handle SIGSEGV for segfaults in ushell

[pogobanane]

No description provided.

[mmisono]

(copy from #17

We need to gracefully shut down (and ideally restart) ushell thread if ushell causes SEGV.
We need to think

• How should we handle traps?
• How should we free memory used by ushell thread?
  • If we have MPK support, then it should be easy to know which memory is allocated by ushell thread
  • But, ushell might pass its memory buffer to unikernel's function (e.g., for device output), and then ushell can crash
  • How should we release such memory? Or such things won't happen usually?
• How to evaluate?
  • In [1] section 5.4.1, there is a fault isolation evaluation by fault injection. I think we can do a similar evaluation.

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• [1] Operating System Support for Safe and Efficient Auxiliary Execution (OSDI'22) https://www.usenix.org/conference/osdi22/presentation/jing

NOTE about orbid ([1])

• Use separate address space for strong isolation
• It's a schedulable entity (having own task_struct)