Accessing user space data from linux kernel

Question

This is an assignment problem which asks for partial implementation of process checkpointing:

The test program allocates an array, does a system call and passes the start and end address of array to the call. In the system call function I have to save the contents in the give range to a file.

From my understanding, I could simply use copy_from_usr function to save the contents from the give range. However since the assignment is based on topic "Process address space", I probably need to walk through page tables. Say I manage to get the struct pages that correspond to given range. How do I get the data corresponding to the pages?

Can I just use page_to_virt function and access data directly? Since the array is contiguous in virtual space, I guess I will just need to translate the starting address to page and then back to virtual address and then just copy the range size of data to file. Is that right?

I think copy_from_usr() is ok. Nothing else needed.

tian_yufeng
– tian_yufeng

2013-03-15 01:46:07 +00:00
Commented Mar 15, 2013 at 1:46 — tian_yufeng
– tian_yufeng, Commented Mar 15, 2013 at 1:46

tian_yufeng · Accepted Answer · 2013-03-15 03:07:01Z

2

I think copy_from_user() is ok, nothing else needed. When executing the system call, although it trap to kernel space, the context is still the process context which doing the system call. The kernel still use the process's page table. So just to use copy_from_user(), and nothing else needed.

Okey, if you want to do this experiment, I think you can use the void __user *vaddr to traverse the mm->pgd(page table), using pgd_offset/pud_offset/pmd_offset/pte_offset to get the page physical address(page size alignment). Then in kernel space, using ioremap() to create a kernel space mapping, then using the kernel virtual address(page size) + offset(inside the page), you get the start virtual address of the array. Now in kernel, you can using the virtual address to access the array.

edited Mar 15, 2013 at 3:07

answered Mar 15, 2013 at 1:49

tian_yufeng

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1 Comment

vaibhav Over a year ago

What if I want to use the other approach- walk the page table and get relevant pages? The assignment requires me to demonstrate knowledge of kernel code relevant to memory management, so I will have to do the translation to pages.

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