# Build-Linux-Kernel-Using-Clang

**Repository Path**: frontcold/Build-Linux-Kernel-Using-Clang

## Basic Information

- **Project Name**: Build-Linux-Kernel-Using-Clang
- **Description**: Build-Linux-Kernel-Using-Clang from github 
- **Primary Language**: Unknown
- **License**: Not specified
- **Default Branch**: master
- **Homepage**: None
- **GVP Project**: No

## Statistics

- **Stars**: 0
- **Forks**: 0
- **Created**: 2019-07-27
- **Last Updated**: 2020-12-19

## Categories & Tags

**Categories**: Uncategorized

**Tags**: None

## README

[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
```shell
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
```shell
git apply path-of-patch/build-linux-kernel-using-clang.patch
```
+ ignore the file generated by clang
```diff
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
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
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
```diff
@@ -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
```diff
@@ -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
```diff
@@ -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
```diff
@@ -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
```diff
@@ -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
```diff
@@ -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
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 += $(call cc-option,-fsanitize-coverage=trace-cmp,)
 endif
```
+ in order to make `fembed-bitcode` work well, need to disable or remove kcov and kasan.
```diff
diff --git a/kernel/Makefile b/kernel/Makefile
index f85ae5dfa474..25c6b6d0fc8b 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -79,7 +79,7 @@ obj-$(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
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
```shell
cd linux-stable
make SHELL=/bin/bash V=1 2>&1 | tee make.log
```
## Link the bc file
```shell
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](https://github.com/ssl-tud/k-miner)