了解nostdlib C程序的ELF二进制大小 [英] Understanding ELF Binary Size for nostdlib C Program

问题描述

我使用的是Ubuntu 20.04， gcc 9.3.0， ld 2.34.我有一个简单的hello world程序，它不使用 glibc 或任何其他库，而仅使用write syscall.尽管如此，我的二进制文件大小仍约为8Kb.我不确定为什么这么大而不说1Kb.

I'm on Ubuntu 20.04, gcc 9.3.0, ld 2.34. I have a simple hello world program that does not use glibc or any other library and just uses write syscall. Despite this, my binary size is roughly 8Kb. I'm unsure as to why it is that large and not say 1Kb.

C程序:

int
x64_syscall_write(int fd, char const *data, unsigned long int data_size)
{
  int result = 0;
  __asm__ __volatile__("syscall"
              : "=a" (result)
              : "a" (1), "D" (fd),
                "S" (data), "d" (data_size)
              : "r11", "rcx", "memory");
  return result;
}

__asm__(".global entry_point\n"
  "entry_point:\n"
  "xor rbp, rbp\n"
  "pop rdi\n"
  "mov rsi, rsp\n"
  "and rsp, 0xfffffffffffffff0\n"
  "call main\n"
  "mov rdi, rax\n"
  "mov rax, 60\n"
  "syscall\n"
  "ret");

int
main(int argc, char *argv[])
{
  x64_syscall_write(1, "hello\n", 6); 
  return 0;
}

内置于:

gcc -ffreestanding -static -nostdlib -no-pie -masm=intel \
-fno-unwind-tables -fno-asynchronous-unwind-tables \
-Wl,--gc-sections -fdata-sections -Os \
hello.c -c -o hello.o

# NOTE: I know more could be done here to shave 
# off a few more bytes, but I feel this is the bulk of it.

ld -e entry_point hello.o -o hello

hello.o 为1.7Kb. hello 是8.4Kb.

hello.o is 1.7Kb. hello is 8.4Kb.

推荐答案

readelf -Wl hello

Elf file type is EXEC (Executable file)
Entry point 0x40101c
There are 6 program headers, starting at offset 64

Program Headers:
  Type           Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
  LOAD           0x000000 0x0000000000400000 0x0000000000400000 0x0001b0 0x0001b0 R   0x1000
  LOAD           0x001000 0x0000000000401000 0x0000000000401000 0x000045 0x000045 R E 0x1000
  LOAD           0x002000 0x0000000000402000 0x0000000000402000 0x000007 0x000007 R   0x1000
  NOTE           0x000190 0x0000000000400190 0x0000000000400190 0x000020 0x000020 R   0x8
  GNU_PROPERTY   0x000190 0x0000000000400190 0x0000000000400190 0x000020 0x000020 R   0x8
  GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW  0x10

 Section to Segment mapping:
  Segment Sections...
   00     .note.gnu.property
   01     .text
   02     .rodata
   03     .note.gnu.property
   04     .note.gnu.property
   05

在这里您可以看到链接器创建了3个 LOAD 段:一个用于 ELF 标头和其他元数据，一个用于 .text 和一个用于 .rodata .

Here you can see that the linker created 3 LOAD segments: one for the ELF header and other metadata, one for .text and one for .rodata.

与 -z鼻梁代码链接所生成的二进制文件要小得多(比 hello.o 小):

Linking with -z noseparate-code results in much smaller binary (smaller than hello.o):

 ls -l hello*
-rwxr-xr-x 1 user user 1384 Apr 26 22:24 hello
-rw-r--r-- 1 user user  603 Apr 26 22:22 hello.c
-rw-r--r-- 1 user user 1680 Apr 26 22:22 hello.o

readelf -Wl hello

Elf file type is EXEC (Executable file)
Entry point 0x40015c
There are 4 program headers, starting at offset 64

Program Headers:
  Type           Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
  LOAD           0x000000 0x0000000000400000 0x0000000000400000 0x00018c 0x00018c R E 0x1000
  NOTE           0x000120 0x0000000000400120 0x0000000000400120 0x000020 0x000020 R   0x8
  GNU_PROPERTY   0x000120 0x0000000000400120 0x0000000000400120 0x000020 0x000020 R   0x8
  GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW  0x10

 Section to Segment mapping:
  Segment Sections...
   00     .note.gnu.property .text .rodata
   01     .note.gnu.property
   02     .note.gnu.property
   03

您可以通过删除 .note.GNU-stack 和 .note.gnu.property 部分来进一步缩小:

You can shrink this further by removing .note.GNU-stack and .note.gnu.property sections:

objcopy -R .note.* hello.o hello1.o
ld -e entry_point hello1.o -o hello1 -z noseparate-code

ls -l hello1*
-rwxr-xr-x 1 user user 1072 Apr 26 22:38 hello1
-rw-r--r-- 1 user user 1440 Apr 26 22:37 hello1.o

readelf -Wl hello1

Elf file type is EXEC (Executable file)
Entry point 0x400094
There is 1 program header, starting at offset 64

Program Headers:
  Type           Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
  LOAD           0x000000 0x0000000000400000 0x0000000000400000 0x0000c4 0x0000c4 R E 0x1000

 Section to Segment mapping:
  Segment Sections...
   00     .text .rodata

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