Initialize LastCallee* of CallContext

[han0110]

This PR is once #278, and aims to initialize last callee's information of CallContext in BeginTx.

For spec, see privacy-scaling-explorations/zkevm-specs#99.

It depends on #292.

[DreamWuGit]

want to know why call last callee in begin_tx ?

[han0110]

To avoid a malicious prover set these fields to read arbitrary call's memory, we need to make sure it's initilized to 0 properly

[ed255]

LGTM!

I have a question though:
LastCallee* rows are always READ right?

But in the same call, we can have different values of LastCallee.
Imagine the following situation

Begin CALL 1
  Begin CALL 2
  End CALL 2
  Begin CALL 3
  End CALL 3
End Call 1

I imagine that the LastCalleeId rows for callId = 1 (in rw_table) will be:

0 rwc	1 isWrite	2 Key0 (Tag)	3 callId	5 Key3	7 Value0
1	false	CallContext	1	LastCalleeId	0
2	false	CallContext	1	LastCalleeId	2
3	false	CallContext	1	LastCalleeId	3

Is this right? If that's the case, then these rows don't follow the READ constraint, because for the same set of keys, the value is changing. Am I missing something?

[han0110]

Fields LastCallee* will be written when callee switch context back to caller (e.g. STOP, RETURN, REVERT, all the other ExecutionState that halt). So in your case it would be like:

0 rwc	1 isWrite	2 Key0 (Tag)	3 callId	5 Key3	7 Value0
1	false	CallContext	1	LastCalleeId	0
2	true	CallContext	1	LastCalleeId	2
3	false	CallContext	1	LastCalleeId	2
4	true	CallContext	1	LastCalleeId	3
5	false	CallContext	1	LastCalleeId	3

There is also implemented in privacy-scaling-explorations/zkevm-specs#100 and the writes are here

[ed255]

Fields LastCallee* will be written when callee switch context back to caller (e.g. STOP, RETURN, REVERT, all the other ExecutionState that halt). So in your case it would be like:
...

Thanks! This resolves my doubt. I had the misunderstanding that LastCalleeId* rows where never WRITE.

[DreamWuGit]

LGTM thanks !

[DreamWuGit]

just thinking how about if we only record LastCalleeId without LastCalleeReturnDataOffset LastCalleeReturnDataOffset ? then we can look up those two items by LastCalleeId as already have ReturnDataOffset ReturnDataOffset , something missing ?

[han0110]

Fields LastCalleeReturnData* are decided by the callee's step which halts.

Fields ReturnData* are decided by the caller's step which triggers the call, to ask callee to copy the ReturnData to the specified range.

So they are for different purposes.

[CPerezz]

Should we update and merge this @han0110 ?