[PATCH] kbuild: apply fixdep logic to link-vmlinux.sh

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

The patch adding CONFIG_VMLINUX_MAP revealed a small defect in the
build system: link-vmlinux.sh takes decisions based on CONFIG_*
options, but changing one of those does not always lead to vmlinux
being linked again.

For most of the CONFIG_* knobs referenced previously, this has
probably been hidden by those knobs also affecting some object file,
hence indirectly also vmlinux.

But CONFIG_VMLINUX_MAP is only handled inside link-vmlinux.sh, and
changing CONFIG_VMLINUX_MAP=n to CONFIG_VMLINUX_MAP=y does not cause
the build system to re-link (and hence have vmlinux.map
emitted). Since that map file is mostly a debugging aid, this is
merely a nuisance which is easily worked around by just deleting
vmlinux and building again.

But one could imagine other (possibly future) CONFIG options that
actually do affect the vmlinux binary but which are not captured
through some object file dependency.

To fix this, make link-vmlinux.sh emit a .vmlinux.d file in the same
format as the dependency files generated by gcc, and apply the fixdep
logic to that. I've tested that this correctly works with both in-tree
and out-of-tree builds.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 Makefile                | 2 +-
 scripts/link-vmlinux.sh | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index b18dbc634690..19d2f7fd088a 100644
--- a/Makefile
+++ b/Makefile
@@ -1192,7 +1192,7 @@ cmd_link-vmlinux =                                                 \
 	$(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)
 
 vmlinux: scripts/link-vmlinux.sh autoksyms_recursive $(vmlinux-deps) FORCE
-	+$(call if_changed,link-vmlinux)
+	+$(call if_changed_dep,link-vmlinux)
 
 targets := vmlinux
 
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 855fd4e6f03e..7d4b7c6f01e8 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -312,6 +312,7 @@ cleanup()
 	rm -f vmlinux
 	rm -f vmlinux.map
 	rm -f vmlinux.o
+	rm -f .vmlinux.d
 }
 
 on_exit()
@@ -421,6 +422,7 @@ if [ -n "${CONFIG_KALLSYMS}" ]; then
 fi
 
 vmlinux_link vmlinux "${kallsymso}" ${btf_vmlinux_bin_o}
+echo "vmlinux: $0" > .vmlinux.d
 
 # fill in BTF IDs
 if [ -n "${CONFIG_DEBUG_INFO_BTF}" -a -n "${CONFIG_BPF}" ]; then
-- 
2.29.2