how would you answer this question?

Hello Everybody!
This is not a real programming question, but it's about linux..
At high level, how would you answer the following question:

"What does it happen in Linux when a process (running in User Mode) tries to address a byte of memory belonging to the address space reserved to the Kernel?"

I'd answer like this:
"The paging circuit will lookup the PAGE DIRECTORY of the process and, not finding the requested address, will raise a PAGE FAULT. The page fault handler will then find out that the address doesn't belong to the process' address space and then terminate the process".

I don't know if the USER/SUPERVISOR flag plays some role in this case..?

Thanks for your comments! Bye!

Quote:

---

and then terminate the process

---

Actually, it will raise a SIGSEGV. The default action for this signal is to terminate the process, but it can be caught and handled.

> I don't know if the USER/SUPERVISOR flag plays some role in this case..?
No, why would it?

Details, assuming x86:
There are several levels of page-table [x86 has 2, 3 or 4 depending on what the mode of the processor is]. At any of these levels, the processor can determine that the page is not accessible, which is signaled by a PAGE FAULT (PF).

The PF handler in the kernel will look at the CR2 (virtual address being accessed) as well as the error code [this indicates the type of error/access attempted, e.g. read, write, not present or privilege violation].

The USER bit in the page-table WILL have an effect here, since the user-mode application attempts to access an address that is [or could be] valid in the kernel-side, so the value value of CS mode bits (lower 2 bits of the CS register) will indicate whether the code is in kernel mode or user-mode. The page-table entry will say if the page is allowed to be accessed in user-mode or only kernel-mode. So if the page is valid, the kernel would have set it as "kernel only".

As discussed, if the PF handle determines that the page should not be used by user-mode, then it will issue a SEGV signal to the user-app.

--
Mats
Mats