From 03fac0ff48c8152e9bcc8cfeed4a6eddb3b0e17f Mon Sep 17 00:00:00 2001
From: Jonathan Hui <jonhui@google.com>
Date: Wed, 26 Jun 2019 03:42:59 -0700
Subject: [PATCH] [message] adjust offset when cloning a message subset (#3944)

---
 src/core/common/message.cpp | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/core/common/message.cpp b/src/core/common/message.cpp
index bc0fcc27e..e323986ba 100644
--- a/src/core/common/message.cpp
+++ b/src/core/common/message.cpp
@@ -637,6 +637,7 @@ Message *Message::Clone(uint16_t aLength) const
 {
     otError  error = OT_ERROR_NONE;
     Message *messageCopy;
+    uint16_t offset;
 
     VerifyOrExit((messageCopy = GetMessagePool()->New(GetType(), GetReserved(), GetPriority())) != NULL,
                  error = OT_ERROR_NO_BUFS);
@@ -644,7 +645,9 @@ Message *Message::Clone(uint16_t aLength) const
     CopyTo(0, 0, aLength, *messageCopy);
 
     // Copy selected message information.
-    messageCopy->SetOffset(GetOffset());
+    offset = GetOffset() < aLength ? GetOffset() : aLength;
+    messageCopy->SetOffset(offset);
+
     messageCopy->SetSubType(GetSubType());
     messageCopy->SetLinkSecurityEnabled(IsLinkSecurityEnabled());
 #if OPENTHREAD_CONFIG_ENABLE_TIME_SYNC