash := hasher.Sum(nil)
 	if !ed25519.Verify(m.publicKey, hash, sig) {
-		return fmt.Errorf("password is incorrect")
+		return ErrHandshakeIncorrectPassword
 	}
 	return nil
 }