[RFC PATCH] kbuild: pass objects instead of archives to linker

[Masahiro Yamada]

This is an experimental patch, driven by the feedback from Jiri Slaby
and Michael Matz. [1]

Michael Matz says:
 "I know of no linker (outside LTO-like modes) that processes
  archives in a different order than first-to-last-member (under
  whole-archive), but that's not guaranteed anywhere. So relying on
  member-order within archives is always brittle."

It is pretty easy to pass the list of objects instead of a thin archive
because the linker supports the '@file' syntax, where command line
arguments are read from 'file'.

Without this patch, the linker receives

  --whole-archive vmlinux.a --no-whole-archive

With this patch, the linker will receive

  @vmlinux.order

Here, vmlinux.order is a text file that lists built-in objects in the
correct link order.

I am not a toolchain expert. I just want to know if this makes any
difference from the linker perspective and from (non-upstreamed) GCC-LTO
perspective.

(I know this patch does not work for Clang LTO because I did not touch
scripts/generate_initcall_order.pl)

This patch may be unneeded because more correct patches were submitted [2]
but I am still curious about "thin archive vs direct object list".

[1]: https://lore.kernel.org/linux-kbuild/alpine.LSU.2.20.2210251210140.29399@xxxxxxxxxxxxx/
[2]: https://lore.kernel.org/all/20221101161529.1634188-1-alexandr.lobakin@xxxxxxxxx/

Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
---

 Makefile                                    | 21 ++++++++++-----------
 scripts/Makefile.modpost                    |  5 +++--
 scripts/Makefile.vmlinux_o                  |  6 +++---
 scripts/clang-tools/gen_compile_commands.py | 21 ++++++++++++++++++++-
 scripts/link-vmlinux.sh                     |  8 ++++----
 5 files changed, 40 insertions(+), 21 deletions(-)

diff --git a/Makefile b/Makefile
index e9e7eff906a5..511484a3dacb 100644
--- a/Makefile
+++ b/Makefile
@@ -1213,19 +1213,18 @@ quiet_cmd_autoksyms_h = GEN     $@
 $(autoksyms_h):
 	$(call cmd,autoksyms_h)
 
-# '$(AR) mPi' needs 'T' to workaround the bug of llvm-ar <= 14
-quiet_cmd_ar_vmlinux.a = AR      $@
-      cmd_ar_vmlinux.a = \
-	rm -f $@; \
-	$(AR) cDPrST $@ $(KBUILD_VMLINUX_OBJS); \
-	$(AR) mPiT $$($(AR) t $@ | sed -n 1p) $@ $$($(AR) t $@ | grep -F -f $(srctree)/scripts/head-object-list.txt)
+quiet_cmd_vmlinux_order = GEN     $@
+      cmd_vmlinux_order = \
+	{ $(foreach m, $(KBUILD_VMLINUX_OBJS), $(AR) t $m;) :; } > $(tmp-target) ; \
+	grep -F -f $(srctree)/scripts/head-object-list.txt $(tmp-target) > $@; \
+	grep -F -f $(srctree)/scripts/head-object-list.txt $(tmp-target) -v >> $@
 
-targets += vmlinux.a
-vmlinux.a: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt autoksyms_recursive FORCE
-	$(call if_changed,ar_vmlinux.a)
+targets += vmlinux.order
+vmlinux.order: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt autoksyms_recursive FORCE
+	$(call if_changed,vmlinux_order)
 
 PHONY += vmlinux_o
-vmlinux_o: vmlinux.a $(KBUILD_VMLINUX_LIBS)
+vmlinux_o: vmlinux.order $(KBUILD_VMLINUX_LIBS)
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.vmlinux_o
 
 vmlinux.o modules.builtin.modinfo modules.builtin: vmlinux_o
@@ -2037,7 +2036,7 @@ quiet_cmd_gen_compile_commands = GEN     $@
       cmd_gen_compile_commands = $(PYTHON3) $< -a $(AR) -o $@ $(filter-out $<, $(real-prereqs))
 
 $(extmod_prefix)compile_commands.json: scripts/clang-tools/gen_compile_commands.py \
-	$(if $(KBUILD_EXTMOD),, vmlinux.a $(KBUILD_VMLINUX_LIBS)) \
+	$(if $(KBUILD_EXTMOD),, vmlinux.order $(KBUILD_VMLINUX_LIBS)) \
 	$(if $(CONFIG_MODULES), $(MODORDER)) FORCE
 	$(call if_changed,gen_compile_commands)
 
diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
index e41dee64d429..1d6847da39bd 100644
--- a/scripts/Makefile.modpost
+++ b/scripts/Makefile.modpost
@@ -70,12 +70,13 @@ quiet_cmd_vmlinux_objs = GEN     $@
 	for f in $(real-prereqs); do	\
 		case $${f} in		\
 		*libgcc.a) ;;		\
-		*) $(AR) t $${f} ;;	\
+		*.a) $(AR) t $${f} ;;	\
+		*) cat $${f} ;;		\
 		esac			\
 	done > $@
 
 targets += .vmlinux.objs
-.vmlinux.objs: vmlinux.a $(KBUILD_VMLINUX_LIBS) FORCE
+.vmlinux.objs: vmlinux.order $(KBUILD_VMLINUX_LIBS) FORCE
 	$(call if_changed,vmlinux_objs)
 
 vmlinux.o-if-present := $(wildcard vmlinux.o)
diff --git a/scripts/Makefile.vmlinux_o b/scripts/Makefile.vmlinux_o
index 0edfdb40364b..6eb07f2bb39f 100644
--- a/scripts/Makefile.vmlinux_o
+++ b/scripts/Makefile.vmlinux_o
@@ -18,7 +18,7 @@ quiet_cmd_gen_initcalls_lds = GEN     $@
 	$(PERL) $(real-prereqs) > $@
 
