In a lot of countries it is illegal.

France:

• vehicles above 3.5 tons longer than 7m: 50 metres distance minimum outside towns if cruising at the same speed (Code de la route R412-12 II)
• it's illegal to attempt overtaking if it would impede the flow of traffic and the relative speeds do not allow for overtaking within a sufficiently short time (Code de la route R414-4 II)
• a lot of busy roads have a sign forbidding overtaking for heavy vehicles, sometimes with specific times listed.

None of it is enforced so you should always expect to be cut off.

Also of note: whenever there are random checks of trucks by police they find around 50% in violation of important regulations like rest times and the administrative situation of a work vehicle. They willfully avoid checking too much because it would mean the means of transport for 85% of freight suddenly becomes way more expensive.