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Module_Requirements.md

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Layer 4.5 Module Requirements

Purpose:

Set the requirements for module development. Not properly handling memory in modules can easily result in a crashed kernel.

RULES:

  1. Modules must register a valid send and recv function pointer. If one path is not desired to customize, then use the no_cust flag to skip the customization in that function.

  2. Do not free the modules send or recv buffer, this is handled by Layer 4.5

  3. If copy_length is set to 0, then no data will be delivered to the application or layer 4. This allows for dropping data. (Needs some testing)

  4. if copy_length is < 0, this signals an error has occurred

  5. Setting the no_cust flag on the buffer allows skipping the customization for this round

  6. Setting set_cust_to_skip will bypass all future customization for this socket (send and recv)

  7. I do not recommend using memcpy when dealing with the src_iter buffer. This buffer is not guaranteed to be linear and you may alter memory that is fragile. Instead use kernel provided methods when dealing with the src_iter structure.
    These include:

    • copy_from_iter_full: which guarantees a full copy is performed or fails and reverts to the previous state

    • iov_iter_advance: adjusts the iter buffer X bytes and properly sets all relevant values of the structure

    • iov_iter_revert: adjust iter buffer X bytes backwards

  8. Do not modify the src_iter structure given to the module, unless done as side effect of standard function (copy_from_iter_full)

  9. to autoload a module on reboot, put the .ko file in /usr/lib/modules/$(uname -r)/layer4_5/customizations folder

  10. Modules now have a activated setting to allow a module to attach to a socket, but not customize the traffic if active_mode is set to false. This setting can be toggled via the NCO/DCA.

  11. A deprecated module will still customize a socket that it was previously attached to when it became deprecated. The module will not attach to new sockets once it has transitioned to a deprecated state.

  12. pid and tgid may have different task names associated with them. tgid likely holds the application name you want, such as dig