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Add <linux/libc-compat.h> a compatibility interface for userspace libc header
coordination.

Origin: kernel-headers provided by sabotage-linux

https://github.com/sabotage-linux/kernel-headers/blob/master/generic/include/linux/libc-compat.h

--- /dev/null
+++ b/include/uapi/linux/libc-compat.h
@@ -0,0 +1,100 @@
+/*
+ * Compatibility interface for userspace libc header coordination:
+ *
+ * Define compatibility macros that are used to control the inclusion or
+ * exclusion of UAPI structures and definitions in coordination with another
+ * userspace C library.
+ *
+ * This header is intended to solve the problem of UAPI definitions that
+ * conflict with userspace definitions. If a UAPI header has such conflicting
+ * definitions then the solution is as follows:
+ *
+ * * Synchronize the UAPI header and the libc headers so either one can be
+ *   used and such that the ABI is preserved. If this is not possible then
+ *   no simple compatibility interface exists (you need to write translating
+ *   wrappers and rename things) and you can't use this interface.
+ *
+ * Then follow this process:
+ *
+ * (a) Include libc-compat.h in the UAPI header.
+ *      e.g. #include <linux/libc-compat.h>
+ *     This include must be as early as possible.
+ *
+ * (b) In libc-compat.h add enough code to detect that the comflicting
+ *     userspace libc header has been included first.
+ *
+ * (c) If the userspace libc header has been included first define a set of
+ *     guard macros of the form __UAPI_DEF_FOO and set their values to 1, else
+ *     set their values to 0.
+ *
+ * (d) Back in the UAPI header with the conflicting definitions, guard the
+ *     definitions with:
+ *     #if __UAPI_DEF_FOO
+ *       ...
+ *     #endif
+ *
+ * This fixes the situation where the linux headers are included *after* the
+ * libc headers. To fix the problem with the inclusion in the other order the
+ * userspace libc headers must be fixed like this:
+ *
+ * * For all definitions that conflict with kernel definitions wrap those
+ *   defines in the following:
+ *   #if !__UAPI_DEF_FOO
+ *     ...
+ *   #endif
+ *
+ * This prevents the redefinition of a construct already defined by the kernel.
+ */
+#ifndef _LIBC_COMPAT_H
+#define _LIBC_COMPAT_H
+
+#ifndef __KERNEL__ /* we're used from userspace */
+
+#ifdef _NETINET_IF_ETHER_H /* musl */
+#define __UAPI_DEF_ETHHDR 0
+#else /* glibc uses __NETINET_IF_ETHER_H, and includes the kernel header. */
+#define __UAPI_DEF_ETHHDR 1
+#endif
+
+/* Coordinate with libc netinet/in.h header. */
+#if defined(_NETINET_IN_H)
+
+/* GLIBC headers included first so don't define anything
+ * that would already be defined. */
+#define __UAPI_DEF_IN6_ADDR		0
+#define __UAPI_DEF_IN6_ADDR_ALT		0
+#define __UAPI_DEF_SOCKADDR_IN6		0
+#define __UAPI_DEF_IPV6_MREQ		0
+#define __UAPI_DEF_IPPROTO_V6		0
+
+#else /* defined(_NETINET_IN_H) */
+
+/* Linux headers included first, and we must define everything
+ * we need. The expectation is that the libc will check the
+ * __UAPI_DEF_* defines and adjust appropriately. */
+#define __UAPI_DEF_IN6_ADDR		1
+/* We unconditionally define the in6_addr macros and libc must
+ * coordinate. */
+#define __UAPI_DEF_IN6_ADDR_ALT		1
+#define __UAPI_DEF_SOCKADDR_IN6		1
+#define __UAPI_DEF_IPV6_MREQ		1
+#define __UAPI_DEF_IPPROTO_V6		1
+
+#endif /* _NETINET_IN_H */
+
+
+/* If we did not see any headers from any supported C libraries,
+ * or we are being included in the kernel, then define everything
+ * that we need. */
+#else /* __KERNEL__ */
+
+/* Definitions for in6.h */
+#define __UAPI_DEF_IN6_ADDR		1
+#define __UAPI_DEF_IN6_ADDR_ALT		1
+#define __UAPI_DEF_SOCKADDR_IN6		1
+#define __UAPI_DEF_IPV6_MREQ		1
+#define __UAPI_DEF_IPPROTO_V6		1
+
+#endif /* __KERNEL__ */
+
+#endif /* _LIBC_COMPAT_H */
--- a/include/uapi/linux/Kbuild
+++ b/include/uapi/linux/Kbuild
@@ -422,3 +422,6 @@
 header-y += xattr.h
 header-y += xfrm.h
 header-y += hw_breakpoint.h
+
+# install compatibility interface for userspace libc header coordination
+header-y += libc-compat.h