How does dynamic dispatch work in WebAssembly?

Someone should make a CPU that does this. Intel CET is a bit of a start, but a CALL instruction that takes a “type” operand and only calls functions of that type would be a big improvement. So would a totally separate return address stack.

There is also pointer authentication in ARM: https://lwn.net/Articles/718888/

It seems pretty difficult to define function types at the ISA level, since anything to do with typing is language/VM-specific. What types can arguments have? Are varargs supported? Multiple parameter returns?

Maybe the type would just be an integer that the ABI would assign meaning to. But if you did that, how would the function's type be determined? Some pseudo-instruction at the CALL target? I guess looking at CET it does have some things like this: the ENDBRANCH instructions notate valid indirect branch targets.

What class of errors do you think this would catch that aren't already covered by CET? Simply bolting on another equality check doesn't strike me as all that useful. The type has to live with the code or data, neither one strikes me as easy, both have huge downsides.

And what about CET's shadow stack is deficient compared to a "totally separate return address stack"?

From playing around with https://mbebenita.github.io/WasmExplorer/ : it seems like any function you ever take the address of gets stored in the table, and wasm just passes the index in that table around. And if you do things like cast an integer to a function pointer, you actually just cast it to an index in the table.

C is high-level enough that IIRC it doesn't guarantee that arbitrary addresses can successfully be used as function pointers; the only valid function pointers are results of addresses of functions, or (possibly) values that have been cast from and then to them. Which is enough for wasm.

Try compiling this (to wat, which is an S-expression format that otherwise resembles the assembly in this article): https://wasdk.github.io/WasmFiddle/?5zbjb