2. Linux Makefiles

2.1. linux/Makefile

Here are some well-known targets in this top-level makefile:

• xconfig, menuconfig, config, oldconfig: generate kernel configuration file linux/.config;

• depend, dep: generate dependency files, like linux/.depend, linux/.hdepend and .depend in subdirectories;

• vmlinux: generate resident kernel image linux/vmlinux, the most important target;

• modules, modules_install: generate and install modules in /lib/modules/$(KERNELRELEASE);

• tags: generate tag file linux/tags, for source browsing with vim.

Overview of linux/Makefile is outlined below:

include .depend include .config include arch/i386/Makefile vmlinux: generate linux/vmlinux /* entry point "stext" defined in arch/i386/kernel/head.S */ $(LD) -T $(TOPDIR)/arch/i386/vmlinux.lds -e stext /* $(HEAD) */ + from arch/i386/Makefile arch/i386/kernel/head.o arch/i386/kernel/init_task.o init/main.o init/version.o init/do_mounts.o --start-group /* $(CORE_FILES) */ + from arch/i386/Makefile arch/i386/kernel/kernel.o arch/i386/mm/mm.o kernel/kernel.o mm/mm.o fs/fs.o ipc/ipc.o /* $(DRIVERS) */ drivers/... char/char.o block/block.o misc/misc.o net/net.o media/media.o cdrom/driver.o and other static linked drivers + from arch/i386/Makefile arch/i386/math-emu/math.o (ifdef CONFIG_MATH_EMULATION) /* $(NETWORKS) */ net/network.o /* $(LIBS) */ + from arch/i386/Makefile arch/i386/lib/lib.a lib/lib.a --end-group -o vmlinux $(NM) vmlinux | grep ... | sort > System.map tags: generate linux/tags for vim modules: generate modules modules_install: install modules clean mrproper distclean: clean up build directory psdocs pdfdocs htmldocs mandocs: generate kernel documents include Rules.make rpm: generate an rpm

"--start-group" and "--end-group" are ld command line options to resolve symbol reference problem. Refer to Using LD, the GNU linker: Command Line Options for details.

Rules.make contains rules which are shared between multiple Makefiles.

2.2. linux/arch/i386/vmlinux.lds

After compilation, ld combines a number of object and archive files, relocates their data and ties up symbol references. linux/arch/i386/vmlinux.lds is designated by linux/Makefile as the linker script used in linking the resident kernel image linux/vmlinux.

/* ld script to make i386 Linux kernel * Written by Martin Mares <mj@atrey.karlin.mff.cuni.cz>; */ OUTPUT_FORMAT("elf32-i386", "elf32-i386", "elf32-i386") OUTPUT_ARCH(i386) /* "ENTRY" is overridden by command line option "-e stext" in linux/Makefile */ ENTRY(_start) /* Output file (linux/vmlinux) layout. * Refer to Using LD, the GNU linker: Specifying Output Sections */ SECTIONS { /* Output section .text starts at address 3G+1M. * Refer to Using LD, the GNU linker: The Location Counter */ . = 0xC0000000 + 0x100000; _text = .; /* Text and read-only data */ .text : { *(.text) *(.fixup) *(.gnu.warning) } = 0x9090 /* Unallocated holes filled with 0x9090, i.e. opcode for "NOP NOP". * Refer to Using LD, the GNU linker: Optional Section Attributes */ _etext = .; /* End of text section */ .rodata : { *(.rodata) *(.rodata.*) } .kstrtab : { *(.kstrtab) } /* Aligned to next 16-bytes boundary. * Refer to Using LD, the GNU linker: Arithmetic Functions */ . = ALIGN(16); /* Exception table */ __start___ex_table = .; __ex_table : { *(__ex_table) } __stop___ex_table = .; __start___ksymtab = .; /* Kernel symbol table */ __ksymtab : { *(__ksymtab) } __stop___ksymtab = .; .data : { /* Data */ *(.data) CONSTRUCTORS } /* For "CONSTRUCTORS", refer to * Using LD, the GNU linker: Option Commands */ _edata = .; /* End of data section */ . = ALIGN(8192); /* init_task */ .data.init_task : { *(.data.init_task) } . = ALIGN(4096); /* Init code and data */ __init_begin = .; .text.init : { *(.text.init) } .data.init : { *(.data.init) } . = ALIGN(16); __setup_start = .; .setup.init : { *(.setup.init) } __setup_end = .; __initcall_start = .; .initcall.init : { *(.initcall.init) } __initcall_end = .; . = ALIGN(4096); __init_end = .; . = ALIGN(4096); .data.page_aligned : { *(.data.idt) } . = ALIGN(32); .data.cacheline_aligned : { *(.data.cacheline_aligned) } __bss_start = .; /* BSS */ .bss : { *(.bss) } _end = . ; /* Output section /DISCARD/ will not be included in the final link output. * Refer to Using LD, the GNU linker: Section Definitions */ /* Sections to be discarded */ /DISCARD/ : { *(.text.exit) *(.data.exit) *(.exitcall.exit) } /* The following output sections are addressed at memory location 0. * Refer to Using LD, the GNU linker: Optional Section Attributes */ /* Stabs debugging sections. */ .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) } }

