Re: [MirageOS-devel] Compiling C components for mirage-xen

[Thomas Leonard <talex5@xxxxxxxxx>]

On 8 January 2016 at 11:19, Tim Cuthbertson <tim@xxxxxxxxxxx> wrote:
> I'd like to use `Bcrypt` from the `safepass` opam module in passe[0].
> There is sadly no `safepass-xen` package, and trying to just use
> `safepass` results in link errors:
>
>
> <...>/lib/safepass/libsafepass_stubs.a(crypt_blowfish.o): In function
> `BF_crypt':
> (.text+0x1b97): undefined reference to `__errno_location'
> <...>/lib/safepass/libsafepass_stubs.a(crypt_blowfish.o): In function
> `BF_crypt':
> (.text+0x2112): undefined reference to `__errno_location'
> <...>/lib/safepass/libsafepass_stubs.a(crypt_blowfish.o): In function
> `_crypt_blowfish_rn':
> (.text+0x21ea): undefined reference to `__errno_location'
> <...>/lib/safepass/libsafepass_stubs.a(crypt_blowfish.o): In function
> `_crypt_gensalt_blowfish_rn':
> (.text+0x2575): undefined reference to `__errno_location'
> <...>/lib/safepass/libsafepass_stubs.a(crypt_blowfish.o): In function
> `_crypt_gensalt_blowfish_rn':
> (.text+0x25a6): undefined reference to `__errno_location'
> <...>/lib/safepass/libsafepass_stubs.a(crypt_blowfish.o):(.text+0x25b7):
> more undefined references to `__errno_location' follow
>
>
> I'm assuming I'd need to customise the build process tp produce an
> appropriate C library as with `gmp-xen`, `zarith-xen` etc. The trouble
> is twofold:
>
>  - I don't really know what the thing I'm doing is. Is mirage-xen a
> different architecture? A different ABI? Do C libraries need to be
> cross-compiled, or just compiled with particular flags so that they'll
> work on xen-minios instead of gnu/linux?

C libraries need to be compiled with different headers and flags (in
particular, -mno-red-zone). Using libraries compiled for Linux will
often appear to work, but may fail when interrupts occur, so the build
makes it hard to use them by accident.

However, OCaml binaries can be shared (ocamlopt doesn't use a red zone
and the OCaml ABI is the same).

>  - The existing scripts seem to be pretty ad-hoc - zarith gets away
> mainly with particular CFLAGS
> (https://github.com/ocaml/opam-repository/blob/master/packages/zarith-xen/zarith-xen.1.4/files/mirage-install.sh),

It's using pkg-config to get the important flags. The others are
optimisations (except -fno-builtin, which is either not needed or
should be supplied by Mini-OS).

> while gmp looks like a full cross-compilation using `--target` (and
> then does some other sneaky things).
>
> safepass uses OASIS for its build system, which doesn't seem to have
> any cross-compilation functionality built in as far as I could tell.
> Can anyone give me pointers on what I'd need to do to safepass' build
> scripts to get this working in mirage-xen, or at the least some
> clarification on why it doesn't work as-is and how cross compilation
> works for C libraries used in mirage-xen?
>
> [0]: https://github.com/timbertson/passe

The io-page library provides a fairly simple template:

https://github.com/mirage/io-page/blob/master/_oasis

It builds io_page_unix and io_page_xen libraries. The C flags are
added to the main META file with:

XMETAExtraLines: xen_linkopts = "-lio_page_xen_stubs"

There's a command to get the Xen flags:

PostConfCommand: ocaml postconf.ml

The Xen source file (stub_alloc_pages_xen.c) is just a symlink to the
Unix one (you need a separate file because OASIS can't compile the
same source path with different flags).


-- 
Dr Thomas Leonard        http://roscidus.com/blog/
GPG: DA98 25AE CAD0 8975 7CDA  BD8E 0713 3F96 CA74 D8BA

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