kbuild: add CONFIG_VMLINUX_MAP expert option

It can be quite useful to have ld emit a link map file, in order to
debug or verify that special sections end up where they are supposed
to, and to see what LD_DEAD_CODE_DATA_ELIMINATION manages to get rid
of.

The only reason I'm not just adding this unconditionally is that the
.map file can be rather large (several MB), and that's a waste of
space when one isn't interested in these things. Also make it depend
on CONFIG_EXPERT.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

.gitignore

@@ -59,6 +59,7 @@ modules.order
 /linux
 /vmlinux
 /vmlinux.32
+/vmlinux.map
 /vmlinux.symvers
 /vmlinux-gdb.py
 /vmlinuz

Documentation/dontdiff

@@ -252,6 +252,7 @@ vmlinux-*
 vmlinux.aout
 vmlinux.bin.all
 vmlinux.lds
+vmlinux.map
 vmlinux.symvers
 vmlinuz
 voffset.h

lib/Kconfig.debug

@@ -449,6 +449,16 @@ config VMLINUX_VALIDATION
 	depends on STACK_VALIDATION && DEBUG_ENTRY && !PARAVIRT
 	default y
 
+config VMLINUX_MAP
+	bool "Generate vmlinux.map file when linking"
+	depends on EXPERT
+	help
+	  Selecting this option will pass "-Map=vmlinux.map" to ld
+	  when linking vmlinux. That file can be useful for verifying
+	  and debugging magic section games, and for seeing which
+	  pieces of code get eliminated with
+	  CONFIG_LD_DEAD_CODE_DATA_ELIMINATION.
+
 config DEBUG_FORCE_WEAK_PER_CPU
 	bool "Force weak per-cpu definitions"
 	depends on DEBUG_KERNEL

scripts/link-vmlinux.sh

@@ -155,6 +155,7 @@ vmlinux_link()
 	local output=${1}
 	local objects
 	local strip_debug
+	local map_option
 
 	info LD ${output}
 
@@ -166,6 +167,10 @@ vmlinux_link()
 		strip_debug=-Wl,--strip-debug
 	fi
 
+	if [ -n "${CONFIG_VMLINUX_MAP}" ]; then
+		map_option="-Map=${output}.map"
+	fi
+
 	if [ "${SRCARCH}" != "um" ]; then
 		if [ -n "${CONFIG_LTO_CLANG}" ]; then
 			# Use vmlinux.o instead of performing the slow LTO
@@ -187,6 +192,7 @@ vmlinux_link()
 		${LD} ${KBUILD_LDFLAGS} ${LDFLAGS_vmlinux}	\
 			${strip_debug#-Wl,}			\
 			-o ${output}				\
+			${map_option}				\
 			-T ${lds} ${objects}
 	else
 		objects="-Wl,--whole-archive			\
@@ -200,6 +206,7 @@ vmlinux_link()
 		${CC} ${CFLAGS_vmlinux}				\
 			${strip_debug}				\
 			-o ${output}				\
+			${map_option:+-Wl,${map_option}}	\
 			-Wl,-T,${lds}				\
 			${objects}				\
 			-lutil -lrt -lpthread
@@ -303,6 +310,7 @@ cleanup()
 	rm -f .tmp_vmlinux*
 	rm -f System.map
 	rm -f vmlinux
+	rm -f vmlinux.map
 	rm -f vmlinux.o
 }