Re: ELF Relocations and OCaml Code

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

On Wed, Jul 25, 2012 at 12:19:08PM +0200, PALI Gabor Janos wrote:
> Hi,
> 
> Yesterday I had a strange issue with unsupported ELF relocations while
> building a kernel module out of OCaml programs for FreeBSD/amd64.  I have
> finally resolved the problem, but Anil suggested me to send in here for the
> record.
> 
> I am working on the FreeBSD port of the Mirage OS module.  Everything seemed
> to compile and link fine, until I wanted to load the module using the
> kldload(8) command:
> 
> # kldload mirage.ko
> 
> Then the console was pounded by lot of linker errors:
> 
> kldload: unexpected relocation type 9
> ...
> kldload: unexpected relocation type 4
> link_elf_obj: symbol camlPervasives__entry undefined
> linker_load_file: Unsupported file type
> 
> kldload: can't load mirage.ko: Exec format error
> 
> By browsing the kernel sources, it turned out that the 64-bit FreeBSD kernel
> does not support ELF relocation types 9 (R_X86_64_GOTPCREL), 10 (R_X86_32) and
> 4 (R_X86_64_PLT32) (among many others).
> 
> So, in order to get rid of these, I had to compile all my OCaml sources as
> position-dependent code, without support for dynamic linking.  This means that
> ocamlopt must be called with the following flags: -fno-PIC and -nodynlink.
> 
> In addition, a custom OCaml stdlib has to be built in the same fashion and
> used when compiling the sources.  (By the way, I use -output-obj to get a
> large C object file with all the referenced code, so it could be linked into
> the kernel module.)  This can be overridden by setting the CAMLLIB
> environmental variable.
> 
> However, note that, in the OCaml run-time sources (which gets finally linked
> to the generated object code) the file called amd64.S should be also modified
> to remove the last pieces of PIC addressing.  That is, the lines
> 
> #define GREL(r) r@GOTPCREL
> #define GCALL(r) r@PLT
> 
> should be rewritten as
> 
> #define GREL(r) r
> #define GCALL(r) r

This change in amd64.S came from:
http://caml.inria.fr/mantis/view.php?id=4795
...which added PIC support for amd64 around ocaml-3.12.

It looks like the way to fix it via a new SYS_kfreeBSD #define, as the
choice of runtime can't be dynamically toggled based on -fPIC at the
moment.  That would be a bigger patch, and probably best left to the OCaml
team to decide if it's necessary or not.

-anil