Dumping the pfn from /proc/<pid>/pagemap does not give the expected content - c

Question

I'm using this code http://fivelinesofcode.blogspot.com/2014/03/how-to-translate-virtual-to-physical.html to dump the pfn related to a given virtual address taken from /proc/"pid"/maps.
Once I get the PFN, I dump it with a specific kernel module. This is a snippet of the code:
static int write_pfn(phys_addr_t pfn)
{
struct page *p;
void *v;
int s =0,ret =0;
p = pfn_to_page((pfn) >> PAGE_SHIFT);
v = kmap(p);
DBG("Writing page %d(mapped addr=0x%lx) - pfn: 0x%lx", p,v,pfn);
s = write_vaddr(v, PAGE_SIZE);
if (s != PAGE_SIZE) {
DBG("Error sending page %d(addr=0x%lx)", s,v);
return (int) s;
ret-=1;
}
kunmap(p);
return ret;
}
However, I have noticed that if I compare the PFN dumped with the kernel module with the real content of the corresponding virtual address inside the process , than the content is completely different.
Note that I dump the content of the process virtual address using the command "x" from (gdb).
Any idea? This is my kernel version:
Linux 3.14.7-rt5 #1 SMP Mon Jun 23 14:55:19 CEST 2014 x86_64 GNU/Linux

Answer 1

Logically your solution is correct, but i'am not sure about correctness of the pagemap reader code. Anyway, there is a way to check it. While you are inside kernel (in the context of target application) you could get page corresponding to specified VA using this kernel API:
1. mm = current->mm
2. struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
3. static inline struct page *follow_page(struct vm_area_struct *vma,
unsigned long address, unsigned int foll_flags)
4. page_to_pfn
You can implement this and compare with your results, Hope this will help.

Answer 2

Thank you Alex.
The problem with your solution is that some symbols are not exported (e.g. follow_page()) therefore I can't use them in my module.
However, I found this solution:
mm = ts->mm;
pgd_t * pgd = pgd_offset(mm, vaddr);
pud_t * pud = pud_offset(pgd, vaddr);
pmd_t * pmd = pmd_offset(pud, vaddr);
pte_t * pte = pte_offset_map(pmd, vaddr);
p = pte_page(*pte);
if(p)
DBG("page frame struct is # %p", p);
v = kmap(p);
DBG("Writing page 0x%lx(mapped addr=0x%lx) - pid: %d", v,vaddr,pidnr);
s = write_vaddr(v, PAGE_SIZE);
if (s != PAGE_SIZE) {
DBG("Error sending page %d(addr=0x%lx pid=%d)", v,vaddr,pidnr);
ret-=1;
}
kunmap(p);
pte_unmap(pte);