[PATCH 10/13] kbuild: build modules in the same way with/without Clang LTO

Masahiro Yamada
2021-08-19T00:59:34Z

When Clang LTO is enabled, additional intermediate files *.lto.o are
created because LLVM bitcode must be converted to ELF before modpost.

For non-LTO builds:

$(LD) $(LD)
objects ---> <modname>.o -----> <modname>.ko
|
<modname>.mod.o ---/

For Clang LTO builds:

$(AR) $(LD) $(LD)
objects ---> <modname>.o ---> <modname>.lto.o -----> <modname>.ko
|
<modname>.mod.o --/

Since the Clang LTO introduction, ugly CONFIG_LTO_CLANG conditionals
are sprinkled everywhere in the kbuild code.

Another confusion for Clang LTO builds is, <modname>.o is an archive
that contains LLVM bitcode files. The suffix should have been .a
instead of .o

To clean up the code, unify the build process of modules, as follows:

$(AR) $(LD) $(LD)
objects ---> <modname>.a ---> <modname>.prelink.o -----> <modname>.ko
|
<modname>.mod.o ------/

Here, 'objects' are either ELF or LLVM bitcode. <modname>.a is an archive,
<modname>.prelink.o is ELF.

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

scripts/Makefile.build | 103 ++++++++++++++++++--------------------
scripts/Makefile.lib | 11 ++--
scripts/Makefile.modfinal | 4 +-
scripts/Makefile.modpost | 7 +--
scripts/mod/modpost.c | 6 +--
scripts/mod/sumversion.c | 6 +--
6 files changed, 61 insertions(+), 76 deletions(-)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 37d6f6da34d6..957addea830b 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -88,9 +88,7 @@ endif

targets-for-modules := $(patsubst %.o, %.mod, $(filter %.o, $(obj-m)))

-ifdef CONFIG_LTO_CLANG
-targets-for-modules += $(patsubst %.o, %.lto.o, $(filter %.o, $(obj-m)))
-endif
+targets-for-modules += $(patsubst %.o, %.prelink.o, $(filter %.o, $(obj-m)))

ifdef need-modorder
targets-for-modules += $(obj)/modules.order
@@ -282,33 +280,12 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) $(objtool_dep) FORCE
$(call if_changed_rule,cc_o_c)
$(call cmd,force_checksrc)

-ifdef CONFIG_LTO_CLANG
-# Module .o files may contain LLVM bitcode, compile them into native code
-# before ELF processing
-quiet_cmd_cc_lto_link_modules = LTO [M] $@
-cmd_cc_lto_link_modules = \
- $(LD) $(ld_flags) -r -o $@ \
- $(shell [ -s $(@:.lto.o=.o.symversions) ] && \
- echo -T $(@:.lto.o=.o.symversions)) \
- --whole-archive $(filter-out FORCE,$^)
-
-ifdef CONFIG_STACK_VALIDATION
-# objtool was skipped for LLVM bitcode, run it now that we have compiled
-# modules into native code
-cmd_cc_lto_link_modules += ; \
- $(objtree)/tools/objtool/objtool $(objtool_args) --module $@
-endif
-
-$(obj)/%.lto.o: $(obj)/%.o FORCE
- $(call if_changed,cc_lto_link_modules)
-endif
-
cmd_mod = { \
echo $(if $($*-objs)$($*-y)$($*-m), $(addprefix $(obj)/, $($*-objs) $($*-y) $($*-m)), $(@:.mod=.o)); \
$(undefined_syms) echo; \
} > $@

-$(obj)/%.mod: $(obj)/%$(mod-prelink-ext).o FORCE
+$(obj)/%.mod: $(obj)/%.prelink.o FORCE
$(call if_changed,mod)

quiet_cmd_cc_lst_c = MKLST $@
@@ -412,17 +389,6 @@ $(obj)/%.asn1.c $(obj)/%.asn1.h: $(src)/%.asn1 $(objtree)/scripts/asn1_compiler
$(subdir-builtin): $(obj)/%/built-in.a: $(obj)/% ;
$(subdir-modorder): $(obj)/%/modules.order: $(obj)/% ;

-# combine symversions for later processing
-ifeq ($(CONFIG_LTO_CLANG) $(CONFIG_MODVERSIONS),y y)
- cmd_update_lto_symversions = \
- rm -f $@.symversions \
- $(foreach n, $(filter-out FORCE,$^), \
- $(if $(shell test -s $(n).symversions && echo y), \
- ; cat $(n).symversions >> $@.symversions))
-else
- cmd_update_lto_symversions = echo >/dev/null
-endif
-
#
# Rule to compile a set of .o files into one .a file (without symbol table)
#
@@ -442,10 +408,10 @@ $(obj)/built-in.a: $(real-obj-y) FORCE
# modules.order unless contained modules are updated.

