I am wondering if a BP crashes, how do the remaining 20 BPs know this?
If they do not know, all block not irreversible so we can be hack? Please give me some details for this. Thanks!
As far as I know, there aren't any alert mechanisms that notify the BPs.
In terms of block production, they skip the BP that crashed(with a moderate time delay).
Given that there are more than 1/3 BPs crashed and were not confirming blocks, they are not included in the LIB. When the BPs return, they confirm the blocks and add to LIB.
In terms of hacking(or modifying blocks I persume), I'm not sure whats the risk, but my guess is if they are able to create a falsified block that is not detected by the EOS software.. maybe?