龙乐乐同学的hello world背后的故事

1. P
Review the True Story of Hello
World？
Are you kidding?
Web Cookie:
Google nativeclient--Native code for web apps
Native Cookie:
Structure and Interpretation of Computer Programs

2. Releated topics
1. Loader and Linker
2. ELF
3. LLVM
4. LIBrary
5. More behind the scene

3. Wrenches
readelf objdump nm file string strace strip ld
NASA done this after Bush cancelled the Back to moon plan

4. news updated

5. You can interrupt me at any time
(rather than Q&A section),
this is more conivenent for context reference。
I am learning,correct me if necessary.
可以在随时打断我，这样方便引用上下文，
以免在Q&A截断重复。
我也只是入门，请纠正出现的错误。
pls read the note before the lecture or along the lecture if you have a
computer before hand now.
请提前阅读讲稿附带的笔记， 如果还没有阅读，如果手头现在有台机
器，也可以跟随演讲进行。

6. The True Story of Hello World
(or at least a good part of it)
original author:Antônio Augusto M. Fröhlich
original story
zh_CN

7. Offtopic:about the author
Prof. Dr. Antônio Augusto Fröhlich
LISHA Software/Hardware
Integration Lab
Federal University of Santa
Catarina
Course taught
Object-Oriented Programming
System Programming
Operating Systems
Computational Biology (parallel
programming)

8. Hello World in C
1 #include <stdio.h> header file directives
2 int main(int argc, char *argv[]) the so-called entry
3 { point
4 printf("Hello world!n"); C library call printf
(buffered)
5 return 0;
tell OS:everything is ok
6 }

9. Let's compile,link,and run it as a beginner
#compile -c get object file(Optimization in GCC and here)
%gcc -Os -c hello.c
#linking
%ld --dynamic-linker /lib/ld-linux.so.2 /usr/lib/crt1.o
/usr/lib/crti.o /usr/lib/crtn.o -lc hello.o -o hello
#run
%./hello

10. How about LLVM?
# compile the C file into a native executable:
% llvm-gcc hello.c -o hello
#compile the C file into a LLVM bitcode file:
% llvm-gcc -O3 -emit-llvm hello.c -c -o hello.bc
#run the program using the just-in-time compiler:
% lli hello.bc
LLVM Getting Started
Writing your own toy compiler
zh_CN 使用Flex Bison 和LLVM编写自己的编译器

11. %objdump -hrt hello.o
1 hello.o: file format elf32-i386
2 Sections:
3 Idx Name Size VMA LMA File off Algn
4 0 .text 00000027 00000000 00000000 00000034 2**2
5 CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
6 1 .data 00000000 00000000 00000000 0000005c 2**2
7 CONTENTS, ALLOC, LOAD, DATA
8 2 .bss 00000000 00000000 00000000 0000005c 2**2
9 ALLOC
10 3 .rodata.str1.1 0000000e 00000000 00000000 0000005c 2**0
11 CONTENTS, ALLOC, LOAD, READONLY, DATA
12 4 .comment 00000024 00000000 00000000 0000006a 2**0
13 CONTENTS, READONLY
14 5 .note.GNU-stack 00000000 00000000 00000000 0000008e 2**0
15 CONTENTS, READONLY
16 SYMBOL TABLE:
17 00000000 l df *ABS* 00000000 hello.c
18 00000000 l d .text 00000000 .text
19 00000000 l d .data 00000000 .data
20 00000000 l d .bss 00000000 .bss
21 00000000 l d .rodata.str1.1 00000000 .rodata.str1.1
22 00000000 l d .note.GNU-stack 00000000 .note.GNU-stack
23 00000000 l d .comment 00000000 .comment
24 00000000 g F .text 00000027 main
25 00000000 *UND* 00000000 __printf_chk
26 RELOCATION RECORDS FOR [.text]:
27 OFFSET TYPE VALUE
28 00000012 R_386_32 .rodata.str1.1
29 00000019 R_386_PC32 __printf_chk