cmd_modules_order = { $(foreach m, $(real-prereqs), \
- $(if $(filter %/modules.order, $m), cat $m, echo $(patsubst %.o,%.ko,$m));) :; } \
+ $(if $(filter %/modules.order, $m), cat $m, echo $(patsubst %.a,%.ko,$m));) :; } \
| $(AWK) '!x[$$0]++' - > $@

-$(obj)/modules.order: $(obj-m) FORCE
+$(obj)/modules.order: $(modules) FORCE
$(call if_changed,modules_order)

#
@@ -454,26 +420,55 @@ $(obj)/modules.order: $(obj-m) FORCE
$(obj)/lib.a: $(lib-y) FORCE
$(call if_changed,ar)

-# NOTE:
-# Do not replace $(filter %.o,^) with $(real-prereqs). When a single object
-# module is turned into a multi object module, $^ will contain header file
-# dependencies recorded in the .*.cmd file.
+#
+# Rule to prelink modules
+#
+
+ifeq ($(CONFIG_LTO_CLANG) $(CONFIG_MODVERSIONS),y y)
+
+cmd_merge_symver = \
+ rm -f $@; \
+ touch $@; \
+ for o in $$($(AR) t $<); do \
+ if [ -s $${o}.symversions ]; then \
+ cat $${o}.symversions >> $@; \
+ fi; \
+ done
+
+$(obj)/%.prelink.symversions: $(obj)/%.a FORCE
+ $(call if_changed,merge_symver)
+
+$(obj)/%.prelink.o: ld_flags += --script=$(filter %.symversions,$^)
+module-symver = $(obj)/%.prelink.symversions
+
+endif
+
+quiet_cmd_ld_o_a = LD [M] $@
+ cmd_ld_o_a = $(LD) $(ld_flags) -r -o $@ --whole-archive $<
+
+$(obj)/%.prelink.o: $(obj)/%.a $(module-symver) FORCE
+ $(call if_changed,ld_o_a)
+
ifdef CONFIG_LTO_CLANG
-quiet_cmd_link_multi-m = AR [M] $@
-cmd_link_multi-m = \
- $(cmd_update_lto_symversions); \
- rm -f $@; \
- $(AR) cDPrsT $@ $(filter %.o,$^)
-else
-quiet_cmd_link_multi-m = LD [M] $@
- cmd_link_multi-m = $(LD) $(ld_flags) -r -o $@ $(filter %.o,$^)
+ifdef CONFIG_STACK_VALIDATION
+# objtool was skipped for LLVM bitcode, run it now that we have compiled
+# modules into native code
+cmd_ld_o_a += ; $(objtool) $(objtool_args) --module $@
endif
+endif
+
+quiet_cmd_ar_module = AR [M] $@
+ cmd_ar_module = rm -f $@; $(AR) cDPrST $@ $(real-prereqs)
+
+$(modules-single): %.a: %.o FORCE
+ $(call if_changed,ar_module)
+
+$(modules-multi): FORCE
+ $(call if_changed,ar_module)
+$(call multi_depend, $(modules-multi), .a, -objs -y -m)

-$(multi-obj-m): FORCE
- $(call if_changed,link_multi-m)
-$(call multi_depend, $(multi-obj-m), .o, -objs -y -m)
+targets += $(modules-single) $(modules-multi)

-targets += $(multi-obj-m)
targets := $(filter-out $(PHONY), $(targets))

# Add intermediate targets:
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 34c4c11c4bc1..f604d2d01cad 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -106,6 +106,10 @@ multi-dtb-y := $(addprefix $(obj)/, $(multi-dtb-y))
real-dtb-y := $(addprefix $(obj)/, $(real-dtb-y))
subdir-ym := $(addprefix $(obj)/,$(subdir-ym))

+modules := $(patsubst %.o, %.a, $(obj-m))
+modules-multi := $(patsubst %.o, %.a, $(multi-obj-m))
+modules-single := $(filter-out $(modules-multi), $(filter %.a, $(modules)))
+
# Finds the multi-part object the current object will be linked into.
# If the object belongs to two or more multi-part objects, list them all.
modname-multi = $(sort $(foreach m,$(multi-obj-ym),\
@@ -225,13 +229,6 @@ dtc_cpp_flags = -Wp,-MMD,$(depfile).pre.tmp -nostdinc \
$(addprefix -I,$(DTC_INCLUDE)) \
-undef -D__DTS__

