How do I check for specific types of error among those returned by ioutil.ReadFile?

According to the current API, ioutil.ReadFile doesn't guarantee any specific behaviour except that it will return err == nil when successful. Even the syscall package doesn't actually guarantee a specific error.

The current implementation of ioutil.ReadFile uses os.Open, which will return *os.PathError when failing to open a file, not os.ErrPermission or anything else. os.PathError contains a field Err which is also an error - in this case, a syscall.Errno. The string "permission denied" is produced from a private table of error messages and it's both architecture and implementation-specific. In my Windows machine it says "Access is denied" instead.

AFAIK, the correct method is to use os.IsPermission(err), which will return true in case of lacking permissions.