riscv-operating-system homework

3-1

使⽤ gcc 编译代码并使⽤ binutils ⼯具对⽣成的⽬标文件和可执⾏文件（ELF 格式）进⾏分析。具体要求如下：

• 编写⼀个简单的打印 “hello world！” 的程序源文件：hello.c
• 对源文件进⾏本地编译，⽣成针对⽀持 x86_64 指令集架构处理器的⽬标文件 hello.o。
• 查看 hello.o 的文件的文件头信息。
• 查看 hello.o 的 Section header table。
• 对 hello.o 反汇编，并查看 hello.c 的 C 程序源码和机器指令的对应关系。

简单地打印 "hello world !"

#include <stdio.h>
int main(){
    printf("hello world !\n");
    return 0;
}

编译为目标文件

gcc hello.cc -o hello.o

查看文件头信息

$ readelf -h hello.o
ELF Header:
  Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              DYN (Shared object file)
  Machine:                           Advanced Micro Devices X86-64
  Version:                           0x1
  Entry point address:               0x1060
  Start of program headers:          64 (bytes into file)
  Start of section headers:          14712 (bytes into file)
  Flags:                             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         13
  Size of section headers:           64 (bytes)
  Number of section headers:         31
  Section header string table index: 30

查看 Section header table

$ readelf -l hello.o

Elf file type is DYN (Shared object file)
Entry point 0x1060
There are 13 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000000040 0x0000000000000040
                 0x00000000000002d8 0x00000000000002d8  R      0x8
  INTERP         0x0000000000000318 0x0000000000000318 0x0000000000000318
                 0x000000000000001c 0x000000000000001c  R      0x1
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x00000000000005f8 0x00000000000005f8  R      0x1000
  LOAD           0x0000000000001000 0x0000000000001000 0x0000000000001000
                 0x00000000000001f5 0x00000000000001f5  R E    0x1000
  LOAD           0x0000000000002000 0x0000000000002000 0x0000000000002000
                 0x0000000000000160 0x0000000000000160  R      0x1000
  LOAD           0x0000000000002db8 0x0000000000003db8 0x0000000000003db8
                 0x0000000000000258 0x0000000000000260  RW     0x1000
  DYNAMIC        0x0000000000002dc8 0x0000000000003dc8 0x0000000000003dc8
                 0x00000000000001f0 0x00000000000001f0  RW     0x8
  NOTE           0x0000000000000338 0x0000000000000338 0x0000000000000338
                 0x0000000000000020 0x0000000000000020  R      0x8
  NOTE           0x0000000000000358 0x0000000000000358 0x0000000000000358
                 0x0000000000000044 0x0000000000000044  R      0x4
  GNU_PROPERTY   0x0000000000000338 0x0000000000000338 0x0000000000000338
                 0x0000000000000020 0x0000000000000020  R      0x8
  GNU_EH_FRAME   0x0000000000002014 0x0000000000002014 0x0000000000002014
                 0x0000000000000044 0x0000000000000044  R      0x4
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     0x10
  GNU_RELRO      0x0000000000002db8 0x0000000000003db8 0x0000000000003db8
                 0x0000000000000248 0x0000000000000248  R      0x1

 Section to Segment mapping:
  Segment Sections...
   00     
   01     .interp 
   02     .interp .note.gnu.property .note.gnu.build-id .note.ABI-tag .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt 
   03     .init .plt .plt.got .plt.sec .text .fini 
   04     .rodata .eh_frame_hdr .eh_frame 
   05     .init_array .fini_array .dynamic .got .data .bss 
   06     .dynamic 
   07     .note.gnu.property 
   08     .note.gnu.build-id .note.ABI-tag 
   09     .note.gnu.property 
   10     .eh_frame_hdr 
   11     
   12     .init_array .fini_array .dynamic .got

反汇编

$ objdump -d hello.o > 3-1.asm
3-1.o:     file format elf64-x86-64


Disassembly of section .init:

