Eliminate the use of commons in the kernel and modules,

simplifying the module linking process and eliminating the risks
associated with doubly-defined variables.

Cases where commons were legitimately used (detection of
compiled-in subsystems) have been converted to use sysinits, and
any new code should use this or an equivalent practice as a
matter of course.

Modules can override this behaviour by substituting -fno-common
out of ${CFLAGS} in cases where commons are necessary
(eg. third-party object modules).  Commons will be resolved and
allocated space when the kld is linked as part of the module
build process, so they will not pose a risk to the kernel or
other modules.

Provide a mechanism for controlling the export of symbols from
the module namespace.  The EXPORT_SYMS variable may be set in the
Makefile to NO (export no symbols), a list of symbols to export,
or the name of a file containing a newline-seperated list of
symbols to be exported.  Non-exported symbols are converted to
local symbols.  If EXPORT_SYMS is not set, all global symbols are
currently exported.  This behaviour is expected to change (to
exporting no symbols) once modules have been converted.

Reviewed by:	peter (in principle)
Obtained from:	green (kmod_syms.awk)

sys/conf/kern.post.mk

@@ -76,7 +76,7 @@ assym.s: $S/kern/genassym.sh genassym.o
 	NM=${NM} OBJFORMAT=elf sh $S/kern/genassym.sh genassym.o > ${.TARGET}
 
 genassym.o: $S/$M/$M/genassym.c
-	${CC} -c ${CFLAGS} $S/$M/$M/genassym.c
+	${CC} -c ${CFLAGS:N-fno-common} $S/$M/$M/genassym.c
 
 ${SYSTEM_OBJS} genassym.o vers.o: opt_global.h
 

sys/conf/kern.pre.mk

@@ -38,7 +38,7 @@ INCLUDES+= -I/usr/include
 .endif
 
 COPTS=	${INCLUDES} ${IDENT} -D_KERNEL -ffreestanding -include opt_global.h
-CFLAGS=	${COPTFLAGS} ${CWARNFLAGS} ${DEBUG} ${COPTS}
+CFLAGS=	${COPTFLAGS} ${CWARNFLAGS} ${DEBUG} ${COPTS} -fno-common
 
 # XXX LOCORE means "don't declare C stuff" not "for locore.s".
 ASM_CFLAGS= -x assembler-with-cpp -DLOCORE ${CFLAGS}
@@ -76,7 +76,7 @@ SYSTEM_SFILES= $S/$M/$M/locore.s
 SYSTEM_DEP= Makefile ${SYSTEM_OBJS}
 SYSTEM_OBJS= locore.o vnode_if.o ${OBJS} hints.o env.o config.o hack.So
 SYSTEM_LD= @${LD} ${FMT} -Bdynamic -T $S/conf/ldscript.$M \
-	-export-dynamic -dynamic-linker /red/herring \
+	-warn-common -export-dynamic -dynamic-linker /red/herring \
 	-o ${.TARGET} -X ${SYSTEM_OBJS} vers.o
 SYSTEM_LD_TAIL= @${OBJCOPY} --strip-symbol gcc2_compiled. ${.TARGET} ; \
 	${SIZE} ${FMT} ${.TARGET} ; chmod 755 ${.TARGET}

sys/conf/kmod.mk

@@ -51,6 +51,11 @@
 # MFILES	Optionally a list of interfaces used by the module.
 #		This file contains a default list of interfaces.
 #
+# EXPORT_SYMS	A list of symbols that should be exported from the module,
+#		or the name of a file containing a list of symbols, or NO
+#		to export no symbols.  If missing, all global symbols are
+#		exported.
+#
 # +++ targets +++
 #
 #       distribute:
@@ -119,6 +124,11 @@ CFLAGS+=	-I${DESTDIR}/usr/include
 CFLAGS+=	-I@/../include -I${DESTDIR}/usr/include
 .endif # @
 
+# Disallow common variables, and if we end up with commons from
+# somewhere unexpected, allocate storage for them in the module itself.
+CFLAGS+=	-fno-common
+LDFLAGS+=	-d -warn-common
+
 CFLAGS+=	${DEBUG_FLAGS}
 
 .if ${OBJFORMAT} == elf
@@ -143,8 +153,25 @@ ${PROG}: ${FULLPROG}
 ${FULLPROG}: ${KMOD}.kld
 	${LD} -Bshareable ${LDFLAGS} -o ${.TARGET} ${KMOD}.kld
 
+.if defined(EXPORT_SYMS)
+CLEANFILES+=	${.OBJDIR}/export_syms
+.endif
+
 ${KMOD}.kld: ${OBJS}
-	${LD} ${LDFLAGS} -r -o ${.TARGET} ${OBJS}
+	${LD} ${LDFLAGS} -r -d -o ${.TARGET} ${OBJS}
+.if defined(EXPORT_SYMS)
+.if ${EXPORT_SYMS} == NO
+	touch ${.OBJDIR}/export_syms
+.elif !exists(${.CURDIR}/${EXPORT_SYMS})
+	echo ${EXPORT_SYMS} > ${.OBJDIR}/export_syms
+.else
+	grep -v '^#' < ${EXPORT_SYMS} >  ${.OBJDIR}/export_syms
+.endif
+	awk -f ${SYSDIR}/conf/kmod_syms.awk ${.TARGET} \
+		${.OBJDIR}/export_syms | \
+	xargs -t -J% ${OBJCOPY} % ${.TARGET}
+.endif
+
 
 .if !defined(NOMAN)
 .if 0

sys/conf/kmod_syms.awk

@@ -0,0 +1,27 @@
+# $FreeBSD$
+
+# Read global symbols from object file.
+BEGIN {
+        while ("nm -g " ARGV[1] | getline) {
+                if (match($0, /^[^[:space:]]+ [^AU] (.*)$/)) {
+                        syms[$3] = $2
+                }
+        }
+}
+
+# De-list symbols from the export list.
+// {
+        if (ARGIND == 1)
+                nextfile
+        delete syms[$0]
+}
+
+# Strip commons, make everything else local.
+END {
+        for (member in syms) {
+                if (syms[member] == "C")
+                        print "-N" member
+                else
+                        print "-L" member
+        }
+}