如何解决objcopy：膨胀的二进制输出文件
在处理引导程序项目时，我注意到生成的二进制文件大于原始ELF文件中每个部分的大小之和。
通过ld链接引导程序映像后，ELF文件的结构如下：
$ readelf -S elfboot.elf
There are 10 section headers,starting at offset 0xa708:
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .text PROGBITS 00007e00 000e00 004bc7 00 AX 0 0 16
[ 2] .rodata PROGBITS 0000c9d0 0059d0 000638 00 A 0 0 32
[ 3] .initcalls PROGBITS 0000ec20 007c20 00000c 00 WA 0 0 4
[ 4] .exitcalls PROGBITS 0000ec30 007c30 00000c 00 WA 0 0 4
[ 5] .data PROGBITS 0000ec40 007c40 0001e0 00 WA 0 0 32
[ 6] .bss NOBITS 0000ee20 007e20 00025c 00 WA 0 0 32
[ 7] .symtab SYMTAB 00000000 007e20 0016e0 10 8 166 4
[ 8] .strtab STRTAB 00000000 009500 0011bb 00 0 0 1
[ 9] .shstrtab STRTAB 00000000 00a6bb 00004a 00 0 0 1
Key to Flags:
W (write),A (alloc),X (execute),M (merge),S (strings),I (info),L (link order),O (extra OS processing required),G (group),T (TLS),C (compressed),x (unknown),o (OS specific),E (exclude),p (processor specific)
我注意到.rodata和.initcalls节之间有很大的差距（〜7KB）。我还检查了每个部分的内容：
$ objdump -s elfboot.elf
[...]
Contents of section .rodata:
[...]
cfc0 23cf0000 80000000 2fcf0000 00010000 #......./.......
cfd0 3bcf0000 00020000 47cf0000 00040000 ;.......G.......
cfe0 54cf0000 00080000 61cf0000 00100000 T.......a.......
cff0 6ecf0000 00200000 7bcf0000 00400000 n.... ..{....@..
d000 89cf0000 00800000 ........
Contents of section .initcalls:
ec20 02b50000 6db50000 feac0000 ....m.......
Contents of section .exitcalls:
[...]
我的链接脚本告诉我们将每个节对齐在16字节（0x10）的边界处。之后
objcopy -O binary elfboot.elf elfboot.bin
生成的二进制文件在偏移量0x5200 (0xd000 - 0x7e00)到偏移量0x6e20 (0xec20 - 0x7e00)处都包含一大块零填充字节。现在，我知道零填充字节来自何处，如何删除它们？作为参考，我添加了链接器步骤的详细输出，其中包括所使用的链接器脚本：
i686-elfboot-gcc -o elfboot.elf -T elfboot.ld ./src/arch/x86/bootstrap.o ./src/arch/x86/a20.o ./src/arch/x86/bda.o ./src/arch/x86/bios.o ./src/arch/x86/copy.o ./src/arch/x86/e820.o ./src/arch/x86/entry.o ./src/arch/x86/idt.o ./src/arch/x86/realmode_jmp.o ./src/arch/x86/setup.o ./src/arch/x86/opmode.o ./src/arch/x86/pic.o ./src/arch/x86/ptrace.o ./src/arch/x86/video.o ./src/core/bdev.o ./src/core/cdev.o ./src/core/elf.o ./src/core/input.o ./src/core/interrupt.o ./src/core/loader.o ./src/core/main.o ./src/core/module.o ./src/core/pci.o ./src/core/printf.o ./src/core/string.o ./src/core/symbol.o ./src/crypto/crc32.o ./src/drivers/ide/ide.o ./src/fs/fs.o ./src/fs/file.o ./src/fs/super.o ./src/fs/ramfs/ramfs.o ./src/fs/isofs/isofs.o ./src/lib/ata/libata.o ./src/lib/tmg/libtmg.o ./src/mm/memblock.o ./src/mm/page_alloc.o ./src/mm/slub.o ./src/mm/util.o -O2 -Wl,--verbose -nostdlib -lgcc
GNU ld (GNU Binutils) 2.31.1
Supported emulations:
elf_i386
elf_iamcu
opened script file elfboot.ld
using external linker script:
==================================================
OUTPUT_FORMAT(elf32-i386)
ENTRY(_arch_start)
SECTIONS
{
/*
* The boot stack starts at 0x6000 and grows towards lower addresses.
* The stack is designed to be only one page in size,which should be
* sufficient for the bootstrap stage.
*/
. = 0x6000;
__stack_start = .;
/*
* Buffer for reading files from the boot device. Usually boot devices
* are designed to read data in chunks of 0x200 (512) or 0x800 (2048)
* bytes. The buffer is large enough to read 4 512 or 2 2048 chunks of
* data from the disk.
*/
__buffer_start = .;
. = 0x7000;
__buffer_end = .;
/*
* Actual bootstrap stage code and data.
*/
. = 0x7E00;
__bootstrap_start = .;
.text ALIGN(0x10) : {
__text_start = .;
*(.text*)
__text_end = .;
}
.rodata ALIGN(0x10) : {
__rodata_start = .;
*(.rodata*)
__rodata_end = .;
}
/*
* This section is dedicated to all built-in modules. If a module will
* be included in the elfboot binary,the module_init function pointer
* of that module is placed in this section.