0000000000001000 <_init>:
    1000:    f3 0f 1e fa              endbr64 
    1004:    48 83 ec 08              sub    $0x8,%rsp
    1008:    48 8b 05 d9 2f 00 00     mov    0x2fd9(%rip),%rax        # 3fe8 <__gmon_start__>
    100f:    48 85 c0                 test   %rax,%rax
    1012:    74 02                    je     1016 <_init+0x16>
    1014:    ff d0                    callq  *%rax
    1016:    48 83 c4 08              add    $0x8,%rsp
    101a:    c3                       retq   

Disassembly of section .plt:

0000000000001020 <.plt>:
    1020:    ff 35 9a 2f 00 00        pushq  0x2f9a(%rip)        # 3fc0 <_GLOBAL_OFFSET_TABLE_+0x8>
    1026:    f2 ff 25 9b 2f 00 00     bnd jmpq *0x2f9b(%rip)        # 3fc8 <_GLOBAL_OFFSET_TABLE_+0x10>
    102d:    0f 1f 00                 nopl   (%rax)
    1030:    f3 0f 1e fa              endbr64 
    1034:    68 00 00 00 00           pushq  $0x0
    1039:    f2 e9 e1 ff ff ff        bnd jmpq 1020 <.plt>
    103f:    90                       nop

Disassembly of section .plt.got:

0000000000001040 <__cxa_finalize@plt>:
    1040:    f3 0f 1e fa              endbr64 
    1044:    f2 ff 25 ad 2f 00 00     bnd jmpq *0x2fad(%rip)        # 3ff8 <__cxa_finalize@GLIBC_2.2.5>
    104b:    0f 1f 44 00 00           nopl   0x0(%rax,%rax,1)

Disassembly of section .plt.sec:

0000000000001050 <puts@plt>:
    1050:    f3 0f 1e fa              endbr64 
    1054:    f2 ff 25 75 2f 00 00     bnd jmpq *0x2f75(%rip)        # 3fd0 <puts@GLIBC_2.2.5>
    105b:    0f 1f 44 00 00           nopl   0x0(%rax,%rax,1)

Disassembly of section .text:

0000000000001060 <_start>:
    1060:    f3 0f 1e fa              endbr64 
    1064:    31 ed                    xor    %ebp,%ebp
    1066:    49 89 d1                 mov    %rdx,%r9
    1069:    5e                       pop    %rsi
    106a:    48 89 e2                 mov    %rsp,%rdx
    106d:    48 83 e4 f0              and    $0xfffffffffffffff0,%rsp
    1071:    50                       push   %rax
    1072:    54                       push   %rsp
    1073:    4c 8d 05 66 01 00 00     lea    0x166(%rip),%r8        # 11e0 <__libc_csu_fini>
    107a:    48 8d 0d ef 00 00 00     lea    0xef(%rip),%rcx        # 1170 <__libc_csu_init>
    1081:    48 8d 3d c1 00 00 00     lea    0xc1(%rip),%rdi        # 1149 <main>
    1088:    ff 15 52 2f 00 00        callq  *0x2f52(%rip)        # 3fe0 <__libc_start_main@GLIBC_2.2.5>
    108e:    f4                       hlt    
    108f:    90                       nop

0000000000001090 <deregister_tm_clones>:
    1090:    48 8d 3d 79 2f 00 00     lea    0x2f79(%rip),%rdi        # 4010 <__TMC_END__>
    1097:    48 8d 05 72 2f 00 00     lea    0x2f72(%rip),%rax        # 4010 <__TMC_END__>
    109e:    48 39 f8                 cmp    %rdi,%rax
    10a1:    74 15                    je     10b8 <deregister_tm_clones+0x28>
    10a3:    48 8b 05 2e 2f 00 00     mov    0x2f2e(%rip),%rax        # 3fd8 <_ITM_deregisterTMCloneTable>
    10aa:    48 85 c0                 test   %rax,%rax
    10ad:    74 09                    je     10b8 <deregister_tm_clones+0x28>
    10af:    ff e0                    jmpq   *%rax
    10b1:    0f 1f 80 00 00 00 00     nopl   0x0(%rax)
    10b8:    c3                       retq   
    10b9:    0f 1f 80 00 00 00 00     nopl   0x0(%rax)

