On Fri, Mar 29, 2013 at 12:45 PM, Jacky <jackyclivia@163.com> wrote:

 Hi All,

When mmaping elf image into memory, why offset vma need to subtract eppnt->p_vaddr as the following code ?

static unsigned long elf_map()
{
    ...
    unsigned long off = eppnt->p_offset - ELF_PAGEOFFSET(eppnt->p_vaddr);
    ...
}

      Assume you have logically divided the contents of the ELF file into chunks of PAGE_SIZE (Typically 4K on x86). Let us name each chunk as file page-synonym to page frame in the physical address space and page in the virtual address space respectively.
Here variable off is the file page base offset - synonym to page base address or page frame base address which are typically multiple of PAGE_SIZE.

      off >> PAGE_SHIFT is the value stored in vm_pgoff in struct vm_area_struct
In a nutshell vm_pgoff is the page number (file page number) in the ELF file where the corresponding PT_LOAD segment starts.

Suppose if a file offset (eppnt->p_offset) of a PT_LOAD segment is 9560 (0x2558) bytes into the file and eppnt->p_vaddr is 0x08048558. Then the file page number base address off is calculated as
 
               off = 0x2558 - 0x558; // ELF_PAGEOFFSET(eppnt->p_vaddr) will expand to 0x558.
Then off is 0x2000 and vm_pgoff is (off >> PAGE_SHIFT) = 2. This means that the PT_LOAD segment starts at 2nd page or 2nd file page in the ELF file.

Thanks in advance.

Jacky

--
Regards,
Prabhunath G
Linux Trainer
Bangalore