Build Linux Kernel Using Clang and Get LLVM bitcode

[TOC]

Build Linux Kernel Using Clang and Get LLVM bitcode

Getting Started

Prerequisites

git
llvm 7.0
clang 7.0

Clone the Linux source tree

git clone git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
cd linux-stable
git reset --hard v4.16

After linux kernel v4.16, it needs compiler support asm-goto and clang do not.

Configure the kernel

O0 can not be used.

Apply the patch

git apply path-of-patch/build-linux-kernel-using-clang.patch

ignore the file generated by clang

diff --git a/.gitignore b/.gitignore
index 1be78fd8163b..6b4d8dec8e97 100644
--- a/.gitignore
+++ b/.gitignore
@@ -33,6 +33,9 @@
 *.order
 *.patch
 *.s
+*.bc
+*.ll
+*.i
 *.so
 *.so.dbg
 *.su

change gcc to clang in makefile

diff --git a/Makefile b/Makefile
index 363dd096e46e..ca51c9f7747e 100644
--- a/Makefile
+++ b/Makefile
@@ -359,8 +359,8 @@ HOST_LFS_CFLAGS := $(shell getconf LFS_CFLAGS)
 HOST_LFS_LDFLAGS := $(shell getconf LFS_LDFLAGS)
 HOST_LFS_LIBS := $(shell getconf LFS_LIBS)
 
-HOSTCC = gcc
-HOSTCXX = g++
+HOSTCC = clang
+HOSTCXX = clang
 HOSTCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 \
   -fomit-frame-pointer -std=gnu89 $(HOST_LFS_CFLAGS)
 HOSTCXXFLAGS := -O2 $(HOST_LFS_CFLAGS)
@@ -370,7 +370,7 @@ HOST_LOADLIBES := $(HOST_LFS_LIBS)
 # Make variables (CC, etc...)
 AS	= $(CROSS_COMPILE)as
 LD	= $(CROSS_COMPILE)ld
-CC	= $(CROSS_COMPILE)gcc
+CC	= $(CROSS_COMPILE)clang
 CPP	= $(CC) -E
 AR	= $(CROSS_COMPILE)ar
 NM	= $(CROSS_COMPILE)nm

get the llvm bit code

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 4f2b25d43ec9..ea1709d89144 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build

add target for built-in

@@ -78,13 +78,13 @@ endif
 
 ifneq ($(strip $(obj-y) $(need-builtin)),)
 builtin-target := $(obj)/built-in.o
+builtin-target_bc := $(obj)/built-in.bc
 endif
 
 modorder-target := $(obj)/modules.order
 
 # We keep a list of all modules in $(MODVERDIR)
-
-__build: $(if $(KBUILD_BUILTIN),$(builtin-target) $(lib-target) $(extra-y)) \
+__build: $(if $(KBUILD_BUILTIN),$(builtin-target) $(builtin-target_bc) $(lib-target) $(extra-y)) \
   $(if $(KBUILD_MODULES),$(obj-m) $(modorder-target)) \
   $(subdir-ym) $(always)
  @:

add -save-temps=obj to get every single llvm bitcode add -fembed-bitcode to get the llvm bitcode after optimization because some files can not do this so disable in those files

@@ -176,7 +176,7 @@ $(obj)/%.symtypes : $(src)/%.c FORCE
 # LLVM assembly
 # Generate .ll files from .c
 quiet_cmd_cc_ll_c = CC $(quiet_modtag) $@
- cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -o $@ $<
+ cmd_cc_ll_c = $(CC) $(c_flags) -fembed-bitcode -save-temps=obj -S -o $@ $<
 
 $(obj)/%.ll: $(src)/%.c FORCE
  $(call if_changed_dep,cc_ll_c)
@@ -188,7 +188,12 @@ $(obj)/%.ll: $(src)/%.c FORCE
 quiet_cmd_cc_o_c = CC $(quiet_modtag) $@
 
 ifndef CONFIG_MODVERSIONS
-cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $<
+cmd_cc_o_c = \
+if [[ $(@) != "scripts/mod/empty.o" ]] && [[ ! $(@) =~ "arch/x86/boot"* ]] && [[ ! $(@) =~ "arch/x86/realmode/rm"* ]] ; then \
+	$(CC) $(c_flags) -fembed-bitcode -save-temps=obj -o $(patsubst %.bc, %.o, $(@)) -c $<; \
+else \
+	$(CC) $(c_flags) -c -o $@ $<; \
+fi
 
 else
 # When module versioning is enabled the following steps are executed:

add rule from c to bc and from s to bc

@@ -324,6 +329,9 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) $(objtool_dep) FORCE
  $(call cmd,force_checksrc)
  $(call if_changed_rule,cc_o_c)
 
