M7: Process management

Currently your operating system has only been able to run one process, probably sosh. In this milestone you will implement the process related system calls: process_create, process_delete, my_id, process_status and process_wait. Obviously each new process should run in its own address space. This will require you to carefully manage seL4 address spaces.

Currently process_create need only run executables that have been archived by the cpio program and placed in the boot image. All the functionality for process creation can be found in the start_first_process() in main.c. You can use this as a guide to create a clean internal SOS interface to process creation and destruction.

sosh has an exec command. This command provides a simple interface to the process_create system call. In a similar style to UNIX shells, if the third argument to exec is an '&' then it will run the process in the background. Otherwise sosh will use process_wait to wait until the child process has finished executing.

Note: the difficult part of this milestone is not process creation, it is process deletion. Now you will discover whether the data structures you have chosen have kept enough information for you to clean up a process and return the resource to the various allocators.

Adding new applications

• Create the following folder structure for your new app, replacing <NEWAPP> with the name of your app (drop the angle brackets):

  mkdir -p projects/aos/apps/<NEWAPP>/src
          

• Use the following CMakeLists.txt for projects/aos/apps/<NEWAPP>/CMakeLists.txt:

  cmake_minimum_required(VERSION 3.7.2)
  
  project(<NEWAPP> C)
  
  set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -u __vsyscall_ptr")
  
  # list all source files here
  add_executable(<NEWAPP> EXCLUDE_FROM_ALL src/<NEWAPP>.c)
  # list libraries to link binary against
  target_link_libraries(<NEWAPP> muslc sel4 sosapi)
  
  # warn about everything
  add_compile_options(-Wall -Werror -W -Wextra)
  
  set_property(GLOBAL APPEND PROPERTY apps_property "$<TARGET_FILE:<NEWAPP>>")
  

• Modify projects/aos/CMakeLists.txt, adding the following line:

   add_subdirectory(apps/sosh)
  +add_subdirectory(apps/<NEWAPP>)
   # add any additional apps here
  

Design issues

As with most milestones, a lot of the design work will be working out suitable data structures to hold process information. You may also need to extend other data structures in your operating system to handle multiple processes.

You must set up the stack properly for every process for the process startup routine from muslc to work. The existing init_process_stack function already does this, use if for all processes.

Processes require some kind of ID. IDs should eventually be re-used, but they should not be re-used to soon to avoid race conditions.

New processes should have stdout and stderr already opened on file descriptors 1 and 2, respectively; this is assumed by muslc. For apps that require stdin, it must be explicitly opened before performing any I/O and must be allocated to file descriptor 0. If you implemented the lowest-available policy, simply open console as the first file syscall in your app. Since SOS implements a single-reader console policy, you must be prepared for this to fail.

Remember, anything allocated while a process runs should be de-allocated when it exits or is killed (e.g. in-kernel TCB, frames, paging file space, etc..).

Assessment

Demonstration

You should show sosh executing a sub-process and show that the ps and kill commands work. Hint: execing and killing multiple instances of sosh is a good test.

As always you should be able to explain the data structures and algorithms used.

Show Stoppers

• Leaking process IDs (e.g. a monotonically increasing counter with no strategy for wrap around).
• Recycling process IDs immediately.
• Only supporting < 16 processes.
• More than constant time lookup from process ID to the PCB.
• Not handling or cleaning up outstanding async requests when a process exists (or after it exits).
• Failing to free paging file space on process destruction.

Better Solutions

• A sound strategy for handling waiting on a process that exited quickly (before the call to wait).