elf - 手动创建的 ELF 可执行文件因 SIGSEGV 而崩溃

Question

我需要学习手动创建 ELF 可执行文件。到目前为止，我一直在使用在线指南，例如:

Manually Creating an ELF Executable

ELF reference

几次失败后，我将我的程序简化为以下内容(它应该以返回代码 0 退出):

0000000: 7f45 4c46 0101 0100 0000 0000 0000 0010  .ELF............
0000010: 0200 0300 0100 0000 8080 0408 3400 0000  ............4...
0000020: 0000 0000 0000 0000 3400 2000 0100 2800  ........4. ...(.
0000030: 0000 0000 0100 0000 5400 0000 8080 0408  ........T.......
0000040: 0000 0000 0c00 0000 0c00 0000 0500 0000  ................
0000050: 0010 0000 b801 0000 00bb 0000 0000 cd80  ................

当我尝试执行它时，它会因 SIGSEGV 崩溃。 GDB 打印:

During startup program terminated with signal SIGSEGV, Segmentation fault.

我做错了什么？

Answer 1

使用你的二进制文件，我从 GDB 得到不同的输出:

(gdb) r
Starting program: /tmp/sample.elf.bad
During startup program terminated with signal SIGKILL, Killed.

查看二进制文件:

readelf -l sample.elf

Elf file type is EXEC (Executable file)
Entry point 0x8048080
There are 1 program headers, starting at offset 52

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  LOAD           0x000054 0x08048080 0x00000000 0x0000c 0x0000c R E 0x1000

在这里，您要求内核 mmap具有文件偏移量 0x54 的段在虚拟地址 0x08048080 .

由于这两个数字不等于彼此模页面大小，内核拒绝:

strace ./sample.elf
execve("./sample.elf", ["./sample.elf"], [/* 42 vars */] <unfinished ...>
+++ killed by SIGKILL +++
Killed

以上 strace 表示内核试图创建进程，不喜欢它所看到的，并以偏见终止它。你的二进制文件没有一条指令被执行。

修复 LOAD虚拟地址和入口点是 0x08048054产生所需的工作可执行文件:

strace ./sample.elf
execve("./sample.elf", ["./sample.elf"], [/* 42 vars */]) = 0
[ Process PID=23172 runs in 32 bit mode. ]
_exit(0)                                = ?
+++ exited with 0 +++

这是它的十六进制转储:

hd ./sample.elf
00000000  7f 45 4c 46 01 01 01 00  00 00 00 00 00 00 00 10  |.ELF............|
00000010  02 00 03 00 01 00 00 00  54 80 04 08 34 00 00 00  |........T...4...|
00000020  00 00 00 00 00 00 00 00  34 00 20 00 01 00 28 00  |........4. ...(.|
00000030  00 00 00 00 01 00 00 00  54 00 00 00 54 80 04 08  |........T...T...|
00000040  00 00 00 00 0c 00 00 00  0c 00 00 00 05 00 00 00  |................|
00000050  00 10 00 00 b8 01 00 00  00 bb 00 00 00 00 cd 80  |................|
00000060