Wrap access to the user address space

[mingzi47]

Hello,

First of all, thank you and the team for your hard work on developing and maintaining the axsignal crate.

Recently, I have been adding a Copy On Write (COW) mechanism in starry-next. The COW mechanism removes the w (write) flags from the user memory space after fork. When a write is needed in the user space, a page_fault is triggered. According to the reference count of the physical page frame, the w flag is either added back to the physical page frame or a new page is copied.

In this crate, direct access to the user address space, which may trigger a page_fault due to COW. Therefore, some checks should be done before accessing the user space.

https://github.com/Starry-OS/axsignal/blob/main/src/api/thread.rs#L70-L90

In starry-next, the user address space is typically accessed using UserPtr. I believe you should also unify the management of user address space access. Perhaps you could pass the construction of UserPtr as a parameter to the functions that need to access the user address space, enabling safe access to user memory.

Thanks again for your excellent work!

[AsakuraMizu]

If we want to do memory address checking in axsignal, we first need a kernel-independent abstraction over UserPtr, which we tried before at https://github.com/Starry-OS/axptr, but it's not ideal. I'll pick this up as soon as possible.

[AsakuraMizu]

Unfortunately, we haven't been able to make any real progress with axptr (soundness, etc.), so let's see if we can solve your problem first.

If I understand correctly, what we need here is to call check_region_access (and populate_area if check_region_access returns false)? But what if populate_area fails here?

[mingzi47]

If I understand correctly, what we need here is to call check_region_access (and populate_area if check_region_access returns false)?

You should do the check like in ptr::check_region.

https://github.com/oscomp/starry-next/blob/main/api/src/ptr.rs#L9

If check_region_access returns true, then call populate_area.

But what if populate_area fails here?

If check_region_access or populate_area returns false, the process should be stopped.

You can also look at how Asterinas does it:

https://github.com/asterinas/asterinas/blob/main/kernel/src/process/signal/mod.rs#L111C1-L126C10

[AsakuraMizu]

ping @Mivik