+$(obj)/%.bc: $(src)/%.c
+	$(call if_changed_rule,cc_o_c)
+
 # Single-part modules are special since we need to mark them in $(MODVERDIR)
 
 $(single-used-m): $(obj)/%.o: $(src)/%.c $(recordmcount_source) $(objtool_dep) FORCE
@@ -390,6 +398,13 @@ $(obj)/%.s: $(src)/%.S FORCE
 
 quiet_cmd_as_o_S = AS $(quiet_modtag) $@
 
+quiet_cmd_as_bc_S = $(CC) -fembed-bitcode -save-temps=obj $(quiet_modtag) $@
+cmd_as_bc_S = $(CC) $(a_flags) -fembed-bitcode -save-temps=obj -c -o $@ $<
+asm_ign := ""
+$(obj)/%.bc: $(src)/%.S FORCE 
+	$(eval asm_ign += $@)
+	$(call if_changed_dep,as_bc_S)
+
 ifndef CONFIG_MODVERSIONS
 cmd_as_o_S = $(CC) $(a_flags) -c -o $@ $<

add rule for link llvm bitcode

@@ -474,9 +489,23 @@ cmd_link_o_target = $(if $(strip $(obj-y)),\
         $(cmd_secanalysis),\
         $(cmd_make_empty_builtin) $@)
 
+tmp = $(patsubst %.o, %.bc, $(obj-y))
+bc-y = $(filter-out $(asm_ign), $(tmp))
+ifneq ($(bc-y),"")
+	cmd_link_bc_target = $(if $(strip $(bc-y)),\
+ llvm-link -o $@ $(filter $(bc-y), $^) \
+ $(cmd_secanalysis),\
+	echo "" > $@)
+else
+	cmd_link_bc_target = echo "" > $@ 
+endif
+
 $(builtin-target): $(obj-y) FORCE
  $(call if_changed,link_o_target)
 
+$(builtin-target_bc): $(bc-y) FORCE
+	$(call if_changed,link_bc_target) 
+
 targets += $(builtin-target)
 endif # builtin-target

deal with some special case when meet some files that can not be built, use this way

@@ -512,6 +541,7 @@ targets += $(lib-target)
 
 dummy-object = $(obj)/.lib_exports.o
 ksyms-lds = $(dot-target).lds
+dummy-object-bc = $(obj)/.lib_exports.o.bc
 ifdef CONFIG_HAVE_UNDERSCORE_SYMBOL_PREFIX
 ref_prefix = EXTERN(_
 else
@@ -524,6 +554,7 @@ cmd_export_list = $(OBJDUMP) -h $< | \
  rm -f $(dummy-object);\
  echo | $(CC) $(a_flags) -c -o $(dummy-object) -x assembler -;\
  $(LD) $(ld_flags) -r -o $@ -T $(ksyms-lds) $(dummy-object);\
+	echo "" > $(patsubst %.o, %.bc, $(@));\
  rm $(dummy-object) $(ksyms-lds)
 
 $(obj)/lib-ksyms.o: $(lib-target) FORCE

add rule for link llvm bitcode

@@ -562,8 +593,12 @@ endif
 quiet_cmd_link_multi-m = LD [M] $@
 cmd_link_multi-m = $(cmd_link_multi-link)
 
+quiet_cmd_link_bc_multi-y = llvm-link $@
+cmd_link_bc_multi-y = llvm-link -o $(patsubst %.o, %.bc, $@) $(patsubst %.o, %.bc, $(link_multi_deps))
+
 $(multi-used-y): FORCE
  $(call if_changed,link_multi-y)
+	$(call if_changed,link_bc_multi-y)
 $(call multi_depend, $(multi-used-y), .o, -objs -y)
 
