Enable DT_RELR in glibc shared libraries and PIEs automatically

Enable DT_RELR in glibc shared libraries and position independent
executables (PIE) automatically if linker supports -z pack-relative-relocs.
Reviewed-by: Florian Weimer <fweimer@redhat.com>

Makeconfig

@@ -362,6 +362,15 @@ relro-LDFLAGS = -Wl,-z,relro
 LDFLAGS.so += $(relro-LDFLAGS)
 LDFLAGS-rtld += $(relro-LDFLAGS)
 
+# Linker options to enable and disable DT_RELR.
+ifeq ($(have-dt-relr),yes)
+dt-relr-ldflag = -Wl,-z,pack-relative-relocs
+no-dt-relr-ldflag = -Wl,-z,nopack-relative-relocs
+else
+dt-relr-ldflag =
+no-dt-relr-ldflag =
+endif
+
 ifeq (no,$(build-pie-default))
 pie-default = $(no-pie-ccflag)
 else # build-pie-default
@@ -370,6 +379,7 @@ pic-default = -DPIC
 pie-default = $(pie-ccflag)
 
 ifeq (yes,$(enable-static-pie))
+static-pie-dt-relr-ldflag = $(dt-relr-ldflag)
 ifeq (yes,$(have-static-pie))
 static-pie-ldflag = -static-pie
 else
@@ -404,6 +414,7 @@ link-extra-libs-tests = $(libsupport)
 # Command for linking PIE programs with the C library.
 ifndef +link-pie
 +link-pie-before-inputs = $(if $($(@F)-no-pie),$(no-pie-ldflag),-pie) \
+	     $(if $($(@F)-no-dt-relr),$(no-dt-relr-ldflag),$(dt-relr-ldflag)) \
 	     -Wl,-O1 -nostdlib -nostartfiles \
 	     $(sysdep-LDFLAGS) $(LDFLAGS) $(LDFLAGS-$(@F)) \
 	     $(relro-LDFLAGS) $(hashstyle-LDFLAGS) \
@@ -436,6 +447,7 @@ endif
 ifndef +link-static
 +link-static-before-inputs = -nostdlib -nostartfiles -static \
 	      $(if $($(@F)-no-pie),$(no-pie-ldflag),$(static-pie-ldflag)) \
+	      $(if $($(@F)-no-dt-relr),$(no-dt-relr-ldflag),$(static-pie-dt-relr-ldflag)) \
 	      $(sysdep-LDFLAGS) $(LDFLAGS) $(LDFLAGS-$(@F))  \
 	      $(firstword $(CRT-$(@F)) $(csu-objpfx)$(real-static-start-installed-name)) \
 	      $(+preinit) $(+prectorT)

Makerules

@@ -536,6 +536,7 @@ lib%.so: lib%_pic.a $(+preinit) $(+postinit) $(link-libc-deps)
 define build-shlib-helper
 $(LINK.o) -shared -static-libgcc -Wl,-O1 $(sysdep-LDFLAGS) \
 	  $(if $($(@F)-no-z-defs)$(no-z-defs),,-Wl,-z,defs) $(rtld-LDFLAGS) \
+	  $(if $($(@F)-no-dt-relr),$(no-dt-relr-ldflag),$(dt-relr-ldflag)) \
 	  $(extra-B-$(@F:lib%.so=%).so) -B$(csu-objpfx) \
 	  $(extra-B-$(@F:lib%.so=%).so) $(load-map-file) \
 	  -Wl,-soname=lib$(libprefix)$(@F:lib%.so=%).so$($(@F)-version) \
@@ -588,6 +589,7 @@ endef
 define build-module-helper
 $(LINK.o) -shared -static-libgcc $(sysdep-LDFLAGS) $(rtld-LDFLAGS) \
 	  $(if $($(@F)-no-z-defs)$(no-z-defs),,-Wl,-z,defs) \
+	  $(if $($(@F)-no-dt-relr),$(no-dt-relr-ldflag),$(dt-relr-ldflag)) \
 	  -B$(csu-objpfx) $(load-map-file) \
 	  $(LDFLAGS.so) $(LDFLAGS-$(@F:%.so=%).so) \
 	  $(link-test-modules-rpath-link) \

elf/Makefile

@@ -1342,6 +1342,7 @@ $(objpfx)ld.so: $(objpfx)librtld.os $(ld-map)
 # if the sanity check below fails.
 	$(LINK.o) -nostdlib -nostartfiles -shared -o $@.new		\
 		  $(LDFLAGS-rtld) -Wl,-z,defs $(z-now-$(bind-now))	\
+		  $(dt-relr-ldflag) \
 		  $(filter-out $(map-file),$^) $(load-map-file)		\
 		  -Wl,-soname=$(rtld-installed-name)
 	$(call after-link,$@.new)
@@ -1666,6 +1667,7 @@ $(objpfx)nodlopen2.out: $(objpfx)nodlopenmod2.so
 
 $(objpfx)filtmod1.so: $(objpfx)filtmod1.os $(objpfx)filtmod2.so
 	$(LINK.o) -shared -o $@ -B$(csu-objpfx) $(LDFLAGS.so) \
+		  $(dt-relr-ldflag) \
 		  -L$(subst :, -L,$(rpath-link)) \
 		  -Wl,-rpath-link=$(rpath-link) \
 		  $< -Wl,-F,$(objpfx)filtmod2.so
@@ -2469,7 +2471,7 @@ $(objpfx)tst-big-note: $(objpfx)tst-big-note-lib.so
 # artificial, large note in tst-big-note-lib.o and invalidate the
 # test.
 $(objpfx)tst-big-note-lib.so: $(objpfx)tst-big-note-lib.o
-	$(LINK.o) -shared -o $@ $(LDFLAGS.so) $<
+	$(LINK.o) -shared -o $@ $(LDFLAGS.so) $(dt-relr-ldflag) $<
 
 $(objpfx)tst-unwind-ctor: $(objpfx)tst-unwind-ctor-lib.so
 
@@ -2778,6 +2780,7 @@ $(objpfx)tst-ro-dynamic: $(objpfx)tst-ro-dynamic-mod.so
 $(objpfx)tst-ro-dynamic-mod.so: $(objpfx)tst-ro-dynamic-mod.os \
   tst-ro-dynamic-mod.map
 	$(LINK.o) -nostdlib -nostartfiles -shared -o $@ \
+		$(dt-relr-ldflag) \
 		-Wl,--script=tst-ro-dynamic-mod.map \
 		$(objpfx)tst-ro-dynamic-mod.os