How to get process's virtual address in Linux kernel

Question

Currently, I'm trying to figure out how to get the virtual address (VA) of a specific process in the Linux kernel, since there are several functions taking VA as an argument related to different page directories, including pgd_offset(), pgd_index(), p4d_offset(), p4d_index()...

1. Could anyone explain the functions of these functions, including xxx_offset(), xxx_index()?(xxx:pgd, p4d, pmd...) And how to use these functions？

2. What does the VA mean when it is taken as an argument of functions mentioned above, is that the virtual address of the process? And how can I get the VA of a specific process? I've already known that we can use process's task_struct->mm->mmap to get the range of the virtual address space, but no idea about how to get a specific virtual address.

3. Is the task_struct->mm->pgd_t indicating the base address of PGD_directory?

Answer 1

Your question doesn't really make sense. You don't "get a virtual address of a process". A process has a virtual address space that serves as a virtual memory map for data, code, stack, heap, etc.

1. Those functions are taking a single virtual address within the process virtual address space and helping with walking through the page tables to find its page table entry and then its physical address (or checking page table entry flags). In Linux, there are 4 page tables levels to go through to get to the page table entry. Normally the levels are pgd (page table directory), pud (page upper directory), pmd (page mid directory), and pte (page table entry). But I think recently p4d was added as an extra page table level. Typically, the address of the page directory (top-level page table) is stored in the CR3 register. So you use that address to access the directory, then use the pgd_index and pgd_offset to find the address of the next level (p4d) you need to look into, and repeat till you hit the pte. A useful file to see this in action is the mm/page_walk.c file.

2. A process accesses memory during its runtime and typically this memory is referred to by virtual addresses. When it accesses an address that isn't in the TLB, the address must be walked through as described above to find out its location and permissions flags. There is no "getting the VA of a process", but when your program uses mmap or malloc and you get addresses of variables, those addresses are typically virtual addresses. You can look in /proc/proc_number/maps to see the virtual address layout of a process with PID proc_number . Note that with address space layout randomization turned on, this map will be different every time you run the same program.

3. I'm not sure, but you probably can test it by comparing that variable with the pgd address used in the page_walk.c file I linked above.