嵌入式Linux之uboot源码make编译逆向分析

一、make编译逆向分析之prepare

在上一篇文章中分析Makefile文件依赖关系时采用自顶向下的方法，从顶层目标开始到最原始的依赖结束。

此处我们采用自下而上的方式，从最原始的依赖开始，一步一步的执行命令生成目标。

完整的prepare的目标依赖：

依次从最右边的依赖说起；

1.1  .config(scripts/kconfig/conf生成的文件)

.config在执行make smdk2410_defconfig配置时生成，文件scripts/kconfig/Makefile中定义了所有匹配%config的目标：

%_defconfig: $(obj)/conf
    $(Q)$< $(silent) --defconfig=arch/$(SRCARCH)/configs/$@ $(Kconfig)

展开为：

smdk2410_defconfig: scripts/kconfig/conf
    $(Q)scripts/kconfig/conf  --defconfig=arch/../configs/smdk2410_defconfig Kconfig

scripts/kconfig/conf会从根目录开始读取Kconfig，输出到根目录下的.config中：

• include/generated/autoconf.h
• include/config/auto.conf.cmd
• include/config/tristate.conf
• include/config/auto.conf

以上4个文件在执行make编译命令的开始会检查%.conf的依赖规则：

include/config/%.conf: $(KCONFIG_CONFIG) include/config/auto.conf.cmd
    $(Q)$(MAKE) -f $(srctree)/Makefile silentoldconfig
    $(Q)$(MAKE) -f $(srctree)/scripts/Makefile.autoconf || 
        { rm -f include/config/auto.conf; false; }
    $(Q)touch include/config/auto.conf

调用make -f ./Makefile silentoldconfig的最终结果是执行scripts/kconfig/Makefile中的规则：

silentoldconfig: $(obj)/conf
    $(Q)mkdir -p include/config include/generated
    $< $(silent) --$@ $(Kconfig)

这个规则展开为：

silentoldconfig: scripts/kconfig/conf
    mkdir -p include/config include/generated
    scripts/kconfig/conf --silentoldconfig Kconfig

scripts/kconfig/conf会从根目录开始读取Kconfig，同时检查并更新配置阶段生成的.config文件，再把最终结果输出到以上的4个文件中。

所生成的4个文件中，include/config/auto.conf依赖于include/config/auto.conf.cmd，但是这里的依赖文件include/config/auto.conf.cmd文件并非由fixdep生成，而是直接由conf工具生成，算是*.cmd文件生成的特例。 

scripts/kconfig/conf生成了图中右侧的依赖：include/config/auto.conf，$(KCONIFG_CONFIG)/.config和include/config/auto.conf.cmd。

1.2 目标include/config/auto.conf的规则

在生成include/config/auto.conf的规则中：

# If .config is newer than include/config/auto.conf, someone tinkered
# with it and forgot to run make oldconfig.
# if auto.conf.cmd is missing then we are probably in a cleaned tree so
# we execute the config step to be sure to catch updated Kconfig files
include/config/%.conf: $(KCONFIG_CONFIG) include/config/auto.conf.cmd
    $(Q)$(MAKE) -f $(srctree)/Makefile silentoldconfig
    @# If the following part fails, include/config/auto.conf should be
    @# deleted so "make silentoldconfig" will be re-run on the next build.
    $(Q)$(MAKE) -f $(srctree)/scripts/Makefile.autoconf || 
        { rm -f include/config/auto.conf; false; }
    @# include/config.h has been updated after "make silentoldconfig".
    @# We need to touch include/config/auto.conf so it gets newer
    @# than include/config.h.
    @# Otherwise, 'make silentoldconfig' would be invoked twice.
    $(Q)touch include/config/auto.conf

除了执行$(MAKE) -f  $(srctree)/Makefile silentoldconfig外，还执行$(MAKE) -f  $(srctree)/scripts/Makefile.autoconf 在scripts/Makefile.autoconf的头部是这样的：