00000000000010c0 <register_tm_clones>:
    10c0:    48 8d 3d 49 2f 00 00     lea    0x2f49(%rip),%rdi        # 4010 <__TMC_END__>
    10c7:    48 8d 35 42 2f 00 00     lea    0x2f42(%rip),%rsi        # 4010 <__TMC_END__>
    10ce:    48 29 fe                 sub    %rdi,%rsi
    10d1:    48 89 f0                 mov    %rsi,%rax
    10d4:    48 c1 ee 3f              shr    $0x3f,%rsi
    10d8:    48 c1 f8 03              sar    $0x3,%rax
    10dc:    48 01 c6                 add    %rax,%rsi
    10df:    48 d1 fe                 sar    %rsi
    10e2:    74 14                    je     10f8 <register_tm_clones+0x38>
    10e4:    48 8b 05 05 2f 00 00     mov    0x2f05(%rip),%rax        # 3ff0 <_ITM_registerTMCloneTable>
    10eb:    48 85 c0                 test   %rax,%rax
    10ee:    74 08                    je     10f8 <register_tm_clones+0x38>
    10f0:    ff e0                    jmpq   *%rax
    10f2:    66 0f 1f 44 00 00        nopw   0x0(%rax,%rax,1)
    10f8:    c3                       retq   
    10f9:    0f 1f 80 00 00 00 00     nopl   0x0(%rax)

0000000000001100 <__do_global_dtors_aux>:
    1100:    f3 0f 1e fa              endbr64 
    1104:    80 3d 05 2f 00 00 00     cmpb   $0x0,0x2f05(%rip)        # 4010 <__TMC_END__>
    110b:    75 2b                    jne    1138 <__do_global_dtors_aux+0x38>
    110d:    55                       push   %rbp
    110e:    48 83 3d e2 2e 00 00     cmpq   $0x0,0x2ee2(%rip)        # 3ff8 <__cxa_finalize@GLIBC_2.2.5>
    1115:    00 
    1116:    48 89 e5                 mov    %rsp,%rbp
    1119:    74 0c                    je     1127 <__do_global_dtors_aux+0x27>
    111b:    48 8b 3d e6 2e 00 00     mov    0x2ee6(%rip),%rdi        # 4008 <__dso_handle>
    1122:    e8 19 ff ff ff           callq  1040 <__cxa_finalize@plt>
    1127:    e8 64 ff ff ff           callq  1090 <deregister_tm_clones>
    112c:    c6 05 dd 2e 00 00 01     movb   $0x1,0x2edd(%rip)        # 4010 <__TMC_END__>
    1133:    5d                       pop    %rbp
    1134:    c3                       retq   
    1135:    0f 1f 00                 nopl   (%rax)
    1138:    c3                       retq   
    1139:    0f 1f 80 00 00 00 00     nopl   0x0(%rax)

0000000000001140 <frame_dummy>:
    1140:    f3 0f 1e fa              endbr64 
    1144:    e9 77 ff ff ff           jmpq   10c0 <register_tm_clones>

0000000000001149 <main>:
    1149:    f3 0f 1e fa              endbr64 
    114d:    55                       push   %rbp
    114e:    48 89 e5                 mov    %rsp,%rbp
    1151:    48 8d 3d ac 0e 00 00     lea    0xeac(%rip),%rdi        # 2004 <_IO_stdin_used+0x4>
    1158:    e8 f3 fe ff ff           callq  1050 <puts@plt>
    115d:    b8 00 00 00 00           mov    $0x0,%eax
    1162:    5d                       pop    %rbp
    1163:    c3                       retq   
    1164:    66 2e 0f 1f 84 00 00     nopw   %cs:0x0(%rax,%rax,1)
    116b:    00 00 00 
    116e:    66 90                    xchg   %ax,%ax

