[message] update 'Write()' to use 'memmove()' instead of 'memcpy()' (#5462)

This change allows `CopyTo()` method to be used to copy bytes backwards
within the same message (i.e., src and dest messages are the same and
the dest offset is before src offset). Unit test `test_message` is
updated in this commit to cover such a use.

However, the `CopyTo()` implementing can still potentially overwrite
the data when bytes are being copied forward within the same message.
This commit adds an assert not allowing such a use and updates the
method documentation mentioning this restriction.

src/core/common/message.cpp

@@ -537,7 +537,7 @@ void Message::Write(uint16_t aOffset, uint16_t aLength, const void *aBuf)
 
     while (chunk.GetLength() > 0)
     {
-        memcpy(chunk.GetData(), bufPtr, chunk.GetLength());
+        memmove(chunk.GetData(), bufPtr, chunk.GetLength());
         bufPtr += chunk.GetLength();
         GetNextChunk(aLength, chunk);
     }
@@ -548,6 +548,13 @@ uint16_t Message::CopyTo(uint16_t aSourceOffset, uint16_t aDestinationOffset, ui
     uint16_t bytesCopied = 0;
     Chunk    chunk;
 
+    // This implementing can potentially overwrite the data when bytes are
+    // being copied forward within the same message, i.e., source and
+    // destination messages are the same, and source offset is smaller than
+    // the destination offset. We assert not allowing such a use.
+
+    OT_ASSERT((&aMessage != this) || (aSourceOffset >= aDestinationOffset));
+
     GetFirstChunk(aSourceOffset, aLength, chunk);
 
     while (chunk.GetLength() > 0)

src/core/common/message.hpp

@@ -532,6 +532,10 @@ public:
     /**
      * This method copies bytes from one message to another.
      *
+     * If source and destination messages are the same, `CopyTo()` can be used to perform a backward copy, but
+     * it MUST not be used to forward copy within the same message (i.e., when source and destination messages are the
+     * same and source offset is smaller than the destination offset).
+     *
      * @param[in] aSourceOffset       Byte offset within the source message to begin reading.
      * @param[in] aDestinationOffset  Byte offset within the destination message to begin writing.
      * @param[in] aLength             Number of bytes to copy.

tests/unit/test_message.cpp

@@ -144,7 +144,27 @@ void TestMessage(void)
         }
     }
 
+    // Verify `CopyTo()` with same source and destination message and a backward copy.
+
+    for (uint16_t srcOffset = 0; srcOffset < kMaxSize; srcOffset++)
+    {
+        uint16_t bytesCopied;
+
+        message->Write(0, kMaxSize, writeBuffer);
+
+        bytesCopied = message->CopyTo(srcOffset, 0, kMaxSize, *message);
+        VerifyOrQuit(bytesCopied == kMaxSize - srcOffset, "CopyTo() failed");
+
+        VerifyOrQuit(message->Read(0, kMaxSize, readBuffer) == kMaxSize, "Message::Read failed");
+
+        VerifyOrQuit(memcmp(&readBuffer[0], &writeBuffer[srcOffset], bytesCopied) == 0,
+                     "CopyTo() changed before srcOffset");
+        VerifyOrQuit(memcmp(&readBuffer[bytesCopied], &writeBuffer[bytesCopied], kMaxSize - bytesCopied) == 0,
+                     "CopyTo() write error");
+    }
+
     message->Free();
+    message2->Free();
 
     testFreeInstance(instance);
 }