musl and kernel headers [was Re: system-images 1.4.2: od is broken; bzip2 is missing]

[Rich Felker <dalias@aerifal.cx>]

On Mon, Oct 05, 2015 at 10:08:10AM -0500, Rob Landley wrote:
> > Re musl: two cases of breakage:
> > 
> >   #include <netinet/in.h>
> >   #include <linux/if_bridge.h>
> > results in:
> >   In file included from /usr/include/linux/if_bridge.h:18,
> >                    from networking/brctl.c:67:
> >   /usr/include/linux/in6.h:32: error: redefinition of 'struct in6_addr'
> >   /usr/include/linux/in6.h:49: error: redefinition of 'struct sockaddr_in6'
> >   /usr/include/linux/in6.h:59: error: redefinition of 'struct ipv6_mreq'
> > 
> > and
> > 
> >   #include <linux/ethtool.h>
> >   #include <net/ethernet.h>
> > results in:
> >   In file included from /usr/include/net/ethernet.h:10,
> >                    from networking/ifplugd.c:41:
> >   /usr/include/netinet/if_ether.h:96: error: redefinition of 'struct ethhdr'
> > 
> 
> That I leave to Rich. :)

Including kernel headers is just really problematic. These days they
try to make it work on glibc with a complex dance between glibc's
headers and the kernel headers. You're likely to have the best luck by
including the libc headers first. I think Sabotage and possibly some
other musl-based dists are patching the kernel headers to make them
behave better; see:

https://github.com/sabotage-linux/sabotage/blob/master/pkg/kernel-headers

But it might be possible that we can just #define whatever macro the
kernel headers expect to suppress redefinitions, if it's in a reserved
namespace. This would only fix the case where the kernel headers are
included _after_ the libc/userspace ones, but it's not too ugly. I
probably won't get around to it myself but if anyone wants to check
what macros we'd need to define (and where), let's discuss it (cc'ing
musl list).

The cleaner approach is just avoiding including both the kernel
headers and libc/userspace headers for the same things in the same
file. In theory this may be hard in some cases, but I find that I can
almost always fix these sorts of errors during a build by commenting
out one or two #include lines.

Rich