Re: kernel header compatibility

[Rich Felker <dalias@libc.org>]

On Tue, Nov 08, 2016 at 08:54:23PM -0500, Felix Janda wrote:
> Rich Felker wrote:
> > On Tue, Nov 08, 2016 at 08:11:45PM -0500, Felix Janda wrote:
> > > The recent commit 04983f2272382af92eb8f8838964ff944fbb8258 (make
> > > netinet/in.h suppress clashing definitions from kernel headers)
> > > intends to address some of the conflicts between the kernel and musl
> > > libc headers. Namely it tries to allow the inclusion of kernel headers
> > > after libc header by defining __UAP_DEF_* macros. However this doesn't
> > > work because the relevant linux headers include <linux/libc-compat.h>,
> > > which unconditionally redefines the constants. For example
> > 
> > Oh, how awful. I missed the whole bogus #else part after the #if
> > defined(__GLIBC__) block.
> > > 
> > > #include <netinet/in.h>
> > > #include <linux/in6.h>
> > > 
> > > leads to
> > > 
> > > #define __UAPI_DEF_IN_ADDR 0 // from <netinet/in.h>
> > > #define __UAPI_DEF_IN_ADDR 1 // from <linux/libc-compat.h>
> > 
> > Conflicting defines should be an error already.
> 
> Strangely gcc-6.2.0 just warns (and the warning is not displayed when
> it is in system headers).
> 
> > > So we still get two conflicting definitions of struct in6_addr.
> > > 
> > > 
> > > By adding the hack "#define _LIBC_COMPAT_H" to <netinet/in.h>, this
> > > particular example compiles.
> > > 
> > > 
> > > Maybe the kernel people can be convinced to add #ifdef guards around
> > > all of the (non glibc) __UAPI_* definitions in <linux/libc-compat.h>.
> > 
> > I think they should, but I don't mind just suppressing the whole
> > header by defining _UAPI_LIBC_COMPAT_H if that works for all kernel
> > versions. It seems to; see:
> > 
> > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/include/uapi/linux/libc-compat.h?id=cfd280c91253cc28e4919e349fa7a813b63e71e8
> > 
> > where the file was introduced.
> 
> Note that for the actually installed kernel headers the "_UAPI" prefix
> is stripped from the include guard. For example "_UAPI__LINUX_KEYBOARD"
> becomes "__LINUX_KEYBOARD".
> 
> Second, defining the include guard in <netinet/in.h> would prevent
> <linux/libc-compat.h> from defining __UAPI_* constants for things
> actually missing from musl. For example, a recent <linux/ipx.h> would
> no longer define struct sockaddr_ipx when included after
> <netinet/in.h>.

Uhg. So there's really no fix except for the kernel to put #ifndef
around its definitions of individual macros, is there? Would you be
willing to propose such a patch? I'd ack it.

Rich