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

[Rich Felker <dalias@aerifal.cx>]

On Fri, Oct 09, 2015 at 09:11:08PM +0200, Denys Vlasenko wrote:
> On Thu, Oct 8, 2015 at 6:58 PM, Rich Felker <dalias@aerifal.cx> wrote:
> >> > 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.
> >>
> >> brctl.c  was including <linux/if_bridge.h> after <netinet/in.h>
> >
> > The problem is linux/libc-compat.h, which should fix this, only works
> > on glibc, by design. See:
> >
> > #ifndef _LIBC_COMPAT_H
> > #define _LIBC_COMPAT_H
> >
> > /* We have included glibc headers... */
> > #if defined(__GLIBC__)
> >
> > /* Coordinate with glibc netinet/in.h header. */
> > #if defined(_NETINET_IN_H)
> >
> > If you patch it like this:
> >
> > -#if defined(__GLIBC__)
> > +#if 1
> >
> > then it should mostly work but it's still all a big hack. I think
> > that's what distros are doing. The problem is that the same header is
> > trying to do two different things:
> >
> > 1. Provide extra linux-kernel-API stuff that's not in the
> >    libc/userspace headers.
> >
> > 2. Provide definitions of the standard types and constants for uClibc
> >    and klibc, which don't have complete libc headers and rely on the
> >    kernel headers for definitions.
> >
> > These two uses really should be separated out into separate headers so
> > that the latter only get included explicitly by uClibc and klibc and
> > otherwise remain completely unused. But that would require coordinated
> > changes/upgrades which are unlikely to happen. :(
> 
> Looking at kernel's libc-compat.h, it looks like you can get away
> with using __UAPI_DEF_foo's like this?
> 
> 
> #if  defined(__UAPI_DEF_SOCKADDR_IN) && __UAPI_DEF_SOCKADDR_IN == 1
> /* kernel already defined the struct, do nothing */
> #else
> struct sockaddr_in {
>         ...
> };

This would address the case where the kernel header is included first,
but it's not a case I or most of the musl community wants to support,
because there's no guarantee that the kernel's definitions of these
structures will actually be compatible with use elsewhere in the libc
headers, etc.

The other direction, suppressing kernel headers' definition of the
structs, is what we want to work, but they've restricted their logic
for that to only work when __GLIBC__ is defined. :(

> #undef __UAPI_DEF_SOCKADDR_IN
> /* tell kernel to not define the struct */
> #define __UAPI_DEF_SOCKADDR_IN 0
> #endif

We could do something like this but then we would need to keep up with
the list of all the __UAPI defines we need to suppress unwanted kernel
definitions.

What if we could get the kernel to change the #if defined(__GLIBC__)
to #if defined(__GLIBC__) || defined(__UAPI_DONTNEED_DEFS) or similar,
so that there would only be one macro we need to define, and the
kernel would then use the same logic it uses with glibc to suppress
all of these.

Rich