Provide libdl.

Create libdl.so.1 as a filter for libc.so.7 which exports public dl*
functions. The functions are resolved from the rtld instead, the goal
of creating library is to avoid errors from the static linker due to
missed libdl. For static binaries, an empty .o is compiled into
libdl.a so that static binaries still get dl stubs from libc.a.

Right now lld cannot create filter objects, disable libdl on arm64
when binutils are not used.

Reviewed by:	bdrewery, dim (previos version); emaste
Exp run:	PR 220525, done by antoine
Sponsored by:	The FreeBSD Foundation
MFC after:	1 month
Differential revision:	https://reviews.freebsd.org/D11504

contrib/mdocml/lib.in

@@ -46,6 +46,7 @@ LINE("libdevctl",	"Device Control Library (libdevctl, \\-ldevctl)")
 LINE("libdevinfo",	"Device and Resource Information Utility Library (libdevinfo, \\-ldevinfo)")
 LINE("libdevstat",	"Device Statistics Library (libdevstat, \\-ldevstat)")
 LINE("libdisk",		"Interface to Slice and Partition Labels Library (libdisk, \\-ldisk)")
+LINE("libdl",		"Dynamic Linker Services Filter (libdl, \\-ldl)")
 LINE("libdm",		"Device Mapper Library (libdm, \\-ldm)")
 LINE("libdwarf",	"DWARF Access Library (libdwarf, \\-ldwarf)")
 LINE("libedit",		"Command Line Editor Library (libedit, \\-ledit)")

lib/Makefile

@@ -40,6 +40,7 @@ SUBDIR=	${SUBDIR_BOOTSTRAP} \
 	libdevctl \
 	libdevinfo \
 	libdevstat \
+	${_libdl} \
 	libdwarf \
 	libedit \
 	libevent \
@@ -183,6 +184,10 @@ _libproc=	libproc
 _librtld_db=	librtld_db
 .endif
 
+.if defined(LINKER_FEATURES) && ${LINKER_FEATURES:Mfilter}
+_libdl=		libdl
+.endif
+
 SUBDIR.${MK_OPENSSL}+=	libmp
 SUBDIR.${MK_PMC}+=	libpmc
 SUBDIR.${MK_RADIUS_SUPPORT}+=	libradius

lib/libc/gen/dlfcn.c

@@ -27,6 +27,8 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
+#if !defined(IN_LIBDL) || defined(PIC)
+
 /*
  * Linkage to services provided by the dynamic linker.
  */
@@ -157,6 +159,7 @@ _rtld_thread_init(void *li __unused)
 	_rtld_error(sorry);
 }
 
+#ifndef IN_LIBDL
 static pthread_once_t dl_phdr_info_once = PTHREAD_ONCE_INIT;
 static struct dl_phdr_info phdr_info;
 
@@ -192,6 +195,7 @@ dl_init_phdr_info(void)
 	}
 	phdr_info.dlpi_adds = 1;
 }
+#endif
 
 #pragma weak dl_iterate_phdr
 int
@@ -199,11 +203,15 @@ dl_iterate_phdr(int (*callback)(struct dl_phdr_info *, size_t, void *) __unused,
     void *data __unused)
 {
 
+#ifndef IN_LIBDL
 	__init_elf_aux_vector();
 	if (__elf_aux_vector == NULL)
 		return (1);
 	_once(&dl_phdr_info_once, dl_init_phdr_info);
 	return (callback(&phdr_info, sizeof(phdr_info), data));
+#else
+	return (0);
+#endif
 }
 
 #pragma weak fdlopen
@@ -251,3 +259,5 @@ _rtld_is_dlopened(void *arg __unused)
 
 	return (0);
 }
+
+#endif /* !defined(IN_LIBDL) || defined(PIC) */

lib/libc/gen/dlopen.3

@@ -32,7 +32,7 @@
 .\" @(#) dlopen.3 1.6 90/01/31 SMI
 .\" $FreeBSD$
 .\"
-.Dd February 14, 2015
+.Dd July 7, 2017
 .Dt DLOPEN 3
 .Os
 .Sh NAME
@@ -377,6 +377,14 @@ option to
 for symbols defined in the executable to become visible to
 .Fn dlsym .
 .Pp
+Other ELF platforms require linking with
+.Lb libdl
+to provide
+.Fn dlopen
+and other functions.
+.Fx
+does not require linking with the library, but supports it for compatibility.
+.Pp
 In previous implementations, it was necessary to prepend an underscore
 to all external symbols in order to gain symbol
 compatibility with object code compiled from the C language.

lib/libdl/Makefile

@@ -0,0 +1,15 @@
+# $FreeBSD$
+
+LIB=dl
+SHLIB_MAJOR=1
+
+.PATH:	${SRCTOP}/lib/libc/gen
+CFLAGS+=-I${SRCTOP}/lib/libc/include
+CFLAGS+=-DIN_LIBDL
+LDFLAGS+=-Wl,-F,libc.so.7
+VERSION_DEF=${SRCTOP}/lib/libc/Versions.def
+SYMBOL_MAPS=${.CURDIR}/Symbol.map
+
+SRCS =	dlfcn.c
+
+.include <bsd.lib.mk>

lib/libdl/Symbol.map

@@ -0,0 +1,20 @@
+/*
+ * $FreeBSD$
+ */
+
+FBSD_1.0 {
+	dladdr;
+	dlclose;
+	dlerror;
+	dlfunc;
+	dlopen;
+	dlsym;
+	dlvsym;
+	dlinfo;
+	dl_iterate_phdr;
+};
+
+
+FBSD_1.3 {
+	fdlopen;
+};

share/mk/bsd.libnames.mk

@@ -56,6 +56,7 @@ LIBDEVDCTL?=	${LIBDESTDIR}${LIBDIR_BASE}/libdevdctl.a
 LIBDEVINFO?=	${LIBDESTDIR}${LIBDIR_BASE}/libdevinfo.a
 LIBDEVSTAT?=	${LIBDESTDIR}${LIBDIR_BASE}/libdevstat.a
 LIBDIALOG?=	${LIBDESTDIR}${LIBDIR_BASE}/libdialog.a
+LIBDL?=		${LIBDESTDIR}${LIBDIR_BASE}/libdl.a
 LIBDNS?=	${LIBDESTDIR}${LIBDIR_BASE}/libdns.a
 LIBDPV?=	${LIBDESTDIR}${LIBDIR_BASE}/libdpv.a
 LIBDTRACE?=	${LIBDESTDIR}${LIBDIR_BASE}/libdtrace.a

share/mk/bsd.linker.mk

@@ -70,6 +70,9 @@ ${X_}LINKER_FEATURES=
 .if ${${X_}LINKER_TYPE} != "bfd" || ${${X_}LINKER_VERSION} > 21750
 ${X_}LINKER_FEATURES+=	build-id
 .endif
+.if ${${X_}LINKER_TYPE} == "bfd"
+${X_}LINKER_FEATURES+=	filter
+.endif
 .endif
 .else
 # Use LD's values

share/mk/src.libnames.mk

@@ -88,6 +88,7 @@ _LIBRARIES=	\
 		devinfo \
 		devstat \
 		dialog \
+		dl \
 		dpv \
 		dtrace \
 		dwarf \