 $(multi-used-m): FORCE

in order to make kcov insert in every basic block. notice that this is only useful in clang.

diff --git a/scripts/Makefile.kcov b/scripts/Makefile.kcov
index 5cc72037e423..256ae15c6f03 100644
--- a/scripts/Makefile.kcov
+++ b/scripts/Makefile.kcov
@@ -1,5 +1,6 @@
 ifdef CONFIG_KCOV
 CFLAGS_KCOV	:= $(call cc-option,-fsanitize-coverage=trace-pc,)
+CFLAGS_KCOV += $(call cc-option,-fsanitize-coverage=no-prune,)
 ifeq ($(CONFIG_KCOV_ENABLE_COMPARISONS),y)
 CFLAGS_KCOV += $(call cc-option,-fsanitize-coverage=trace-cmp,)
 endif

in order to make fembed-bitcode work well, need to disable or remove kcov and kasan.

diff --git a/kernel/Makefile b/kernel/Makefile
index f85ae5dfa474..25c6b6d0fc8b 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -79,7 +79,7 @@ obj-$(CONFIG_AUDITSYSCALL) += auditsc.o
 obj-$(CONFIG_AUDIT_WATCH) += audit_watch.o audit_fsnotify.o
 obj-$(CONFIG_AUDIT_TREE) += audit_tree.o
 obj-$(CONFIG_GCOV_KERNEL) += gcov/
-obj-$(CONFIG_KCOV) += kcov.o
+# obj-$(CONFIG_KCOV) += kcov.o
 obj-$(CONFIG_KPROBES) += kprobes.o
 obj-$(CONFIG_FAIL_FUNCTION) += fail_function.o
 obj-$(CONFIG_KGDB) += debug/

diff --git a/mm/Makefile b/mm/Makefile
index e669f02c5a54..a2b925c69315 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -69,7 +69,7 @@ obj-$(CONFIG_KSM) += ksm.o
 obj-$(CONFIG_PAGE_POISONING) += page_poison.o
 obj-$(CONFIG_SLAB) += slab.o
 obj-$(CONFIG_SLUB) += slub.o
-obj-$(CONFIG_KASAN)	+= kasan/
+# obj-$(CONFIG_KASAN)	+= kasan/
 obj-$(CONFIG_FAILSLAB) += failslab.o
 obj-$(CONFIG_MEMORY_HOTPLUG) += memory_hotplug.o
 obj-$(CONFIG_MEMTEST)	+= memtest.o

Build the kernel

cd linux-stable
make SHELL=/bin/bash V=1 2>&1 | tee make.log

Link the bc file

llvm-link -o built-in.bc arch/x86/kernel/head64.bc arch/x86/kernel/ebda.bc arch/x86/kernel/platform-quirks.bc init/built-in.bc usr/built-in.bc arch/x86/built-in.bc kernel/built-in.bc certs/built-in.bc mm/built-in.bc fs/built-in.bc ipc/built-in.bc security/built-in.bc crypto/built-in.bc block/built-in.bc lib/built-in.bc arch/x86/lib/built-in.bc drivers/built-in.bc sound/built-in.bc firmware/built-in.bc arch/x86/pci/built-in.bc arch/x86/power/built-in.bc arch/x86/video/built-in.bc net/built-in.bc virt/built-in.bc

Get file used for analysis

llvm-dis built-in.bc
cat `find -name "*.s"` >> built-in.s

Acknowledgments

k-miner

Name		Name	Last commit message	Last commit date
Latest commit History 4 Commits
README.md		README.md
build-linux-kernel-using-clang.patch		build-linux-kernel-using-clang.patch

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Build Linux Kernel Using Clang and Get LLVM bitcode

Getting Started

Prerequisites

Clone the Linux source tree

Configure the kernel

Apply the patch

Build the kernel

Link the bc file

Get file used for analysis

Acknowledgments

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Build Linux Kernel Using Clang and Get LLVM bitcode

Getting Started

Prerequisites

Clone the Linux source tree

Configure the kernel

Apply the patch

Build the kernel

Link the bc file

Get file used for analysis

Acknowledgments

About

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