2.3. linux/arch/i386/Makefile

linux/arch/i386/Makefile is included by linux/Makefile to provide i386 specific items and terms.

All the following targets depend on target vmlinux of linux/Makefile. They are accomplished by making corresponding targets in linux/arch/i386/boot/Makefile with some options.

It is worth noticing that this makefile redefines some environment variables which are exported by linux/Makefile, specifically:

OBJCOPY=$(CROSS_COMPILE)objcopy -O binary -R .note -R .comment -S

The effect will be passed to subdirectory makefiles and will change the tool's behavior. Refer to GNU Binary Utilities: objcopy for objcopy command line option details.

Not sure why $(LIBS) includes "$(TOPDIR)/arch/i386/lib/lib.a" twice:

LIBS := $(TOPDIR)/arch/i386/lib/lib.a $(LIBS) $(TOPDIR)/arch/i386/lib/lib.a

It may be employed to work around linking problems with some toolchains.

2.4. linux/arch/i386/boot/Makefile

linux/arch/i386/boot/Makefile is somehow independent as it is not included by either linux/arch/i386/Makefile or linux/Makefile.

However, they do have some relationship:

• linux/Makefile: provides resident kernel image linux/vmlinux;

• linux/arch/i386/boot/Makefile: provides bootstrap;

• linux/arch/i386/Makefile: makes sure linux/vmlinux is ready before the bootstrap is constructed, and exports targets (like bzImage) to linux/Makefile.

$(BOOTIMAGE) value, which is for target zdisk, zlilo or zdisk, comes from linux/arch/i386/Makefile.

$(OBJCOPY) compressed/vmlinux compressed/vmlinux.out
tools/build bootsect setup compressed/vmlinux.out $(ROOT_DEV) > zImage

$(OBJCOPY) compressed/bvmlinux compressed/bvmlinux.out
tools/build -b bbootsect bsetup compressed/bvmlinux.out $(ROOT_DEV) \
        > bzImage

dd bs=8192 if=$(BOOTIMAGE) of=/dev/fd0

if [ -f $(INSTALL_PATH)/vmlinuz ]; then mv $(INSTALL_PATH)/vmlinuz
        $(INSTALL_PATH)/vmlinuz.old; fi
if [ -f $(INSTALL_PATH)/System.map ]; then mv $(INSTALL_PATH)/System.map
        $(INSTALL_PATH)/System.old; fi
cat $(BOOTIMAGE) > $(INSTALL_PATH)/vmlinuz
cp $(TOPDIR)/System.map $(INSTALL_PATH)/
if [ -x /sbin/lilo ]; then /sbin/lilo; else /etc/lilo/install; fi

sh -x ./install.sh $(KERNELRELEASE) $(BOOTIMAGE) $(TOPDIR)/System.map
        "$(INSTALL_PATH)"

2.5. linux/arch/i386/boot/compressed/Makefile

This makefile handles image (de)compression mechanism.