# This helper makefile is used for creating
#  - symbolic links (arch/$ARCH/include/asm/arch
#  - include/autoconf.mk, {spl,tpl}/include/autoconf.mk
#  - include/config.h
#
# When our migration to Kconfig is done
# (= When we move all CONFIGs from header files to Kconfig)
# this makefile can be deleted.
#
# SPDX-License-Identifier:  GPL-2.0
#

__all: include/autoconf.mk include/autoconf.mk.dep

ifeq ($(shell grep -q '^CONFIG_SPL=y' include/config/auto.conf 2>/dev/null && echo y),y)
__all: spl/include/autoconf.mk
endif

ifeq ($(shell grep -q '^CONFIG_TPL=y' include/config/auto.conf 2>/dev/null && echo y),y)
__all: tpl/include/autoconf.mk
endif

此处没有设置CONFIG_SPL=y或CONFIG_TPL=y，所以整个makefile的__all的依赖有：

• include/autoconf.mk
• include/autoconf.mk.dep

然而include/autoconf.mk还要进一步依赖于config.h。

(1) include/config.h的规则

所有的autoconf.mk都依赖于include/config.h：

include/autoconf.mk include/autoconf.mk.dep 
    spl/include/autoconf.mk tpl/include/autoconf.mk: include/config.h

实际上include/config.h由宏filechk_config_h生成：

# include/config.h
# Prior to Kconfig, it was generated by mkconfig. Now it is created here.
define filechk_config_h
    (echo "/* Automatically generated - do not edit */";        
    for i in $$(echo $(CONFIG_SYS_EXTRA_OPTIONS) | sed 's/,/ /g'); do 
        echo #define CONFIG_$$i                
        | sed '/=/ {s/=/    /;q; } ; { s/$$/    1/; }'; 
    done;                               
    echo #define CONFIG_BOARDDIR board/$(if $(VENDOR),$(VENDOR)/)$(BOARD);
    echo #include <config_defaults.h>;               
    echo #include <config_uncmd_spl.h>;              
    echo #include <configs/$(CONFIG_SYS_CONFIG_NAME).h>;     
    echo #include <asm/config.h>;                
    echo #include <config_fallbacks.h>;)
endef

include/config.h: scripts/Makefile.autoconf create_symlink FORCE
    $(call filechk,config_h)

最终生成的include/config.h也比较简单，不妨看看：

/* Automatically generated - do not edit */
#define CONFIG_BOARDDIR board/samsung/smdk2410
#include <config_defaults.h>
#include <config_uncmd_spl.h>
#include <configs/smdk2410.h>
#include <asm/config.h>
#include <config_fallbacks.h>

生成config.h之前，还要应用create_symlink生成相应的符号链接。

(2) create_symlink的规则

# symbolic links
# If arch/$(ARCH)/mach-$(SOC)/include/mach exists,
# make a symbolic link to that directory.
# Otherwise, create a symbolic link to arch/$(ARCH)/include/asm/arch-$(SOC).
PHONY += create_symlink
create_symlink:
ifdef CONFIG_CREATE_ARCH_SYMLINK
ifneq ($(KBUILD_SRC),)
    $(Q)mkdir -p include/asm
    $(Q)if [ -d $(KBUILD_SRC)/arch/$(ARCH)/mach-$(SOC)/include/mach ]; then 
        dest=arch/$(ARCH)/mach-$(SOC)/include/mach;         
    else                                    
        dest=arch/$(ARCH)/include/asm/arch-$(if $(SOC),$(SOC),$(CPU));  
    fi;                                 
    ln -fsn $(KBUILD_SRC)/$$dest include/asm/arch
else
    $(Q)if [ -d arch/$(ARCH)/mach-$(SOC)/include/mach ]; then   
        dest=../../mach-$(SOC)/include/mach;            
    else                                
        dest=arch-$(if $(SOC),$(SOC),$(CPU));           
    fi;                             
    ln -fsn $$dest arch/$(ARCH)/include/asm/arch
endif
endif

注释已经很好解释了create_symlink的行为：

• 如果arch/$(ARCH)/mach-$(SOC)/include/mach存在，则生成符号链接：

arch/$(ARCH)/include/asm/arch --> arch/$(ARCH)/mach-$(SOC)

