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Message-ID: <20161109015423.GA1597@nyan>
Date: Tue, 8 Nov 2016 20:54:23 -0500
From: Felix Janda <felix.janda@...teo.de>
To: musl@...ts.openwall.com
Subject: Re: kernel header compatibility
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>.
Felix
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