kheaders: rebuild kheaders_data.tar.xz when a file is modified within a minute

When a header file is changed, kernel/gen_kheaders.sh may fail to update
kernel/kheaders_data.tar.xz.

[steps to reproduce]

[1] Build kernel/kheaders_data.tar.xz

  $ make -j$(nproc) kernel/kheaders.o
    DESCEND objtool
    INSTALL libsubcmd_headers
    CALL    scripts/checksyscalls.sh
    CHK     kernel/kheaders_data.tar.xz
    GEN     kernel/kheaders_data.tar.xz
    CC      kernel/kheaders.o

[2] Modify a header without changing the file size

  $ sed -i s/0xdeadbeef/0xfeedbeef/ include/linux/elfnote.h

[3] Rebuild kernel/kheaders_data.tar.xz

  $ make -j$(nproc) kernel/kheaders.o
    DESCEND objtool
    INSTALL libsubcmd_headers
    CALL    scripts/checksyscalls.sh
    CHK     kernel/kheaders_data.tar.xz

kernel/kheaders_data.tar.xz is not updated if steps [1] - [3] are run
within the same minute.

The headers_md5 variable stores the MD5 hash of the 'ls -l' output
for all header files. This hash value is used to determine whether
kheaders_data.tar.xz needs to be rebuilt. However, 'ls -l' prints the
modification times with minute-level granularity. If a file is modified
within the same minute and its size remains the same, the MD5 hash does
not change.

To reliably detect file modifications, this commit rewrites
kernel/gen_kheaders.sh to output header dependencies to
kernel/.kheaders_data.tar.xz.cmd. Then, Make compares the timestamps
and reruns kernel/gen_kheaders.sh when necessary. This is the standard
mechanism used by Make and Kbuild.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

1 files changed, 42 insertions, 5 deletions

diff --git a/kernel/Makefile b/kernel/Makefile
index 32e80dd626af..9a9ff405ea89 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -158,11 +158,48 @@ filechk_cat = cat $<
 $(obj)/config_data: $(KCONFIG_CONFIG) FORCE
 	$(call filechk,cat)
 
+# kheaders_data.tar.xz
 $(obj)/kheaders.o: $(obj)/kheaders_data.tar.xz
 
-quiet_cmd_genikh = CHK     $(obj)/kheaders_data.tar.xz
-      cmd_genikh = $(CONFIG_SHELL) $(srctree)/kernel/gen_kheaders.sh $@
-$(obj)/kheaders_data.tar.xz: FORCE
-	$(call cmd,genikh)
+quiet_cmd_kheaders_data = GEN     $@
+      cmd_kheaders_data = "$<" "$@" "$(obj)/kheaders-srclist" "$(obj)/kheaders-objlist"
+      cmd_kheaders_data_dep = cat $(depfile) >> $(dot-target).cmd; rm -f $(depfile)
 
-clean-files := kheaders_data.tar.xz kheaders.md5
+define rule_kheaders_data
+	$(call cmd_and_savecmd,kheaders_data)
+	$(call cmd,kheaders_data_dep)
+endef
+
+targets += kheaders_data.tar.xz
+$(obj)/kheaders_data.tar.xz: $(src)/gen_kheaders.sh $(obj)/kheaders-srclist $(obj)/kheaders-objlist $(obj)/kheaders.md5 FORCE
+	$(call if_changed_rule,kheaders_data)
+
+# generated headers in objtree
+#
+# include/generated/utsversion.h is ignored because it is generated
+# after gen_kheaders.sh is executed. (utsversion.h is unneeded for kheaders)
+filechk_kheaders_objlist = \
+	for d in include "arch/$(SRCARCH)/include"; do \
+		find "$${d}/generated" ! -path "include/generated/utsversion.h" -a -name "*.h" -print; \
+	done
+
+$(obj)/kheaders-objlist: FORCE
+	$(call filechk,kheaders_objlist)
+
+# non-generated headers in srctree
+filechk_kheaders_srclist = \
+	for d in include "arch/$(SRCARCH)/include"; do \
+		find "$(srctree)/$${d}" -path "$(srctree)/$${d}/generated" -prune -o -name "*.h" -print; \
+	done
+
+$(obj)/kheaders-srclist: FORCE
+	$(call filechk,kheaders_srclist)
+
+# Some files are symlinks. If symlinks are changed, kheaders_data.tar.xz should
+# be rebuilt.
+filechk_kheaders_md5sum = xargs -r -a $< stat -c %N | md5sum
+
+$(obj)/kheaders.md5: $(obj)/kheaders-srclist FORCE
+	$(call filechk,kheaders_md5sum)
+
+clean-files := kheaders.md5 kheaders-srclist kheaders-objlist