Hello,
Please find the proposal for the Negative Grants project attached along with this e-mail. It would be great to hear your opinions and suggestions on the approach proposed here, before delving into its implementation. 
Some context about the project:

The current privilege system allows access in this manner:

GRANT SELECT ON some_database.* TO a_user@%;

Any revoke would revoke access from the entire database. We want to support a revoke that would disable select on a specific table, like:

REVOKE SELECT ON some_database.secret_table FROM a_user@%;

Reference:

[1] https://jira.mariadb.org/browse/MDEV-14443

[2] https://mariadb.com/kb/en/library/google-summer-of-code-2018/

Thanks,

Rutuja