How to get a physical address from a linear one using a different pgd

["Martin Candurra" <martincad@xxxxxxxxx>]

I had to get some data from a user process within a kernel module. That process wasn't the "current" so the process pgd was not the same as the current pgd (for this reason I couldn't use copy_from_user function).

 

I had been looking for some straight method (or function) to do it, but I could not find any. I finally did it translating the address  throw  the corresponding page directories and page tables, using the following code:

 

         "from" is a linear address in user space of a process using pgd (task->mm->pgd).

 

         if (!pgd_none(*pgd)) {
            pud = pud_offset(pgd, (unsigned long) from);
            if (!pud_none(*pud)) {
                pmd = pmd_offset(pud, (unsigned long) from);
                if (!pmd_none(*pmd)) {
                    ptep = pte_offset_map(pmd, (unsigned long) from);
                    pte = *ptep;
                    if (pte_present(pte)) {
                        page = pte_page(pte);
                    }
                    pte_unmap(ptep);
                }
            }
        }

 

Does somebody know if there is some kernel function that do this ? Or some copy_from_user function that allows to specify a different pgd (or a different task struct)?

 

Thanks in advance.

Best Regards.