[klibc] Mips cross-compiling whos

[Ben Hutchings]

On Tue, 2022-07-26 at 22:30 +0200, Olliver Schinagl wrote:
> Hey list,
> 
> I'm trying to cross-compile klibc on mips. As I packaged it for alpine 
> before, I figured it shouldn't be that hard ;) Sadly, alpine doesn't 
> support mips[32|64] so I have to cross-compile it.

Cross-compilation should work, and I regularly test this for all the
architectures that Debian provides cross-compilers for.

> I'm doing this from within an alpine based docker container, using a 
> simple script (setup vars, unzip, make).
> 
> 
> There's a few things I don't understand why they are the way they are, 
> but have a somewhat easy work-around (haven't spend time to figure out 
> if it is the correct one at all or not :p).
> 
> 
> First, this work-around I have also in the alpine packages, since it's 
> just shuffling stuff around, but keeping all definitions, I don't 
> mind/care much.
> 
> ```
> 
>          if [ ! -e 'usr/include/sys/sysinfo.h.orig' ]; then
>              mv 'usr/include/sys/sysinfo.h' \
>                 'usr/include/sys/sysinfo.h.orig'
>          fi
>          cat \
>              '/usr/include/linux/sysinfo.h' \
>              'usr/include/sys/sysinfo.h.orig' > \
>              'usr/include/sys/sysinfo.h'
> ```

This doesn't make any sense to me.  Our sys/sysinfo.h already includes
<linux/kernel.h>, which includes <linux/sysinfo.h>.

> 
> excuse the uglyness :p
> 
> 
> Secondly, specificaly on mips, I get re-definition errors, I haven't 
> looked into detail why `f_owner_ex` is redefined, but I use the 
> following here:
> 
> ```
> 
>          sed -i \
>              -e '/^typedef struct flock {$/i # define 
> HAVE_ARCH_STRUCT_FLOCK/' \
>              -e '/^struct f_owner_ex {$/,+6d' \
>              'usr/include/arch/mips/klibc/archfcntl.h'
> 
> ```

It's also defined in <asm/fcntl.h>, which I think is "wrong" on MIPS. 
We try to prevent including both <klibc/archfcntl.h> and <asm/fcntl.h>,
though it looks like that check broke about 10 years ago in the UAPI
split!

> Lastly, _KLIBC_USE_RT_SIG supposedly 'can' be used on mips, but it 
> causes a `-1` unnamed array definition error, so 'can' means 'does not 
> needed to be', so I just disabled it.
> 
> 
> ```
> 
>          sed -i 's|^\(#define _KLIBC_USE_RT_SIG \).*$|\1 0|' 
> 'usr/include/arch/mips/klibc/archconfig.h'
> 
> ```

This is not a configuration option.  You can't change it without
changing the architecture implementation too.

[...]
> Now, to compile, I use
> 
> ```
> 
>          make \
>               KBUILD_REPRODUCIBLE=1 \
>               KLIBCARCH="mips" \
>               KLIBCKERNELSRC='/usr/' \
>               ;
> 
> ```
[...]

This causes klibc to use the kernel headers for the build machine's
architecture, which is wrong for cross-builds.  This is probably the
source of all or most of the errors.

If you have a working cross-compiler then it must have a copy of the
kernel headers for the target architecture installed somewhere, and you
need to tell klibc where that is or link to it.  I don't know how
Alpine packages cross-tools, but this is what we do in Debian:
https://salsa.debian.org/kernel-team/klibc/-/blob/master/debian/rules#L58

Ben.

-- 
Ben Hutchings
Larkinson's Law: All laws are basically false.
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