linux: For better compatibility, provide compatible endian.h

Add endian.h. This includes sys/endian.h and then adds extra defines
that glibc defines with double underscores for our
_{BIG,BYTE,LITTLE,PDP}_ENDIAN macros. We also define __FLOAT_WORD_ORDER
to be the same as _BYTE_ENDIAN since FreeBSD doesn't currently define
this, and the default with glibc is exactly this for our platforms.
Move common parts of endian.h and sys/endian.h into sys/_endian.h
to limit namespace pollution from endian.h

All this gives us good compatibility with Linux. There may be one or two
upstreams that haven't integrated the patches I tried to send up.

There are some minor differences:
	o The extra glibc macros are not defined. These are all
	  controlled with either __ at the start, or only defined
	  when glibc is being built. We also don't define macros
	  that are used internally in glibc that would pollute
	  the namespace.
	o For complete compatibility, this change must also be
	  paired with providing a glibc-compatible byteswap.h.

Sponsored by:		Netflix
Reviewed by:		mhorne, markj, jhb
Differential Revision:	https://reviews.freebsd.org/D31962

(cherry picked from commit 30e0d2a51026830e5141ce3dd43854819bba910c)

4 files changed, 86 insertions, 39 deletions

diff --git a/include/Makefile b/include/Makefile
index 6f30598fedab..b7aa723a562f 100644
--- a/include/Makefile
+++ b/include/Makefile
@@ -11,8 +11,8 @@ SUBDIR= arpa protocols rpcsvc rpc xlocale
 SUBDIR_PARALLEL=
 INCS=	a.out.h ar.h assert.h bitstring.h complex.h cpio.h _ctype.h ctype.h \
 	db.h \
-	dirent.h dlfcn.h elf.h elf-hints.h err.h fmtmsg.h fnmatch.h fstab.h \
-	fts.h ftw.h getopt.h glob.h grp.h \
+	dirent.h dlfcn.h elf.h elf-hints.h endian.h err.h fmtmsg.h fnmatch.h \
+	fstab.h fts.h ftw.h getopt.h glob.h grp.h \
 	ieeefp.h ifaddrs.h \
 	inttypes.h iso646.h kenv.h langinfo.h libgen.h limits.h link.h \
 	locale.h malloc.h malloc_np.h memory.h monetary.h mpool.h mqueue.h \
diff --git a/include/endian.h b/include/endian.h
new file mode 100644
index 000000000000..c7b78217cedd
--- /dev/null
+++ b/include/endian.h
@@ -0,0 +1,43 @@
+/*-
+ * Copyright (c) 2021 M. Warner Losh <imp@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+/*
+ * A mostly Linux/glibc-compatible endian.h
+ */
+
+#ifndef _ENDIAN_H_
+#define _ENDIAN_H_
+
+/*
+ * FreeBSD's sys/_endian.h is very close to the interface provided on Linux by
+ * glibc's endian.h.
+ */
+#include <sys/_endian.h>
+
+/*
+ * glibc uses double underscore for these symbols. Define these unconditionally.
+ * The compiler defines __BYTE_ORDER__ these days, so we don't do anything
+ * with that since sys/endian.h defines _BYTE_ORDER based on it.
+ */
+#define __BIG_ENDIAN		_BIG_ENDIAN
+#define __BYTE_ORDER		_BYTE_ORDER
+#define __LITTLE_ENDIAN		_LITTLE_ENDIAN
+#define __PDP_ENDIAN		_PDP_ENDIAN
+
+/*
+ * FreeBSD's sys/endian.h and machine/endian.h doesn't define a separate
+ * byte order for floats. Use the host non-float byte order.
+ */
+#define __FLOAT_WORD_ORDER	_BYTE_ORDER
+
+/*
+ * We don't define BIG_ENDI, LITTLE_ENDI, HIGH_HALF and LOW_HALF macros that
+ * glibc's endian.h defines since those appear to be internal to internal to
+ * glibc. We also don't try to emulate the various helper macros that glibc
+ * uses to limit namespace visibility.
+ */
+
+#endif /* _ENDIAN_H_ */
diff --git a/sys/sys/_endian.h b/sys/sys/_endian.h
index 936962cc729f..72e1a40a8deb 100644
--- a/sys/sys/_endian.h
+++ b/sys/sys/_endian.h
@@ -32,7 +32,7 @@
 #ifndef _SYS__ENDIAN_H_
 #define	_SYS__ENDIAN_H_
 
-#ifndef _MACHINE_ENDIAN_H_
+#if !defined(_MACHINE_ENDIAN_H_) && !defined(_ENDIAN_H_)
 #error "sys/_endian.h should not be included directly"
 #endif
 
