On Fri, Mar 29, 2013 at 3:03 PM, Jacky <jackyclivia@163.com> wrote:

Great ! Thanks Prabhunath.

Let me make sure I understand you correctly:

Substracting ELF_PAGEOFFSET(eppnt->p_vaddr) from variable off makes vma map the page-aligned file contents, right ?

Small change : Substracting ELF_PAGEOFFSET(eppnt->p_vaddr) from variable eppnt->p_offset makes vma map the page-aligned file contents

在 2013-03-29 16:56:29，"Prabhu nath" <gprabhunath@gmail.com> 写道：

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

-- Regards, Prabhunath G Linux Trainer Bangalore