0000000000001170 <__libc_csu_init>:
    1170:    f3 0f 1e fa              endbr64 
    1174:    41 57                    push   %r15
    1176:    4c 8d 3d 3b 2c 00 00     lea    0x2c3b(%rip),%r15        # 3db8 <__frame_dummy_init_array_entry>
    117d:    41 56                    push   %r14
    117f:    49 89 d6                 mov    %rdx,%r14
    1182:    41 55                    push   %r13
    1184:    49 89 f5                 mov    %rsi,%r13
    1187:    41 54                    push   %r12
    1189:    41 89 fc                 mov    %edi,%r12d
    118c:    55                       push   %rbp
    118d:    48 8d 2d 2c 2c 00 00     lea    0x2c2c(%rip),%rbp        # 3dc0 <__do_global_dtors_aux_fini_array_entry>
    1194:    53                       push   %rbx
    1195:    4c 29 fd                 sub    %r15,%rbp
    1198:    48 83 ec 08              sub    $0x8,%rsp
    119c:    e8 5f fe ff ff           callq  1000 <_init>
    11a1:    48 c1 fd 03              sar    $0x3,%rbp
    11a5:    74 1f                    je     11c6 <__libc_csu_init+0x56>
    11a7:    31 db                    xor    %ebx,%ebx
    11a9:    0f 1f 80 00 00 00 00     nopl   0x0(%rax)
    11b0:    4c 89 f2                 mov    %r14,%rdx
    11b3:    4c 89 ee                 mov    %r13,%rsi
    11b6:    44 89 e7                 mov    %r12d,%edi
    11b9:    41 ff 14 df              callq  *(%r15,%rbx,8)
    11bd:    48 83 c3 01              add    $0x1,%rbx
    11c1:    48 39 dd                 cmp    %rbx,%rbp
    11c4:    75 ea                    jne    11b0 <__libc_csu_init+0x40>
    11c6:    48 83 c4 08              add    $0x8,%rsp
    11ca:    5b                       pop    %rbx
    11cb:    5d                       pop    %rbp
    11cc:    41 5c                    pop    %r12
    11ce:    41 5d                    pop    %r13
    11d0:    41 5e                    pop    %r14
    11d2:    41 5f                    pop    %r15
    11d4:    c3                       retq   
    11d5:    66 66 2e 0f 1f 84 00     data16 nopw %cs:0x0(%rax,%rax,1)
    11dc:    00 00 00 00 

00000000000011e0 <__libc_csu_fini>:
    11e0:    f3 0f 1e fa              endbr64 
    11e4:    c3                       retq   

Disassembly of section .fini:

00000000000011e8 <_fini>:
    11e8:    f3 0f 1e fa              endbr64 
    11ec:    48 83 ec 08              sub    $0x8,%rsp
    11f0:    48 83 c4 08              add    $0x8,%rsp
    11f4:    c3                       retq   

3-2

如下例⼦ C 语⾔代码：

#include <stdio.h>
int global_init = 0x11111111;
const int global_const = 0x22222222;
void main()
{
    static int static_var = 0x33333333;
    static int static_var_uninit;
    int auto_var = 0x44444444;
    printf("hello world!\n");
    return;
}

请问编译为 .o 文件后，global_init, global_const, static_var, static_var_uninit, auto_var 这些变 量分别存放在那些 section ⾥，"hello world!\n" 这个字符串⼜在哪⾥？并尝试⽤⼯具查看并验证你的猜测。

• global_init 和 static_var 在 .data 段

Disassembly of section .data:

0000000000004000 <__data_start>:
    ...

0000000000004008 <__dso_handle>:
    4008:    08 40 00                 or     %al,0x0(%rax)
    400b:    00 00                    add    %al,(%rax)
    400d:    00 00                    add    %al,(%rax)
    ...

0000000000004010 <global_init>:
    4010:    11 11                    adc    %edx,(%rcx)
    4012:    11 11                    adc    %edx,(%rcx)

0000000000004014 <static_var.2317>:
    4014:    33 33                    xor    (%rbx),%esi
    4016:    33 33                    xor    (%rbx),%esi

• global_const 在 rodata 段

Disassembly of section .rodata:

0000000000002000 <_IO_stdin_used>:
    2000:    01 00                    add    %eax,(%rax)
    2002:    02 00                    add    (%rax),%al

0000000000002004 <global_const>:
    2004:    22 22                    and    (%rdx),%ah
    2006:    22 22                    and    (%rdx),%ah
    2008:    68 65 6c 6c 6f           pushq  $0x6f6c6c65
    200d:    20 77 6f                 and    %dh,0x6f(%rdi)
    2010:    72 6c                    jb     207e <__GNU_EH_FRAME_HDR+0x66>
    2012:    64 21 00                 and    %eax,%fs:(%rax)

• static_var_uninit 在 bss 段

Disassembly of section .bss:

0000000000004018 <completed.8060>:
    4018:    00 00                    add    %al,(%rax)
    ...

