The WTO may have rules in place to discipline domestic agricultural support, but in practice this is quite difficult given the propensity of countries to evade or fail to fully implement the rules given what they perceive as being their national interests. Three leading agricultural economists have produced a report on the subject which covers four developed countries (including the US, EU and Japan) and four developing countries (including India and China). It can be found here: Agricultural support
The report raises the question of the legitimacy of green box support which has been discussed on this page before. It notes that this has been treated as 'decoupled income support by the United States, the European Union, and China. There are large differences in the levels of such payments. The extent to which decoupled income support affects production remains uncertain but may be consequential. Limits might therefore be envisioned for this type of support to achieve a balanced set of future commitments.'
The EU has always taken the view that the SFP can be protected by putting it in the Green Box, but is always possible that this might be challenged in the WTO's Dispute Settlement Mechanism - although this would incur political costs for the country concerned.