How do you prevent an attacker from getting the seed? This would just create a possible attack vector that could be used to effectively disable address space randomization.

e.g. only root can get access to the seed. Then the attacker would already need to have root, so then you're in huge trouble anyways. And yes, it may be weaker than a full-random solution. But a pseudorandom system that gets accepted is more secure than a full-random system that doesn't get accepted.

This would sort of negate the purpose of ASLR, as afaik the whole point is an attacker would not know the mem layout. The very fact its not reproducible is the solution and the problem!

No. If the seed would be made available only in a kernel bug report.

Yes, that is what frederikvs probably means but access to the seed could become a new weakest link of ASLR. Presumably only available to CAP_SYS_ADMIN/uid 0, but it's worth a great deal of caution in designing the feature that allows determining what the seed was.

Further adding to that, the seed could be changed before that bug report was submitted. A dedicated tool with a FSM and minimal privileges does it. That lets us verify it strongly.