On Fri, Mar 29, 2013 at 3:03 PM, Jacky <[email protected]> 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" <[email protected]> 写道： > > > > On Fri, Mar 29, 2013 at 12:45 PM, Jacky <[email protected]> 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
_______________________________________________ Kernelnewbies mailing list [email protected] http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
