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.
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