p9: avoid sending a partial dirent when the Rreaddir response exceeds…

… message limit

Ported from gVisor. When the Rreaddir response is too large to fit in
the 9P response message, the last dirent entry may get truncated. This
causes decode errors on 9P clients with more stringent checks such the
Linux kernel. This change sets the payload length to the exact number of
bytes used to encode all directory entries.

Signed-off-by: Chris Koch <[email protected]>

p9/messages.go

@@ -1593,17 +1593,17 @@ func (r *rreaddir) decode(b *buffer) {
 // encode implements encoder.encode.
 func (r *rreaddir) encode(b *buffer) {
 	entriesBuf := buffer{}
+	payloadSize := 0
 	for _, d := range r.Entries {
-		prev := len(entriesBuf.data)
 		d.encode(&entriesBuf)
-		if len(entriesBuf.data) >= int(r.Count) {
-			entriesBuf.data = entriesBuf.data[:prev]
+		if len(entriesBuf.data) > int(r.Count) {
 			break
 		}
+		payloadSize = len(entriesBuf.data)
 	}
-	r.Count = uint32(len(entriesBuf.data))
-	r.payload = entriesBuf.data
-	b.Write32(uint32(r.Count))
+	r.Count = uint32(payloadSize)
+	r.payload = entriesBuf.data[:payloadSize]
+	b.Write32(r.Count)
 }
 
 // typ implements message.typ.

p9/messages_test.go

@@ -197,7 +197,7 @@ func TestEncodeDecode(t *testing.T) {
 		},
 		&rreaddir{
 			// Count must be sufficient to encode a dirent.
-			Count:   0x40,
+			Count:   0x1a,
 			Entries: []Dirent{{QID: QID{Type: 2}}},
 		},
 		&tfsync{