BMessage::ReplaceData: potential corruption on realloc fail

The method would update the field offsets first, and would not restore
them in case of realloc failure, leaving them pointing to the wrong
place.

headers/os/app/Message.h

@@ -510,6 +510,7 @@ private:
 
 			status_t			_ValidateMessage();
 
+			void				_UpdateOffsets(uint32 offset, int32 change);
 			status_t			_ResizeData(uint32 offset, int32 change);
 
 			uint32				_HashName(const char* name) const;

src/kits/app/Message.cpp

@@ -1567,13 +1567,10 @@ BMessage::PopSpecifier()
 }
 
 
-status_t
-BMessage::_ResizeData(uint32 offset, int32 change)
+void
+BMessage::_UpdateOffsets(uint32 offset, int32 change)
 {
-	if (change == 0)
-		return B_OK;
-
-	/* optimize for the most usual case: appending data */
+	// Update the header to match the new position of the fields
 	if (offset < fHeader->data_size) {
 		field_header* field = fFields;
 		for (uint32 i = 0; i < fHeader->field_count; i++, field++) {
@@ -1581,19 +1578,37 @@ BMessage::_ResizeData(uint32 offset, int32 change)
 				field->offset += change;
 		}
 	}
+}
+
+
+status_t
+BMessage::_ResizeData(uint32 offset, int32 change)
+{
+	if (change == 0)
+		return B_OK;
+
+	/* optimize for the most usual case: appending data */
 
 	if (change > 0) {
+		// We need to make the field bigger
+		// check if there is enough free space allocated
 		if (fDataAvailable >= (uint32)change) {
+			// In this case, we just need to move the data after the growing
+			// field to get the space at the right place
 			if (offset < fHeader->data_size) {
 				memmove(fData + offset + change, fData + offset,
 					fHeader->data_size - offset);
 			}
 
+			_UpdateOffsets(offset, change);
+
 			fDataAvailable -= change;
 			fHeader->data_size += change;
 			return B_OK;
 		}
 
+		// We need to grow the buffer. We try to optimize reallocations by
+		// preallocating space for more fields.
 		size_t size = fHeader->data_size * 2;
 		size = min_c(size, fHeader->data_size + MAX_DATA_PREALLOCATION);
 		size = max_c(size, fHeader->data_size + change);
@@ -1625,6 +1640,7 @@ BMessage::_ResizeData(uint32 offset, int32 change)
 			uint8* newData = (uint8*)realloc(fData, size);
 			if (size > 0 && newData == NULL) {
 				// this is strange, but not really fatal
+				_UpdateOffsets(offset, change);
 				return B_OK;
 			}
 
@@ -1633,6 +1649,7 @@ BMessage::_ResizeData(uint32 offset, int32 change)
 		}
 	}
 
+	_UpdateOffsets(offset, change);
 	return B_OK;
 }
 
@@ -3133,7 +3150,7 @@ BMessage::ReplaceRef(const char* name, int32 index, const entry_ref* ref)
 	status_t error = BPrivate::entry_ref_flatten(buffer, &size, ref);
 
 	if (error >= B_OK)
-		error = ReplaceData(name, B_REF_TYPE, index, &buffer, size);
+		error = ReplaceData(name, B_REF_TYPE, index, buffer, size);
 
 	return error;
 }