基于linux2.6.38.8内核zImage文件的自解压详解

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作者：EasyWave                                                                                 时间：2012.02.18

类别：linux驱动开发                                                                           声明：转载，请保留链接

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内核编译完成后会生成zImage内核镜像文件。zImage是如何解压的呢？本文将结合关键代码，讲解zImage的解压过程。还是先来看看zImage的组成吧。在内核编译完成后会在arch/arm/boot/下生成zImage。 

在arch/arm/boot/Makefile中，如下代码：

#
# arch/arm/boot/Makefile
#
# This file is included by the global makefile so that you can add your own
# architecture-specific flags and dependencies.
#
# This file is subject to the terms and conditions of the GNU General Public
# License.  See the file "COPYING" in the main directory of this archive
# for more details.
#
# Copyright (C) 1995-2002 Russell King
#

MKIMAGE         := $(srctree)/scripts/mkuboot.sh

ifneq ($(MACHINE),)
include $(srctree)/$(MACHINE)/Makefile.boot
endif

# Note: the following conditions must always be true:
#   ZRELADDR == virt_to_phys(PAGE_OFFSET + TEXT_OFFSET)
#   PARAMS_PHYS must be within 4MB of ZRELADDR
#   INITRD_PHYS must be in RAM
ZRELADDR    := $(zreladdr-y)
PARAMS_PHYS := $(params_phys-y)
INITRD_PHYS := $(initrd_phys-y)

export ZRELADDR INITRD_PHYS PARAMS_PHYS

targets := Image zImage xipImage bootpImage uImage

ifeq ($(CONFIG_XIP_KERNEL),y)

$(obj)/xipImage: vmlinux FORCE
	$(call if_changed,objcopy)
	@echo '  Kernel: $@ is ready (physical address: $(CONFIG_XIP_PHYS_ADDR))'

$(obj)/Image $(obj)/zImage: FORCE
	@echo 'Kernel configured for XIP (CONFIG_XIP_KERNEL=y)'
	@echo 'Only the xipImage target is available in this case'
	@false

else

$(obj)/xipImage: FORCE
	@echo 'Kernel not configured for XIP (CONFIG_XIP_KERNEL!=y)'
	@false

$(obj)/Image: vmlinux FORCE
	$(call if_changed,objcopy)
	@echo '  Kernel: $@ is ready'

$(obj)/compressed/vmlinux: $(obj)/Image FORCE
	$(Q)$(MAKE) $(build)=$(obj)/compressed $@

$(obj)/zImage:	$(obj)/compressed/vmlinux FORCE
	$(call if_changed,objcopy)
	@echo '  Kernel: $@ is ready'

endif

quiet_cmd_uimage = UIMAGE  $@
      cmd_uimage = $(CONFIG_SHELL) $(MKIMAGE) -A arm -O linux -T kernel \
		   -C none -a $(LOADADDR) -e $(STARTADDR) \
		   -n 'Linux-$(KERNELRELEASE)' -d {1}lt; $@

ifeq ($(CONFIG_ZBOOT_ROM),y)
$(obj)/uImage: LOADADDR=$(CONFIG_ZBOOT_ROM_TEXT)
else
$(obj)/uImage: LOADADDR=$(ZRELADDR)
endif

$(obj)/uImage: STARTADDR=$(LOADADDR)

$(obj)/uImage:	$(obj)/zImage FORCE
	$(call if_changed,uimage)
	@echo '  Image $@ is ready'

$(obj)/bootp/bootp: $(obj)/zImage initrd FORCE
	$(Q)$(MAKE) $(build)=$(obj)/bootp $@
	@:

$(obj)/bootpImage: $(obj)/bootp/bootp FORCE
	$(call if_changed,objcopy)
	@echo '  Kernel: $@ is ready'

PHONY += initrd FORCE
initrd:
	@test "$(INITRD_PHYS)" != "" || \
	(echo This machine does not support INITRD; exit -1)
	@test "$(INITRD)" != "" || \
	(echo You must specify INITRD; exit -1)

install: $(obj)/Image
	$(CONFIG_SHELL) $(srctree)/$(src)/install.sh $(KERNELRELEASE) \
	$(obj)/Image System.map "$(INSTALL_PATH)"

zinstall: $(obj)/zImage
	$(CONFIG_SHELL) $(srctree)/$(src)/install.sh $(KERNELRELEASE) \
	$(obj)/zImage System.map "$(INSTALL_PATH)"

zi:
	$(CONFIG_SHELL) $(srctree)/$(src)/install.sh $(KERNELRELEASE) \
	$(obj)/zImage System.map "$(INSTALL_PATH)"

i:
	$(CONFIG_SHELL) $(srctree)/$(src)/install.sh $(KERNELRELEASE) \
	$(obj)/Image System.map "$(INSTALL_PATH)"

subdir-	    := bootp compressed

我们将上面的部分内容抓取出来，独立来分析，这样就不会被其它的代码干扰到，见下面的代码：

$(obj)/Image: vmlinux FORCE
$(call if_changed,objcopy)
@echo '  Kernel: $@ is ready'

$(obj)/zImage: $(obj)/compressed/vmlinux FORCE
$(call if_changed,objcopy)
@echo '  Kernel: $@ is ready'
由此可见，zImage是由arch/arm/boot/compressed/vmlinux二进制化得到的，与此同时在arch/armboot/compressed/Makefile中：
$(obj)/vmlinux: $(obj)/vmlinux.lds $(obj)/$(HEAD) $(obj)/piggy.$(suffix_y).o \
$(addprefix $(obj)/, $(OBJS)) $(lib1funcs) FORCE
$(call if_changed,ld)
@:

$(obj)/piggy.$(suffix_y): $(obj)/../Image FORCE
$(call if_changed,$(suffix_y))

$(obj)/piggy.$(suffix_y).o:  $(obj)/piggy.$(suffix_y) FORCE
其中Image是由内核顶层目录下的vmlinux二进制化后得到的[见上面蓝色加粗部分代码]，其中在arch/armboot/compressed/Makefile链接选项中有个 –fpic参数：
EXTRA_CFLAGS  := -fpic -fno-builtin
EXTRA_AFLAGS  := -Wa,-march=all

总结一下zImage的组成，它是由一个压缩后的内核piggy.o，连接上一段初始化及解压功能的代码（head.o misc.o）,组成的。见下面的代码：

#
# linux/arch/arm/boot/compressed/Makefile
#
# create a compressed vmlinuz image from the original vmlinux
#
......................
......................
#
# We now have a PIC decompressor implementation.  Decompressors running
# from RAM should not define ZTEXTADDR.  Decompressors running directly
# from ROM or Flash must define ZTEXTADDR (preferably via the config)
# FIXME: Previous assignment to ztextaddr-y is lost here. See SHARK
ifeq ($(CONFIG_ZBOOT_ROM),y)
ZTEXTADDR	:= $(CONFIG_ZBOOT_ROM_TEXT)
ZBSSADDR	:= $(CONFIG_ZBOOT_ROM_BSS)
else
ZTEXTADDR	:= 0
ZBSSADDR	:= ALIGN(8)
endif

SEDFLAGS	= s/TEXT_START/$(ZTEXTADDR)/;s/BSS_START/$(ZBSSADDR)/

suffix_$(CONFIG_KERNEL_GZIP) = gzip
suffix_$(CONFIG_KERNEL_LZO)  = lzo
suffix_$(CONFIG_KERNEL_LZMA) = lzma

targets       := vmlinux vmlinux.lds \
		 piggy.$(suffix_y) piggy.$(suffix_y).o \
		 font.o font.c head.o misc.o $(OBJS)

# Make sure files are removed during clean
extra-y       += piggy.gzip piggy.lzo piggy.lzma lib1funcs.S

targets       := vmlinux vmlinux.lds \
piggy.$(suffix_y) piggy.$(suffix_y).o \
font.o font.c head.o misc.o $(OBJS)
那么内核是从什么地方开始运行的呢？这个要看lds文件。其实zImage的生成经历了两次链接过程：一是顶层vmlinux的生成，在arch/arm/boot/vmlinux.lds（这个lds文件是由arch/arm/kernel/vmlinux.lds.S生成的）中；另一次是arch/arm/boot/compressed/vmlinux的生成，在arch/arm/boot/compressed/vmlinux.lds.in中。zImage的入口点应该由arch/arm/boot/compressed/vmlinux.lds.in决定。从中可以看出入口点为‘_start’

/*
 *  linux/arch/arm/boot/compressed/vmlinux.lds.in
 *
 *  Copyright (C) 2000 Russell King
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */
OUTPUT_ARCH(arm)
ENTRY(_start)
SECTIONS
{
  /DISCARD/ : {
    *(.ARM.exidx*)
    *(.ARM.extab*)
    /*
     * Discard any r/w data - this produces a link error if we have any,
     * which is required for PIC decompression.  Local data generates
     * GOTOFF relocations, which prevents it being relocated independently
     * of the text/got segments.
     */
    *(.data)
  }

  . = TEXT_START;
  _text = .;

  .text : {
    _start = .;
    *(.start)
    *(.text)
    *(.text.*)
    *(.fixup)
    *(.gnu.warning)
    *(.rodata)
    *(.rodata.*)
    *(.glue_7)
    *(.glue_7t)
    *(.piggydata)
    . = ALIGN(4);
  }

  _etext = .;

  /* Assume size of decompressed image is 4x the compressed image */
  _image_size = (_etext - _text) * 4;

  _got_start = .;
  .got			: { *(.got) }
  _got_end = .;
  .got.plt		: { *(.got.plt) }
  _edata = .;

  . = BSS_START;
  __bss_start = .;
  .bss			: { *(.bss) }
  _end = .;

  . = ALIGN(8);		/* the stack must be 64-bit aligned */
  .stack		: { *(.stack) }

  .stab 0		: { *(.stab) }
  .stabstr 0		: { *(.stabstr) }
  .stab.excl 0		: { *(.stab.excl) }
  .stab.exclstr 0	: { *(.stab.exclstr) }
  .stab.index 0		: { *(.stab.index) }
  .stab.indexstr 0	: { *(.stab.indexstr) }
  .comment 0		: { *(.comment) }
}

在arch/arm/boot/compressed/head.S中找到入口点。
看看head.S会做些什么样的工作： [下面是从网络上摘录]
1: 对于各种Arm CPU的DEBUG输出设定，通过定义宏来统一操作；
2: 设置kernel开始和结束地址，保存architecture ID；
3: 如果在ARM2以上的CPU中，用的是普通用户模式，则升到超级用户模式，然后关中断
4: 分析LC0结构delta offset，判断是否需要重载内核地址(r0存入偏移量，判断r0是否为零)。
5: 需要重载内核地址，将r0的偏移量加到BSS region和GOT table中的每一项。
   对于位置无关的代码，程序是通过GOT表访问全局数据目标的，也就是说GOT表中中记录的是全局数据目标的绝对地址，所以其中的每一项也需要重载。
6: 清空bss堆栈空间r2－r3
7: 建立C程序运行需要的缓存
8: 这时r2是缓存的结束地址，r4是kernel的最后执行地址，r5是kernel境象文件的开始地址
9: 用文件misc.c的函数decompress_kernel()，解压内核于缓存结束的地方(r2地址之后)。