x86/vdso: Create .build-id links for unstripped vdso files

With this change, doing 'make vdso_install' and telling gdb:

set debug-file-directory /lib/modules/KVER/vdso

will enable vdso debugging with symbols.  This is useful for
testing, but kernel RPM builds will probably want to manually delete
these symlinks or otherwise do something sensible when they strip
the vdso/*.so files.

If ld does not support --build-id, then the symlinks will not be
created.

Note that kernel packagers that use vdso_install may need to adjust
their packaging scripts to accomdate this change.  For example,
Fedora's scripts create build-id symlinks themselves in a different
location, so the spec should probably be updated to remove the
symlinks created by make vdso_install.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/a424b189ce3ced85fe1e82d032a20e765e0fe0d3.1403291930.git.luto@amacapital.net
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/vdso/Makefile

@@ -169,14 +169,24 @@ quiet_cmd_vdso = VDSO    $@
 		 sh $(srctree)/$(src)/checkundef.sh '$(NM)' '$@'
 
 VDSO_LDFLAGS = -fPIC -shared $(call cc-ldoption, -Wl$(comma)--hash-style=sysv) \
-	-Wl,-Bsymbolic $(LTO_CFLAGS)
+	$(call cc-ldoption, -Wl$(comma)--build-id) -Wl,-Bsymbolic $(LTO_CFLAGS)
 GCOV_PROFILE := n
 
 #
-# Install the unstripped copies of vdso*.so.
+# Install the unstripped copies of vdso*.so.  If our toolchain supports
+# build-id, install .build-id links as well.
 #
 quiet_cmd_vdso_install = INSTALL $(@:install_%=%)
-      cmd_vdso_install = cp $< $(MODLIB)/vdso/$(@:install_%=%)
+define cmd_vdso_install
+	cp $< "$(MODLIB)/vdso/$(@:install_%=%)"; \
+	if readelf -n $< |grep -q 'Build ID'; then \
+	  buildid=`readelf -n $< |grep 'Build ID' |sed -e 's/^.*Build ID: \(.*\)$$/\1/'`; \
+	  first=`echo $$buildid | cut -b-2`; \
+	  last=`echo $$buildid | cut -b3-`; \
+	  mkdir -p "$(MODLIB)/vdso/.build-id/$$first"; \
+	  ln -sf "../../$(@:install_%=%)" "$(MODLIB)/vdso/.build-id/$$first/$$last.debug"; \
+	fi
+endef
 
 vdso_img_insttargets := $(vdso_img_sodbg:%.dbg=install_%)