See #3280 and #3369.

This is a ~equivalent version of #3369 using Lua expressions instead of CEL expressions. This uses the "vendored" feature of mlua so that a system Lua interpreter doesn't need to be installed. I chose the "lua54" feature because 54 was the biggest number.

Unfortunately, I don't see a way to compile the expressions ahead of time, so they're compiled each time they're evaluated. (Compiled functions are non-send non-sync Function<'lua> where 'lua is the lifetime of the interpreter, which leads to issues.)

I wrote this PR without much thinking and without knowing anything about Lua, but it seems to work.

experimental.syscall_overrides

Default: {}
Type: Object

Override the behaviour of emulated syscalls.

A map of syscall numbers to lists of Lua expressions. For each syscall
number listed, a list of expressions will be evaluated before running Shadow's
syscall handler. If an expression returns a non-nil signed integer result, that
result will be used as the return value for the syscall. Otherwise if all
expressions return nil, Shadow will perform the syscall as usual.

The following variables are available in expressions:

• args: Array of syscall arguments cast as integers.
• host: The hostname of the current host.
• process: The name of the current process.
• pid: The pid of the current process.
• tid: The tid of the current thread.

Example:

experimental:
  syscall_overrides:
    # SYS_SETSOCKOPT = 54, SOL_IP = 0, IP_BIND_ADDRESS_NO_PORT = 24, IP_TTL = 2
    54:
    - # setsockopt(_, SOL_IP, IP_BIND_ADDRESS_NO_PORT, ...) -> 0
      "(args[1] == 0 and args[2] == 24) and 0 or nil"
    - # setsockopt(_, SOL_IP, IP_TTL, ...) -> 0
      "(args[1] == 0 and args[2] == 2) and 0 or nil"

You must not modify global state or do anything weird within an expression. In
other words, there should be no side effects. The Lua interpreter may be reused.

To easily find the integer value for a given libc constant, you can search for
the constant name within the libc crate's documentation. For example to find
the integer value of SO_REUSEADDR, you can search for
SO_REUSEADDR at https://docs.rs/libc/latest/libc/.