• 否则生成符号链接：

arch/$(ARCH)/include/asm/arch -->arch/$(ARCH)/include/asm/arch-$(SOC) 

对基于arm v9架构的s3c24x0芯片，arch/arm/mach-s3c2440文件夹不存在，所以生成链接：

arch/arm/include/asm/arch --> arch/arm/include/asm/arch-s3c24x0

简单说来，create_symlink就是将芯片arch/arm/include/asm/arch-s3c24x0链接到跟芯片名字无关的arch/arm/include/asm下。 

(3) include/autoconf.mk的规则

# We are migrating from board headers to Kconfig little by little.
# In the interim, we use both of
#  - include/config/auto.conf (generated by Kconfig)
#  - include/autoconf.mk      (used in the U-Boot conventional configuration)
# The following rule creates autoconf.mk
# include/config/auto.conf is grepped in order to avoid duplication of the
# same CONFIG macros
quiet_cmd_autoconf = GEN     $@
      cmd_autoconf = 
    $(CPP) $(c_flags) $2 -DDO_DEPS_ONLY -dM $(srctree)/include/common.h > $@.tmp && { 
        sed -n -f $(srctree)/tools/scripts/define2mk.sed $@.tmp |       
        while read line; do                         
            if [ -n "${KCONFIG_IGNORE_DUPLICATES}" ] ||         
               ! grep -q "$${line%=*}=" include/config/auto.conf; then  
                echo "$$line";                      
            fi                              
        done > $@;                              
        rm $@.tmp;                              
    } || {                                      
        rm $@.tmp; false;                           
    }

include/autoconf.mk: FORCE
    $(call cmd,autoconf)

从cmd_autoconf来看，这里会根据include/common.h的依赖，然后调用tools/scripts/define2mk.sed，并合并之前生成的include/config/auto.conf生成最终的autoconf.mk。

(4) include/autoconf.mk.dep的规则

quiet_cmd_autoconf_dep = GEN     $@
      cmd_autoconf_dep = $(CC) -x c -DDO_DEPS_ONLY -M -MP $(c_flags) 
    -MQ include/config/auto.conf $(srctree)/include/common.h > $@ || {  
        rm $@; false;                           
    }
include/autoconf.mk.dep: FORCE
    $(call cmd,autoconf_dep)

这个规则比较简单，由于autoconf.mk由common.h和auto.conf而来，因此直接处理这两个文件的依赖并合并到autoconf.mk.dep中。

1.3 include/config/uboot.release

define filechk_uboot.release
    echo "$(UBOOTVERSION)$$($(CONFIG_SHELL) $(srctree)/scripts/setlocalversion $(srctree))"
endef

# Store (new) UBOOTRELEASE string in include/config/uboot.release
include/config/uboot.release: include/config/auto.conf FORCE
    $(call filechk,uboot.release)

命令$(call filechk,uboot.release)展开后就是调用宏filechk_uboot.release，最终将字符串2016.09写入include/config/uboot.release中。

1.4 timestamp.h和version.h的规则

ersion_h := include/generated/version_autogenerated.h
timestamp_h := include/generated/timestamp_autogenerated.h

...

# Generate some files
# ---------------------------------------------------------------------------

define filechk_version.h
    (echo #define PLAIN_VERSION "$(UBOOTRELEASE)"; 
    echo #define U_BOOT_VERSION "U-Boot " PLAIN_VERSION; 
    echo #define CC_VERSION_STRING "$$(LC_ALL=C $(CC) --version | head -n 1)"; 
    echo #define LD_VERSION_STRING "$$(LC_ALL=C $(LD) --version | head -n 1)"; )
endef

