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

[Tim Cuthbertson <tim@xxxxxxxxxxx>]

On Fri, Jan 8, 2016 at 11:14 PM, Thomas Leonard <talex5@xxxxxxxxx> wrote:
> 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).

Thanks, Thomas. - that helps quite a lot

I think I've followed the same pattern now, but still haven't been
able to get safepass building.

I'm using the following for CCOpt:

`pkg-config --cflags --static mirage-xen-ocaml`

Now the linking phase fails with:

+ ocamlfind ocamlmklib -o src/safepass_stubs src/crypt_blowfish.o
src/bcrypt_stub.o
ld: errno: TLS definition in /nix/store/<...>-glibc-2.21/lib/libc.so.6
section .tbss mismatches non-TLS reference in src/crypt_blowfish.o
/nix/store/<...>-glibc-2.21/lib/libc.so.6: could not read symbols: Bad value
collect2: error: ld returned 1 exit status
Command exited with code 2.
E: Failure("Command ''ocamlbuild' src/libsafepass_stubs.a
src/dllsafepass_stubs.so src/safepass.cma src/safepass.cmxa
src/safepass.a src/safepass.cmxs -tag debug' terminated with error
code 10")

This seems to be something special about errno being thread-local in
glibc. Googling turns up a lot of advice on fixing this problem, but
all of it assumes you're just building on linux for linux.
bcrypt_stubs.c does <errno.h>, which I believe is being provided by
minios-xen-0.8/include/minios-xen/errno.h.

However, at link time I don't know where errno is supposed to come
from. It seems to be coming from glibc, but shouldn't it be coming
from minios instead given that's where the errno header comes from? I
poked around with `nm` in minios' built files but couldn't find
anything that actually defined errno.

 -- other things I've tried:

1) I tried using `pkg-config --cflags mirage-xen-ocaml` for CCLib,
however `ocamlfind ocamlmklib` didn't seem to like that - it didn't
understand "-m elf_x86_64" nor
"-T/nix/store/<...>-minios-xen-0.8/lib/minios-xen/libminios.lds"

2) I notice that minios-xen/errno.h has:

#ifdef HAVE_LIBC
#include <mini-os/sched.h>
extern int errno;
#define ERRNO
#define errno (get_current()->reent._errno)
#endif

So just to see what happened, I tried adding -DHAVE_LIBC. But then I
get errors because sys/reent.h can't be found, so that's probably not
a useful path to explore.

Any advice where to go from here?

Thanks,
 - Tim.

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