Query regarding ELF loader arg style

Eric W. Biederman ebiederm at xmission.com
Fri Jan 17 13:03:10 EST 2014

Vivek Goyal <vgoyal at redhat.com> writes:

> On Thu, Jan 16, 2014 at 03:27:13PM -0800, Eric W. Biederman wrote:
> [..]
>> > - ELF does not tell anything whether it is self relocating or not. So we
>> >   are forced to load it at a address it has been compiled for (In case of
>> >   kdump). And that address is already occupied by current running kernel
>> >   so it does not work.
>> ELF does tell you if it is relocatable in the e_type field of the main
>> elf header.  In particular ET_DYN vs ET_EXEC.  ET_DYN can be loaded
>> anywhere, and ET_EXEC must be loaded at the specified address.  It would
>> not surprise me if the kernel build process is using ET_EXEC in error.
>> ET_DYN is also what is used for PIE executables and shared libraires.
>> The rule with ET_DYN without a dynamic linker is that a constant offset
>> must be added to the load address of all of the program segments but
>> otherwise nothing changes.  I thought I had support for that form in
>> kexec-tools.
>> Perhaps not.
> Hi Eric,
> Kexec-tools supports ET_DYN. I saw code for that. Current kernel vmlinux
> is ET_EXEC though and expects to be loaded at the address for it has been
> compiled for.
> I think in intial version of patches I will just support ET_EXEC and then
> support for ET_DYN can be added later. It should really be simple as  all
> the core structure to find a memory hole and load buffers there will
> already be present. And at that time one can look into converting vmlinux
> to a ET_DYN instead of ET_EXEC.

Sounds good.


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