# The SOURCE_DATE_EPOCH mechanism requires a date that behaves like GNU date.
# The BSD date on the other hand behaves different and would produce errors
# with the misused '-d' switch.  Respect that and search a working date with
# well known pre- and suffixes for the GNU variant of date.
define filechk_timestamp.h
    (if test -n "$${SOURCE_DATE_EPOCH}"; then 
        SOURCE_DATE="@$${SOURCE_DATE_EPOCH}"; 
        DATE=""; 
        for date in gdate date.gnu date; do 
            $${date} -u -d "$${SOURCE_DATE}" >/dev/null 2>&1 && DATE="$${date}"; 
        done; 
        if test -n "$${DATE}"; then 
            LC_ALL=C $${DATE} -u -d "$${SOURCE_DATE}" +'#define U_BOOT_DATE "%b %d %C%y"'; 
            LC_ALL=C $${DATE} -u -d "$${SOURCE_DATE}" +'#define U_BOOT_TIME "%T"'; 
            LC_ALL=C $${DATE} -u -d "$${SOURCE_DATE}" +'#define U_BOOT_TZ "%z"'; 
            LC_ALL=C $${DATE} -u -d "$${SOURCE_DATE}" +'#define U_BOOT_DMI_DATE "%m/%d/%Y"'; 
        else 
            return 42; 
        fi; 
    else 
        LC_ALL=C date +'#define U_BOOT_DATE "%b %d %C%y"'; 
        LC_ALL=C date +'#define U_BOOT_TIME "%T"'; 
        LC_ALL=C date +'#define U_BOOT_TZ "%z"'; 
        LC_ALL=C date +'#define U_BOOT_DMI_DATE "%m/%d/%Y"'; 
    fi)
endef

$(version_h): include/config/uboot.release FORCE
    $(call filechk,version.h)

$(timestamp_h): $(srctree)/Makefile FORCE
    $(call filechk,timestamp.h)

(1) include/generated/version_autogenerated.h

根据include/config/uboot.release文件，规则调用filechk_version.h宏生成版本相关字符串文件include/generated/version_autogenerated.h，如下：

#define PLAIN_VERSION "2016.05"
#define U_BOOT_VERSION "U-Boot " PLAIN_VERSION
#define CC_VERSION_STRING "arm-none-linux-gnueabi-gcc (Sourcery G++ Lite 2008q3-72) 4.3.2"
#define LD_VERSION_STRING "GNU ld (Sourcery G++ Lite 2008q3-72) 2.18.50.20080215

(2) include/generated/timestamp_autogenerated.h

调用宏filechk_timestamp.h生成编译的时间戳文件，如下：

#define U_BOOT_DATE "Jun 07 2021"
#define U_BOOT_TIME "01:58:59"
#define U_BOOT_TZ "+0800"
#define U_BOOT_DMI_DATE "06/07/2021"

1.5 outputmakfile的规则

PHONY += outputmakefile
# outputmakefile generates a Makefile in the output directory, if using a
# separate output directory. This allows convenient use of make in the
# output directory.
outputmakefile:
ifneq ($(KBUILD_SRC),)
    $(Q)ln -fsn $(srctree) source
    $(Q)$(CONFIG_SHELL) $(srctree)/scripts/mkmakefile 
        $(srctree) $(objtree) $(VERSION) $(PATCHLEVEL)
endif

• 如果编译没有设置O，即输出和代码都在同一个目录下，则outputmakefile的规则什么都不做；
• 如果编译指定了输出目录O，则调用scripts/mkmakefile在O选项指定的目录下生成一个简单的makefile

1.6 scripts_basic的规则

# Basic helpers built in scripts/
PHONY += scripts_basic
scripts_basic:
    $(Q)$(MAKE) $(build)=scripts/basic
    $(Q)rm -f .tmp_quiet_recordmcount

scripts_basic的执行结果就是编译生成scripts/basic/fixdep工具，该工具是u-boot编译系统中最常用的工具，用于在编译过程中修正每一个生成文件的依赖关系。

1.7 parepare0的规则

prepare0: archprepare FORCE
    $(Q)$(MAKE) $(build)=.

展开后为：

prepare0: archprepare FORCE
    make -f ./scripts/Makefile.build obj=.