@@ -89,4 +89,38 @@
 #define	__ntohs(x)	((__uint16_t)(x))
 #endif
 
+/*
+ * Host to big endian, host to little endian, big endian to host, and little
+ * endian to host byte order functions as detailed in byteorder(9).
+ */
+#if _BYTE_ORDER == _LITTLE_ENDIAN
+#define	htobe16(x)	__bswap16((x))
+#define	htobe32(x)	__bswap32((x))
+#define	htobe64(x)	__bswap64((x))
+#define	htole16(x)	((uint16_t)(x))
+#define	htole32(x)	((uint32_t)(x))
+#define	htole64(x)	((uint64_t)(x))
+
+#define	be16toh(x)	__bswap16((x))
+#define	be32toh(x)	__bswap32((x))
+#define	be64toh(x)	__bswap64((x))
+#define	le16toh(x)	((uint16_t)(x))
+#define	le32toh(x)	((uint32_t)(x))
+#define	le64toh(x)	((uint64_t)(x))
+#else /* _BYTE_ORDER != _LITTLE_ENDIAN */
+#define	htobe16(x)	((uint16_t)(x))
+#define	htobe32(x)	((uint32_t)(x))
+#define	htobe64(x)	((uint64_t)(x))
+#define	htole16(x)	__bswap16((x))
+#define	htole32(x)	__bswap32((x))
+#define	htole64(x)	__bswap64((x))
+
+#define	be16toh(x)	((uint16_t)(x))
+#define	be32toh(x)	((uint32_t)(x))
+#define	be64toh(x)	((uint64_t)(x))
+#define	le16toh(x)	__bswap16((x))
+#define	le32toh(x)	__bswap32((x))
+#define	le64toh(x)	__bswap64((x))
+#endif /* _BYTE_ORDER == _LITTLE_ENDIAN */
+
 #endif /* _SYS__ENDIAN_H_ */
diff --git a/sys/sys/endian.h b/sys/sys/endian.h
index faac0458ae2c..d91c442e9238 100644
--- a/sys/sys/endian.h
+++ b/sys/sys/endian.h
@@ -56,48 +56,19 @@ typedef	__uint64_t	uint64_t;
 #endif
 
 /*
+ * Note: While tempting to try to avoid namespace pollution from this file,
+ * several software packages assume these marcos are defined, even when it
+ * defines _POSIX_C_SOURCE to request an unpolluted namespace.
+ */
+
+/*
  * General byte order swapping functions.
  */
 #define	bswap16(x)	__bswap16(x)
 #define	bswap32(x)	__bswap32(x)
 #define	bswap64(x)	__bswap64(x)
 
-/*
- * Host to big endian, host to little endian, big endian to host, and little
- * endian to host byte order functions as detailed in byteorder(9).
- */
-#if _BYTE_ORDER == _LITTLE_ENDIAN
-#define	htobe16(x)	__bswap16((x))
-#define	htobe32(x)	__bswap32((x))
-#define	htobe64(x)	__bswap64((x))
-#define	htole16(x)	((uint16_t)(x))
-#define	htole32(x)	((uint32_t)(x))
-#define	htole64(x)	((uint64_t)(x))
-
-#define	be16toh(x)	__bswap16((x))
-#define	be32toh(x)	__bswap32((x))
-#define	be64toh(x)	__bswap64((x))
-#define	le16toh(x)	((uint16_t)(x))
-#define	le32toh(x)	((uint32_t)(x))
-#define	le64toh(x)	((uint64_t)(x))
-#else /* _BYTE_ORDER != _LITTLE_ENDIAN */
-#define	htobe16(x)	((uint16_t)(x))
-#define	htobe32(x)	((uint32_t)(x))
-#define	htobe64(x)	((uint64_t)(x))
-#define	htole16(x)	__bswap16((x))
-#define	htole32(x)	__bswap32((x))
-#define	htole64(x)	__bswap64((x))
-
-#define	be16toh(x)	((uint16_t)(x))
-#define	be32toh(x)	((uint32_t)(x))
-#define	be64toh(x)	((uint64_t)(x))
-#define	le16toh(x)	__bswap16((x))
-#define	le32toh(x)	__bswap32((x))
-#define	le64toh(x)	__bswap64((x))
-#endif /* _BYTE_ORDER == _LITTLE_ENDIAN */
-
 /* Alignment-agnostic encode/decode bytestream to/from little/big endian. */
-
 static __inline uint16_t
 be16dec(const void *pp)
 {
@@ -203,5 +174,4 @@ le64enc(void *pp, uint64_t u)
 	le32enc(p, (uint32_t)(u & 0xffffffffU));
 	le32enc(p + 4, (uint32_t)(u >> 32));
 }
-
 #endif	/* _SYS_ENDIAN_H_ */