Re: Idea to create a elf executable from running program [process2executable]

[Cedric Le Goater <clg@fr.ibm.com>]

vamsi krishna wrote:
> Hello All,
> 
> I have been working on an idea of creating an executable from a
> running process image.
> 
> MOTIVATION:
> Process migration among the nodes in distributed computing,
> checkpointing process state.
> 
> BASIS:
> 
> The basis of my idea would be update the existing executable with
> extra PHDRS (Program Headers) with type PT_LOAD and each of these
> headers corresponding the vaddr mapping from /proc/<pid>/maps.
> 
> I have done some basic study of kernels loders code in
> 'fs/binfmt_elf.c' especially code in 'load_elf_binary' function, the
> following is my understanding.
> <------------------------------------------>
> bss=0;
> brk=0;
> foreach (phdr in elf_header){
> 
>    if(phdr->type == PT_LOAD){
>         if( phdr->filesize < phdr->memsize){
>            /* Segment with .bss, so update brk and bss*/
>         }
>        else {
>           /* Just map it*/
>        }
>     }
> /*Update brk bss*/
> }
> <------------------------------------>
> 
> from the above the kernel is updating brk, thus creating the start of
> sbrk(0) only when it sees a  PT_LOAD segment with filesize<memsize. So
> if I create a elf executable with all PT_LOAD segments with out any
> segments with filesize < memsize. The kernel will set brk base i.e
> sbrk(0) to the value phdr->vaddr+phdr->memsize of the last PT_LOAD
> segment its mapping? so do I need to reoder my PT_LOAD segments so
> that the heap goes as the last PT_LOAD segment?

Why don't you let execve() finish its job before modifying the mapping ?

Once execve returns, the segments are mapped and you are free to remap them
however you want and fill them in with a state previously saved on disk.

C.