编译时，命令make -f ./scripts/Makefile.build obj=.不会生成任何目标。

1.8 prepare系列目标总结

prepare阶段主要做了以下工作：

• scripts_basic规则生成fixdep工具，用于对整个系统生成目标文件相应依赖文件的更新；
• 配置阶段，scripts/kconfig/conf根据传入的指定配置文件在根目录下生成.config文件；
• 编译阶段，scripts/kconfig/conf读取配置阶段生成的.config，并检查最新配置生成以下文件：

1. • include/generated/autoconf.h
   • include/config/auto.conf.cmd
   • include/config/tristate.conf
   • include/config/auto.conf

• 调用宏filechk_config_h生成include/config.h文件；
• 调用命令cmd_autoconf_dep生成autoconf.mk和autoconf.mk.cmd文件；
• 调用宏filechk_uboot.release生成include/config/uboot.release文件；
• 调用宏filechk_version.h生成include/generated/version_autogenerated.h文件；
• 调用宏filechk_timestamp.h生成include/generated/timestamp_autogenerated.h文件；
• 调用宏create_symlink就是将芯片arch/$(ARCH)/include/asm/arch-$(SOC)链接到跟芯片名字无关的arch/$(ARCH)/include/asm下。

二、make编译逆向分析之u-boot

完整的u-boot的目标依赖：

从图上可见，除了prepare依赖外，u-boot还依赖于文件$(head-y)，$(libs-y)和$(LDSCRIPT)，即依赖于：

• 启动文件arch/arm/cpu/$(CPU)/start.o ；
• 各个目录下的build-in.o ；
• 链接脚本文件arch/arm/cpu/u-boot.lds；

2.1 启动文件start.o

$(head-y)在arch/arm/Makefile中被直接指定：

head-y := arch/arm/cpu/$(CPU)/start.o

在顶层makefile中被指定给变量u-boot-init：

u-boot-init := $(head-y)

2.2 各目录下的build-in.o

$(libs-y)在顶层的makefile中被指定为各个子目录下的build-in.o的集合：

libs-y += lib/
...
libs-y += fs/
libs-y += net/
libs-y += disk/
libs-y += drivers/
...

libs-y += $(if $(BOARDDIR),board/$(BOARDDIR)/)

libs-y := $(sort $(libs-y))

...

libs-y      := $(patsubst %/, %/built-in.o, $(libs-y))

...
u-boot-main := $(libs-y)

以上脚本中，先将$(libs-y)设置为各子目录的集合，最后调用patsubst函数将$(libs-y)设置为这些目录下的built-in.o文件的集合，最后赋值给变量u-boot-main作为链接的主体文件。

各目录下的built-in.o是如何生成的呢？

以drivers/mmc/built-in.o为例，先查看生成的依赖文件drivers/mmc/.built-in.o.cmd：

cmd_drivers/mmc/built-in.o :=  rm -f drivers/mmc/built-in.o; arm-linux-ar rcs drivers/mmc/built-in.o

生成built-in.o的规则在scripts/Makefile.build中定义：

#
# Rule to compile a set of .o files into one .o file
#
ifdef builtin-target
quiet_cmd_link_o_target = LD      $@
# If the list of objects to link is empty, just create an empty built-in.o
cmd_link_o_target = $(if $(strip $(obj-y)),
              $(LD) $(ld_flags) -r -o $@ $(filter $(obj-y), $^) 
              $(cmd_secanalysis),
              rm -f $@; $(AR) rcs$(KBUILD_ARFLAGS) $@)

$(builtin-target): $(obj-y) FORCE
    $(call if_changed,link_o_target)

targets += $(builtin-target)
endif # builtin-target

2.3 链接脚本u-boot.lds

链接脚本的规则如下：

quiet_cmd_cpp_lds = LDS     $@
cmd_cpp_lds = $(CPP) -Wp,-MD,$(depfile) $(cpp_flags) $(LDPPFLAGS) 
        -D__ASSEMBLY__ -x assembler-with-cpp -P -o $@ $<

u-boot.lds: $(LDSCRIPT) prepare FORCE
    $(call if_changed_dep,cpp_lds)

2.4 生成u-boot规则

顶层Makefile中定义了生成u-boot文件的规则：

# Rule to link u-boot
# May be overridden by arch/$(ARCH)/config.mk
quiet_cmd_u-boot__ ?= LD      $@
      cmd_u-boot__ ?= $(LD) $(LDFLAGS) $(LDFLAGS_u-boot) -o $@ 
      -T u-boot.lds $(u-boot-init)                             
      --start-group $(u-boot-main) --end-group                 
      $(PLATFORM_LIBS) -Map u-boot.map

