zfs/module/lua

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Introduction
------------

This README describes the Lua interpreter source code that lives in the ZFS
source tree to enable execution of ZFS channel programs, including its
maintenance policy, the modifications that have been made to it, and how it
should (and should not) be used.

For a description of the Lua language and features exposed by ZFS channel
programs, please refer to the zfs-program(1m) man page instead.


Maintenance policy
------------------

The Lua runtime is considered stable software. Channel programs don't need much
complicated logic, so updates to the Lua runtime from upstream are viewed as
nice-to-have, but not required for channel programs to be well-supported. As
such, the Lua runtime in ZFS should be updated on an as-needed basis for
security vulnerabilities, but not much else.


Modifications to Lua
--------------------

The version of the Lua runtime we're using in ZFS has been modified in a variety
of ways to make it more useful for the specific purpose of running channel
programs. These changes include:

1. "Normal" Lua uses floating point for all numbers it stores, but those aren't
   useful inside ZFS / the kernel. We have changed the runtime to use int64_t
   throughout for all numbers.
2. Some of the Lua standard libraries do file I/O or spawn processes, but
   neither of these make sense from inside channel programs. We have removed
   those libraries rather than reimplementing them using kernel APIs.
3. The "normal" Lua runtime handles errors by failing fatally, but since this
   version of Lua runs inside the kernel we must handle these failures and
   return meaningful error codes to userland. We have customized the Lua
   failure paths so that they aren't fatal.
4. Running poorly-vetted code inside the kernel is always a risk; even if the
   ability to do so is restricted to the root user, it's still possible to write
   an incorrect program that results in an infinite loop or massive memory use.
   We've added new protections into the Lua interpreter to limit the runtime
   (measured in number of Lua instructions run) and memory overhead of running
   a channel program.
5. The Lua bytecode is not designed to be secure / safe, so it would be easy to
   pass invalid bytecode which can panic the kernel. By comparison, the parser
   is hardened and fails gracefully on invalid input. Therefore, we only accept
   Lua source code at the ioctl level and then interpret it inside the kernel.

Each of these modifications have been tested in the zfs-test suite. If / when
new modifications are made, new tests should be added to the suite located in
zfs-tests/tests/functional/channel_program/lua_core.


How to use this Lua interpreter
-------------------------------

From the above, it should be clear that this is not a general-purpose Lua
interpreter. Additional work would be required to extricate this custom version
of Lua from ZFS and make it usable by other areas of the kernel.