Implement SharedMemory

[roblabla]

This PR depends on #28 . DO NOT MERGE BEFORE MERGING #28

• Implements Create/Map/Unmap shared memory syscalls in kernelspace
• Implements the equivalent syscall in userspace
• Add SharedMemory and MappedSharedMemory userspace types.
• MappedSharedMemory contains some unsafe functions that should probably not
  be used...

[todo]

RE the switch: can we map a subsection of a shared memory?

https://github.com/roblabla42/KFS/blob/341469f276d7fcaeec4ab6884a7d960b96f84cf0/kernel/src/interrupts/syscalls.rs#L246-L256

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This comment was generated by todo based on a TODO comment in 341469f in #30. cc @roblabla.

[todo]

Can we unmap a subsection of a shared memory?

https://github.com/roblabla42/KFS/blob/341469f276d7fcaeec4ab6884a7d960b96f84cf0/kernel/src/interrupts/syscalls.rs#L264-L274

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This comment was generated by todo based on a TODO comment in 341469f in #30. cc @roblabla.

[todo]

Handle MemoryAttributes and refcounts in query_memory

https://github.com/roblabla42/KFS/blob/341469f276d7fcaeec4ab6884a7d960b96f84cf0/kernel/src/interrupts/syscalls.rs#L296-L306

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This comment was generated by todo based on a TODO comment in 341469f in #30. cc @roblabla.

[todo]

PageInfo Handling

https://github.com/roblabla42/KFS/blob/341469f276d7fcaeec4ab6884a7d960b96f84cf0/kernel/src/interrupts/syscalls.rs#L305-L313

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This comment was generated by todo based on a TODO comment in 341469f in #30. cc @roblabla.

[todo]

Extend MappingType to cover all MemoryTypes

https://github.com/roblabla42/KFS/blob/341469f276d7fcaeec4ab6884a7d960b96f84cf0/kernel/src/paging/mapping.rs#L48-L58

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This comment was generated by todo based on a TODO comment in 341469f in #30. cc @roblabla.

[Orycterope]

So, if I understand correctly, closing a SharedMemory handle leaves it mapped, but makes it impossible for the process to unmap it. The SharedMemory is not dropped, because an Arc remains in the process' bookkeeping.
Is that it ?

[roblabla]

@Orycterope no. When you map a SharedMemory, MappedSharedMemory gets ownership of the underlying handle. As such, the handle cannot be closed until the MappedSharedMemory gets unmapped (which will close the handle).

[todo]

Frame Allocator should take lengths in bytes.

https://github.com/roblabla42/KFS/blob/c6d95b32cbd1b6947ed251463c1f76731be73d70/kernel/src/interrupts/syscalls.rs#L235-L245

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This comment was generated by todo based on a TODO comment in c6d95b3 in #30. cc @roblabla.

[Orycterope]

👍