[PATCH v1 2/2] kbuild: Mark the initcall ordering recipe as recursive

[Kees Cook]

generate_initcall_order.pl, which orders initcalls for Clang LTO, forks
a child per input file, up to the job count scripts/jobserver-exec
passes it in PARALLELISM. It used to run from link-vmlinux.sh, which
the top-level Makefile invokes as a recursive recipe, but
commit 5d45950dfbb15 ("kbuild: move vmlinux.o link to
scripts/Makefile.vmlinux_o") moved it into a recipe without "+".

GNU Make passes the jobserver pipe only to recipes it treats as
recursive, and the make 4.3 in CentOS Stream 9 also hides it in
sub-makes (a backport of the fix for GNU Make bug 58232), so
jobserver-exec cannot open it and prints:

  WARNING: Unable to reopen jobserver read-side pipe: FileNotFoundError(2, 'No such file or directory')

PARALLELISM is then left unset, and the script caps its children at the
number of online CPUs instead of the free job slots.

Mark the recipe as recursive again.

Tested ARCH=x86_64 defconfig with CONFIG_LTO_CLANG_THIN=y, LLVM=1
(Clang 24.0.0git), and GNU Make 4.3 plus CentOS Stream 9's
make-4.3-cloexec.patch.

Fixes: 5d45950dfbb15 ("kbuild: move vmlinux.o link to scripts/Makefile.vmlinux_o")
Assisted-by: LLM
Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
---
 scripts/Makefile.vmlinux_o | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/Makefile.vmlinux_o b/scripts/Makefile.vmlinux_o
index 24a3a4fd271c..0b37bd5dc3a4 100644
--- a/scripts/Makefile.vmlinux_o
+++ b/scripts/Makefile.vmlinux_o
@@ -19,7 +19,7 @@ quiet_cmd_gen_initcalls_lds = GEN     $@
 
 .tmp_initcalls.lds: $(srctree)/scripts/generate_initcall_order.pl \
 		vmlinux.a $(KBUILD_VMLINUX_LIBS) FORCE
-	$(call if_changed,gen_initcalls_lds)
+	+$(call if_changed,gen_initcalls_lds)
 
 targets := .tmp_initcalls.lds
 
-- 
2.55.0