000000000000401c <static_var_uninit.2318>:
    401c:    00 00                    add    %al,(%rax)
    ...

• auto_var 为局部变量，不在任何 section 中，在代码段中直接使用

4-1

针对 rv32ima 指令集架构，重复 3-1 的操作

指令用法与 3-1 基本相同

$riscv64-unknown-elf-gcc 3-1.c -march=rv32ima -mabi=ilp32 -g -Wall -o 4-1.o
$readelf -h 4-1.o 
ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF32
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           RISC-V
  Version:                           0x1
  Entry point address:               0x10090
  Start of program headers:          52 (bytes into file)
  Start of section headers:          26136 (bytes into file)
  Flags:                             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         2
  Size of section headers:           40 (bytes)
  Number of section headers:         21
  Section header string table index: 20
$readelf -l 4-1.o 

Elf file type is EXEC (Executable file)
Entry point 0x10090
There are 2 program headers, starting at offset 52

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  LOAD           0x000000 0x00010000 0x00010000 0x0362c 0x0362c R E 0x1000
  LOAD           0x00362c 0x0001462c 0x0001462c 0x00858 0x008b0 RW  0x1000

 Section to Segment mapping:
  Segment Sections...
   00     .text .rodata 
   01     .eh_frame .init_array .fini_array .data .sdata .sbss .bss
$riscv64-unknown-elf-objdump -d 4-1.o > 4-1.asm

• 反汇编节选

4-1.o:     file format elf32-littleriscv


Disassembly of section .text:

00010074 <register_fini>:
   10074:    ffff0797              auipc    a5,0xffff0
   10078:    f8c78793              addi    a5,a5,-116 # 0 <register_fini-0x10074>
   1007c:    00078863              beqz    a5,1008c <register_fini+0x18>
   10080:    00000517              auipc    a0,0x0
   10084:    14050513              addi    a0,a0,320 # 101c0 <__libc_fini_array>
   10088:    0f00006f              j    10178 <atexit>
   1008c:    00008067              ret

00010090 <_start>:
   10090:    00005197              auipc    gp,0x5
   10094:    db018193              addi    gp,gp,-592 # 14e40 <__global_pointer$>
   10098:    04418513              addi    a0,gp,68 # 14e84 <_edata>
   1009c:    09c18613              addi    a2,gp,156 # 14edc <__BSS_END__>
   100a0:    40a60633              sub    a2,a2,a0
   100a4:    00000593              li    a1,0
   100a8:    20c000ef              jal    ra,102b4 <memset>
   100ac:    00000517              auipc    a0,0x0
   100b0:    11450513              addi    a0,a0,276 # 101c0 <__libc_fini_array>
   100b4:    0c4000ef              jal    ra,10178 <atexit>
   100b8:    168000ef              jal    ra,10220 <__libc_init_array>
   100bc:    00012503              lw    a0,0(sp)
   100c0:    00410593              addi    a1,sp,4
   100c4:    00000613              li    a2,0
   100c8:    07c000ef              jal    ra,10144 <main>
   100cc:    0c00006f              j    1018c <exit>
...
...
00010144 <main>:
   10144:    ff010113              addi    sp,sp,-16
   10148:    00112623              sw    ra,12(sp)
   1014c:    00812423              sw    s0,8(sp)
   10150:    01010413              addi    s0,sp,16
   10154:    000137b7              lui    a5,0x13
   10158:    61878513              addi    a0,a5,1560 # 13618 <__modsi3+0x30>
   1015c:    304000ef              jal    ra,10460 <puts>
   10160:    00000793              li    a5,0
   10164:    00078513              mv    a0,a5
   10168:    00c12083              lw    ra,12(sp)
   1016c:    00812403              lw    s0,8(sp)
   10170:    01010113              addi    sp,sp,16
   10174:    00008067              ret
...
...

qemu-riscv32 模拟运行

$ qemu-riscv32 4-1.o
hello world !