...

u-boot: $(u-boot-init) $(u-boot-main) u-boot.lds FORCE
    $(call if_changed,u-boot__)
...

u-boot文件的生成很简单，调用ld命令，将$(u-boot-init)和$(u-boot-main)指定的一系列文件通过脚本u-boot.lds连接起来。

生成了u-boot文件后，后续就是针对u-boot文件的各种处理了。

三、make编译逆向分析之 顶层目标依赖

顶层目标依赖：

 

显然，在生成了u-boot的基础上，进一步生成所需要的各种目标文件：

3.1 u-boot.srec

# Normally we fill empty space with 0xff
quiet_cmd_objcopy = OBJCOPY $@
cmd_objcopy = $(OBJCOPY) --gap-fill=0xff $(OBJCOPYFLAGS) 
    $(OBJCOPYFLAGS_$(@F)) $< $@
...
OBJCOPYFLAGS_u-boot.hex := -O ihex

OBJCOPYFLAGS_u-boot.srec := -O srec

u-boot.hex u-boot.srec: u-boot FORCE
    $(call if_changed,objcopy)

调用objcopy命令，通过-O ihex或-O srec指定生成u-boot.hex或u-boot.srec格式文件。

3.2 u-boot.sym

quiet_cmd_sym ?= SYM     $@
      cmd_sym ?= $(OBJDUMP) -t $< > $@
u-boot.sym: u-boot FORCE
    $(call if_changed,sym)

调用$(OBJDUMP)命令生成符号表文件u-boot.sym。

3.3 System.map

SYSTEM_MAP = 
        $(NM) $1 | 
        grep -v '(compiled)|(.o$$)|( [aUw] )|(..ng$$)|(LASH[RL]DI)' | 
        LC_ALL=C sort
System.map: u-boot
        @$(call SYSTEM_MAP,$<) > $@

调用$(NM)命令打印u-boot文件的符号表，并用grep -v处理后得到System.map文件，里面包含了最终使用到的各个符号的位置信息。

3.4 u-boot.bin和u-boot-nodtb.bin

PHONY += dtbs
dtbs: dts/dt.dtb
    @:
dts/dt.dtb: checkdtc u-boot
    $(Q)$(MAKE) $(build)=dts dtbs

quiet_cmd_copy = COPY    $@
      cmd_copy = cp $< $@

ifeq ($(CONFIG_OF_SEPARATE),y)
u-boot-dtb.bin: u-boot-nodtb.bin dts/dt.dtb FORCE
    $(call if_changed,cat)

u-boot.bin: u-boot-dtb.bin FORCE
    $(call if_changed,copy)
else
u-boot.bin: u-boot-nodtb.bin FORCE
    $(call if_changed,copy)
endif

由于这里没有使用device tree设置，即编译没有定义CONFIG_OF_SEPARATE，因此u-boot.bin和u-boot-nodtb.bin是一样的。

至于生成u-boot-nodtb.bin的规则：

u-boot-nodtb.bin: u-boot FORCE
    $(call if_changed,objcopy)
    $(call DO_STATIC_RELA,$<,$@,$(CONFIG_SYS_TEXT_BASE))
    $(BOARD_SIZE_CHECK)

显然，u-boot-nodtb.bin是u-boot文件通过objcopy得到。

3.5 u-boot.cfg

u-boot.cfg中包含了所有用到的宏定义，其生成规则如下：

# Create a file containing the configuration options the image was built with
quiet_cmd_cpp_cfg = CFG     $@
cmd_cpp_cfg = $(CPP) -Wp,-MD,$(depfile) $(cpp_flags) $(LDPPFLAGS) -ansi 
    -DDO_DEPS_ONLY -D__ASSEMBLY__ -x assembler-with-cpp -P -dM -E -o $@ $<
...
u-boot.cfg: include/config.h FORCE
    $(call if_changed,cpp_cfg)

因此，阅读源码时如果不确定某个宏的值，可以检查u-boot.cfg文件。 自此，生成了所有的目标文件，完成了整个编译过程的分析。