*
* Make sure to initialize built-in filesystems first as we need it to
* setup the root file system node.
*/
.initcalls ALIGN(0x10) : {
__initcalls_vfs_start = .;
/* Built-in file systems */
*(.initcalls_vfs*)
__initcalls_vfs_end = .;
__initcalls_dev_start = .;
/* Built-in devices */
*(.initcalls_dev*)
__initcalls_dev_end = .;
__initcalls_start = .;
/* Built-in modules */
*(.initcalls*)
__initcalls_end = .;
}
.exitcalls ALIGN(0x10) : {
__exitcalls_start = .;
*(.exitcalls*)
__exitcalls_end = .;
}
.data ALIGN(0x10) : {
__data_start = .;
*(.data*)
__data_end = .;
}
.bss ALIGN(0x10) : {
__bss_start = .;
*(.bss*)
__bss_end = .;
}
__bootstrap_end = .;
}
==================================================
attempt to open ./src/arch/x86/bootstrap.o succeeded
./src/arch/x86/bootstrap.o
attempt to open ./src/arch/x86/a20.o succeeded
./src/arch/x86/a20.o
attempt to open ./src/arch/x86/bda.o succeeded
./src/arch/x86/bda.o
attempt to open ./src/arch/x86/bios.o succeeded
./src/arch/x86/bios.o
attempt to open ./src/arch/x86/copy.o succeeded
./src/arch/x86/copy.o
attempt to open ./src/arch/x86/e820.o succeeded
./src/arch/x86/e820.o
attempt to open ./src/arch/x86/entry.o succeeded
./src/arch/x86/entry.o
attempt to open ./src/arch/x86/idt.o succeeded
./src/arch/x86/idt.o
attempt to open ./src/arch/x86/realmode_jmp.o succeeded
./src/arch/x86/realmode_jmp.o
attempt to open ./src/arch/x86/setup.o succeeded
./src/arch/x86/setup.o
attempt to open ./src/arch/x86/opmode.o succeeded
./src/arch/x86/opmode.o
attempt to open ./src/arch/x86/pic.o succeeded
./src/arch/x86/pic.o
attempt to open ./src/arch/x86/ptrace.o succeeded
./src/arch/x86/ptrace.o
attempt to open ./src/arch/x86/video.o succeeded
./src/arch/x86/video.o
attempt to open ./src/core/bdev.o succeeded
./src/core/bdev.o
attempt to open ./src/core/cdev.o succeeded
./src/core/cdev.o
attempt to open ./src/core/elf.o succeeded
./src/core/elf.o
attempt to open ./src/core/input.o succeeded
./src/core/input.o
attempt to open ./src/core/interrupt.o succeeded
./src/core/interrupt.o
attempt to open ./src/core/loader.o succeeded
./src/core/loader.o
attempt to open ./src/core/main.o succeeded
./src/core/main.o
attempt to open ./src/core/module.o succeeded
./src/core/module.o
attempt to open ./src/core/pci.o succeeded
./src/core/pci.o
attempt to open ./src/core/printf.o succeeded
./src/core/printf.o
attempt to open ./src/core/string.o succeeded
./src/core/string.o
attempt to open ./src/core/symbol.o succeeded
./src/core/symbol.o
attempt to open ./src/crypto/crc32.o succeeded
./src/crypto/crc32.o
attempt to open ./src/drivers/ide/ide.o succeeded
./src/drivers/ide/ide.o
attempt to open ./src/fs/fs.o succeeded
./src/fs/fs.o
attempt to open ./src/fs/file.o succeeded
./src/fs/file.o
attempt to open ./src/fs/super.o succeeded
./src/fs/super.o
attempt to open ./src/fs/ramfs/ramfs.o succeeded
./src/fs/ramfs/ramfs.o
attempt to open ./src/fs/isofs/isofs.o succeeded
./src/fs/isofs/isofs.o
attempt to open ./src/lib/ata/libata.o succeeded
./src/lib/ata/libata.o
attempt to open ./src/lib/tmg/libtmg.o succeeded
./src/lib/tmg/libtmg.o
attempt to open ./src/mm/memblock.o succeeded
./src/mm/memblock.o
attempt to open ./src/mm/page_alloc.o succeeded
./src/mm/page_alloc.o
attempt to open ./src/mm/slub.o succeeded
./src/mm/slub.o
attempt to open ./src/mm/util.o succeeded
./src/mm/util.o
attempt to open /home/croemheld/Repositories/elfboot/elfboot-toolchain/lib/gcc/i686-elfboot/8.2.0/libgcc.so failed
attempt to open /home/croemheld/Repositories/elfboot/elfboot-toolchain/lib/gcc/i686-elfboot/8.2.0/libgcc.a succeeded
(/home/croemheld/Repositories/elfboot/elfboot-toolchain/lib/gcc/i686-elfboot/8.2.0/libgcc.a)_umoddi3.o
(/home/croemheld/Repositories/elfboot/elfboot-toolchain/lib/gcc/i686-elfboot/8.2.0/libgcc.a)_udivmoddi4.o
请请注意，由于第一阶段的引导加载程序（引导扇区）仅以与引导设备指定的偏移量加载整个文件，因此无法将这些段分别加载到内存中。为了减小第二阶段引导加载程序映像的大小，我想尝试在链接时或如果可能的话，在链接后（objcopy，...）消除间隙。
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