Seems to make sense. If they didn't allocate small/big frames but
exactly just enough space for the arguments this will indeed fail.
You could also change the DNU to reallocate the frame on demand; maybe
even only if the argsize is 0? I don't know the implementation details
obviously, but it seems like you would waste less space to do it that
way; although you will get a slight performance hit for the DNU, but
that seems less relevant.
cheers,
Toon
On 05/06/2011 07:39 PM, Eliot Miranda wrote:
Hi All,
could you check my reasoning here. I think I've found a serious
bug in the VM with doesNotUnderstand: processing for zero-argument
sends and the calculation of the necessary stack size in
MethodNode>>generate:* (senders of CompiledMethod>>needsFrameSize:).
The bug causes a push beyond the end of a context, overwriting the
header of the following object, when an MNU occurs for a zero-argument
send when the stack is full. If you look at
Interpreter>>createActualMessageTo: it does the following to push the
message created as the argument of the doesNotUnderstand:
self pop: argumentCount thenPush: message
If argumentCount is zero this pushes an item on the stack. If the
stack is full at this point (the receiver of the not-understood
message is the last item in the context) then the push will be out of
bound of the activeContext's stack and overwrite the first word of the
following object's header.
We observe this with a Newspeak application at Cadence and see a
subsequent VM crash.
1. am I right?
2. is this an old bug?
3. did this, or did this not, affect Smalltalk-80 v2 and e.g. the
Dorado implementation, and if so why (e.g. was the message of an MNU
not pushed onto the stack, and instead MNUs activated by special
code?, or was the activeContext not on the heap? or...)
4. what's the right fix? It seems to me that the right fix is simply
to add one to the needed stack depth, with an explanatory comment, but
in senders of CompiledMethod>>needsFrameSize: or
in CompiledMethod>>needsFrameSize:? Trying to avoid pushing the
argument of an MNU is going to involve lots of changes to the VM and
will be tricky and likely have a bug tail.
best,
Eliot
