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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(配置阶段)

.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%_defconfig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

在配置阶段，scripts/kconfig/conf搜集所有默认的Kconfig配置，然后用指定的smdk2410_defconfig配置进行更新并输出到根目录的.config文件中：

1.2 include/config/auto.conf的规则

在编译阶段，scripts/kconfig/conf读取配置阶段生成的.config，并检查最新配置生成以下4个文件：

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

在生成include/config/auto.conf的规则中：

复制代码
# 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
复制代码

(1) 首先执行$(MAKE) -f  $(srctree)/Makefile silentoldconfig外，即调用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

(2) 然后执行$(MAKE) -f  $(srctree)/scripts/Makefile.autoconf，在scripts/Makefile.autoconf的文件头部定义有：

复制代码
# 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
复制代码

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。

1.2.1 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：

复制代码
# 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)
复制代码

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。

1.2.2 create_symlink的规则
复制代码
# 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
复制代码

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。 

1.2.3 include/autoconf.mk的规则
复制代码
# 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)
复制代码

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。

1.2.4 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

复制代码
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)
复制代码
1.4.1 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
1.4.2 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

复制代码
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
复制代码
  • 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：
    • 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：

复制代码
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)
复制代码

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：

复制代码
#
# 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
复制代码

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

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