-ifeq ($(CONFIG_LTO_CLANG),y)
-# With CONFIG_LTO_CLANG, .o files in modules might be LLVM bitcode, so we
-# need to run LTO to compile them into native code (.lto.o) before further
-# processing.
-mod-prelink-ext := .lto
-endif
-
# Useful for describing the dependency of composite objects
# Usage:
# $(call multi_depend, multi_used_targets, suffix_to_remove, suffix_to_add)
diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index ff805777431c..1b6401f53662 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -9,7 +9,7 @@ __modfinal:
include include/config/auto.conf
include $(srctree)/scripts/Kbuild.include

-# for c_flags and mod-prelink-ext
+# for c_flags
include $(srctree)/scripts/Makefile.lib

# find all modules listed in modules.order
@@ -55,7 +55,7 @@ if_changed_except = $(if $(call newer_prereqs_except,$(2))$(cmd-check), \

# Re-generate module BTFs if either module's .ko or vmlinux changed
-$(modules): %.ko: %$(mod-prelink-ext).o %.mod.o scripts/module.lds $(if $(KBUILD_BUILTIN),vmlinux) FORCE
+$(modules): %.ko: %.prelink.o %.mod.o scripts/module.lds $(if $(KBUILD_BUILTIN),vmlinux) FORCE
+$(call if_changed_except,ld_ko_o,vmlinux)
ifdef CONFIG_DEBUG_INFO_BTF_MODULES
+$(if $(newer-prereqs),$(call cmd,btf_ko))
diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
index eef56d629799..11883b31c615 100644
--- a/scripts/Makefile.modpost
+++ b/scripts/Makefile.modpost
@@ -41,9 +41,6 @@ __modpost:
include include/config/auto.conf
include $(srctree)/scripts/Kbuild.include

-# for mod-prelink-ext
-include $(srctree)/scripts/Makefile.lib
-
MODPOST = scripts/mod/modpost \
$(if $(CONFIG_MODVERSIONS),-m) \
$(if $(CONFIG_MODULE_SRCVERSION_ALL),-a) \
@@ -128,9 +125,9 @@ endif
# Read out modules.order to pass in modpost.
# Otherwise, allmodconfig would fail with "Argument list too long".
quiet_cmd_modpost = MODPOST $@
- cmd_modpost = sed 's/\.ko$$/$(mod-prelink-ext)\.o/' $< | $(MODPOST) -T -
+ cmd_modpost = sed 's/ko$$/prelink.o/' $< | $(MODPOST) -T -

-$(output-symdump): $(MODORDER) $(input-symdump) $(modules:.ko=$(mod-prelink-ext).o) FORCE
+$(output-symdump): $(MODORDER) $(input-symdump) $(modules:ko=prelink.o) FORCE
$(call if_changed,modpost)

targets += $(output-symdump)
diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 270a7df898e2..8c63c52af88d 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -1991,9 +1991,9 @@ static void read_symbols(const char *modname)
/* strip trailing .o */
tmp = NOFAIL(strdup(modname));
tmp[strlen(tmp) - 2] = '\0';
- /* strip trailing .lto */
- if (strends(tmp, ".lto"))
- tmp[strlen(tmp) - 4] = '\0';
+ /* strip trailing .prelink */
+ if (strends(tmp, ".prelink"))
+ tmp[strlen(tmp) - 8] = '\0';
mod = new_module(tmp);
free(tmp);
}
diff --git a/scripts/mod/sumversion.c b/scripts/mod/sumversion.c
index 760e6baa7eda..8ea0f7b23c63 100644
--- a/scripts/mod/sumversion.c
+++ b/scripts/mod/sumversion.c
@@ -391,14 +391,10 @@ void get_src_version(const char *modname, char sum[], unsigned sumlen)
struct md4_ctx md;
char *fname;
char filelist[PATH_MAX + 1];
- int postfix_len = 1;
-
- if (strends(modname, ".lto.o"))
- postfix_len = 5;

/* objects for a module are listed in the first line of *.mod file. */
snprintf(filelist, sizeof(filelist), "%.*smod",
- (int)strlen(modname) - postfix_len, modname);
+ (int)(strlen(modname) - strlen("prelink.o")), modname);

buf = read_text_file(filelist);

--
2.30.2