 .tmp_initcalls.lds: $(srctree)/scripts/generate_initcall_order.pl \
-		vmlinux.a $(KBUILD_VMLINUX_LIBS) FORCE
+		vmlinux.order $(KBUILD_VMLINUX_LIBS) FORCE
 	$(call if_changed,gen_initcalls_lds)
 
 targets := .tmp_initcalls.lds
@@ -48,7 +48,7 @@ quiet_cmd_ld_vmlinux.o = LD      $@
       cmd_ld_vmlinux.o = \
 	$(LD) ${KBUILD_LDFLAGS} -r -o $@ \
 	$(addprefix -T , $(initcalls-lds)) \
-	--whole-archive vmlinux.a --no-whole-archive \
+	@vmlinux.order \
 	--start-group $(KBUILD_VMLINUX_LIBS) --end-group \
 	$(cmd_objtool)
 
@@ -57,7 +57,7 @@ define rule_ld_vmlinux.o
 	$(call cmd,gen_objtooldep)
 endef
 
-vmlinux.o: $(initcalls-lds) vmlinux.a $(KBUILD_VMLINUX_LIBS) FORCE
+vmlinux.o: $(initcalls-lds) vmlinux.order $(KBUILD_VMLINUX_LIBS) FORCE
 	$(call if_changed_rule,ld_vmlinux.o)
 
 targets += vmlinux.o
diff --git a/scripts/clang-tools/gen_compile_commands.py b/scripts/clang-tools/gen_compile_commands.py
index d800b2c0af97..c8ba9f084bd0 100755
--- a/scripts/clang-tools/gen_compile_commands.py
+++ b/scripts/clang-tools/gen_compile_commands.py
@@ -57,7 +57,7 @@ def parse_arguments():
     parser.add_argument('-a', '--ar', type=str, default='llvm-ar', help=ar_help)
 
     paths_help = ('directories to search or files to parse '
-                  '(files should be *.o, *.a, or modules.order). '
+                  '(files should be *.o, *.a, or *.order). '
                   'If nothing is specified, the current directory is searched')
     parser.add_argument('paths', type=str, nargs='*', help=paths_help)
 
@@ -124,6 +124,23 @@ def cmdfiles_for_a(archive, ar):
         yield to_cmdfile(obj)
 
 
+def cmdfiles_for_vmlinux_order(vmlinux_order):
+    """Generate the iterator of .cmd files associated with the vmlinux.order.
+
+    Parse the given vmlinux.order, and yield every .cmd file used to build the
+    contained modules.
+
+    Args:
+        vmlinux_order: The vmlinux.order file to parse
+
+    Yields:
+        The path to every .cmd file found
+    """
+    with open(vmlinux_order) as f:
+        for line in f:
+            yield to_cmdfile(line.rstrip())
+
+
 def cmdfiles_for_modorder(modorder):
     """Generate the iterator of .cmd files associated with the modules.order.
 
@@ -203,6 +220,8 @@ def main():
             cmdfiles = cmdfiles_in_dir(path)
         elif path.endswith('.a'):
             cmdfiles = cmdfiles_for_a(path, ar)
+        elif path.endswith('vmlinux.order'):
+            cmdfiles = cmdfiles_for_vmlinux_order(path)
         elif path.endswith('modules.order'):
             cmdfiles = cmdfiles_for_modorder(path)
         else:
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 918470d768e9..617ed443e377 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -3,15 +3,15 @@
 #
 # link vmlinux
 #
-# vmlinux is linked from the objects in vmlinux.a and $(KBUILD_VMLINUX_LIBS).
-# vmlinux.a contains objects that are linked unconditionally.
+# vmlinux is linked from the objects in vmlinux.order and $(KBUILD_VMLINUX_LIBS).
+# vmlinux.order is a text file that contains objects that are linked unconditionally.
 # $(KBUILD_VMLINUX_LIBS) are archives which are linked conditionally
 # (not within --whole-archive), and do not require symbol indexes added.
 #
 # vmlinux
 #   ^
 #   |
-#   +--< vmlinux.a
+#   +--< vmlinux.order
 #   |
 #   +--< $(KBUILD_VMLINUX_LIBS)
 #   |    +--< lib/lib.a + more
@@ -65,7 +65,7 @@ vmlinux_link()
 		objs=vmlinux.o
 		libs=
 	else
-		objs=vmlinux.a
+		objs=@vmlinux.order
 		libs="${KBUILD_VMLINUX_LIBS}"
 	fi
 
-- 
2.34.1