It is good to separate (de)compression from bootstrap. This divide-and-conquer solution allows us to easily improve (de)compression mechanism or to adopt a new bootstrap method.

Directory linux/arch/i386/boot/compressed/ contains two source files: head.S and misc.c.

$(CC) $(CFLAGS) -DKBUILD_BASENAME=$(subst $(comma),_,$(subst -,_,$(*F)))
        -c misc.c[b]

tmppiggy=_tmp_$$$$piggy; \
rm -f $$tmppiggy $$tmppiggy.gz $$tmppiggy.lnk; \
$(OBJCOPY) $(SYSTEM) $$tmppiggy; \
gzip -f -9 < $$tmppiggy > $$tmppiggy.gz; \
echo "SECTIONS { .data : { input_len = .; \
        LONG(input_data_end - input_data) input_data = .; \
        *(.data) input_data_end = .; }}" > $$tmppiggy.lnk; \
$(LD) -r -o piggy.o -b binary $$tmppiggy.gz -b elf32-i386 \
        -T $$tmppiggy.lnk; \
rm -f $$tmppiggy $$tmppiggy.gz $$tmppiggy.lnk

piggy.o contains variable input_len and gzipped linux/vmlinux. input_len is at the beginning of piggy.o, and it is equal to the size of piggy.o excluding input_len itself. Refer to Using LD, the GNU linker: Section Data Expressions for "LONG(expression)" in piggy.o linker script.

To be exact, it is not linux/vmlinux itself (in ELF format) that is gzipped but its binary image, which is generated by objcopy command. Note that $(OBJCOPY) has been redefined by linux/arch/i386/Makefile in Section 2.3 to output raw binary using "-O binary" option.

When linking {bootsect, setup} or {bbootsect, bsetup}, $(LD) specifies "--oformat binary" option to output them in binary format. When making zImage (or bzImage), $(OBJCOPY) generates an intermediate binary output from compressed/vmlinux (or compressed/bvmlinux) too. It is vital that all components in zImage or bzImage are in raw binary format, so that the image can run by itself without asking a loader to load and relocate it.

Both vmlinux and bvmlinux prepend head.o and misc.o before piggy.o, but they are linked against different start addresses (0x1000 vs 0x100000).

2.6. linux/arch/i386/tools/build.c

linux/arch/i386/tools/build.c is a host utility to generate zImage or bzImage.

In linux/arch/i386/boot/Makefile:

tools/build bootsect setup compressed/vmlinux.out $(ROOT_DEV) > zImage tools/build -b bbootsect bsetup compressed/bvmlinux.out $(ROOT_DEV) > bzImage

"-b" means is_big_kernel, used to check whether system image is too big.

tools/build outputs the following components to stdout, which is redirected to zImage or bzImage:

1. bootsect or bbootsect: from linux/arch/i386/boot/bootsect.S, 512 bytes;

2. setup or bsetup: from linux/arch/i386/boot/setup.S, 4 sectors or more, sector aligned;

3. compressed/vmlinux.out or compressed/bvmlinux.out, including:

   1. head.o: from linux/arch/i386/boot/compressed/head.S;

   2. misc.o: from linux/arch/i386/boot/compressed/misc.c;

   3. piggy.o: from input_len and gzipped linux/vmlinux.

tools/build will change some contents of bootsect or bbootsect when outputting to stdout:

In the following chapters, compressed/vmlinux will be referred as vmlinux and compressed/bvmlinux as bvmlinux, if not confusing.

2.7. Reference

• Linux Kernel Makefiles: linux/Documentation/kbuild/makefiles.txt

• The Linux Kernel HOWTO

• GNU make

• Using LD, the GNU linker

• GNU Binary Utilities

• GNU Bash