12. %readelf -l hello
1 Elf file type is EXEC (Executable file)
2 Entry point 0x80482e0
3 There are 7 program headers, starting at offset 52
4 Program Headers:
5 Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
6 PHDR 0x000034 0x08048034 0x08048034 0x000e0 0x000e0 R E 0x4
7 INTERP 0x000114 0x08048114 0x08048114 0x00013 0x00013 R 0x1
8 [Requesting program interpreter: /lib/ld-linux.so.2]
9 LOAD 0x000000 0x08048000 0x08048000 0x003d6 0x003d6 R E 0x1000
10 LOAD 0x0003d8 0x080493d8 0x080493d8 0x000e8 0x000e8 RW 0x1000
11 DYNAMIC 0x0003d8 0x080493d8 0x080493d8 0x000c8 0x000c8 RW 0x4
12 NOTE 0x000128 0x08048128 0x08048128 0x00020 0x00020 R 0x4
13 GNU_STACK 0x000000 0x00000000 0x00000000 0x00000 0x00000 RW 0x4
14 Section to Segment mapping:
15 Segment Sections...
16 00
17 01 .interp
18 02 .interp .note.ABI-tag .hash .dynsym .dynstr .gnu.version .gnu.version_r .rel.
dyn .rel.plt .init .plt .text .fini .rodata
19 03 .dynamic .got .got.plt .data
20 04 .dynamic
21 05 .note.ABI-tag
22 06

14. More explained one

15. An example of an LKM with various ELF sections

16. Executable and Linking Format (ELF)
%file hello
ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked
(uses shared libs), for GNU/Linux 2.6.15, not stripped
1,AMD-64
ELF 64-bit LSB executable, x86-64, version 1 (SYSV), for GNU/Linux 2.6.8,
dynamically linked (uses shared libs), not stripped
2,RISC_6000
executable (RISC System/6000 V3.1) or obj module not stripped
3,UltraSPARC
ELF 32-bit MSB executable, SPARC32PLUS, V8+ Required, version 1 (SYSV),
dynamically linked (uses shared libs), not stripped
4,loongson
ELF 32-bit LSB executable, MIPS, MIPS-I version 1 (SYSV), for GNU/Linux
2.4.1, dynamically linked (uses shared libs), not stripped
above file format data is collected from unix-center.net

17. Sections and Segments

18. Hello World in assembly
%gcc -Os -S hello.c -o -
1 .file "hello.c"
2 .section .rodata.str1.1,"aMS",@progbits,1
3 .LC0:
4 .string "Hello world!n"
5 .text
6 .globl main
7 .type main, @function
8 main:
9 leal 4(%esp), %ecx
10 andl $-16, %esp
11 pushl -4(%ecx)
12 pushl %ebp
13 movl %esp, %ebp
14 pushl %ecx
15 subl $12, %esp
16 pushl $.LC0
17 pushl $1
18 call __printf_chk
19 movl -4(%ebp), %ecx
20 xorl %eax, %eax
21 leave
22 leal -4(%ecx), %esp
23 ret
24 .size main, .-main
25 .ident "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"
26 .section .note.GNU-stack,"",@progbits

19. Look into
%objdump -s hello
Contents of section .interp:
8048114 2f6c6962 2f6c642d 6c696e75 782e736f /lib/ld-linux.so
8048124 2e3200 .2.
Contents of section .dynstr:
80481c0 006c6962 632e736f 2e36005f 494f5f73 .libc.so.6._IO_s
80481d0 7464696e 5f757365 64005f5f 6c696263 tdin_used.__libc
80481e0 5f737461 72745f6d 61696e00 5f5f7072 _start_main.__pr
80481f0 696e7466 5f63686b 005f5f67 6d6f6e5f intf_chk.__gmon_
8048200 73746172 745f5f00 474c4942 435f322e start__.GLIBC_2.
8048210 3000474c 4942435f 322e332e 3400 0.GLIBC_2.3.4.
.......
Contents of section .rodata:
80483c0 03000000 01000200 48656c6c 6f20776f ........Hello wo
80483d0 726c6421 0a00 rld!..
Contents of section .comment:
0000 4743433a 20285562 756e7475 20342e34 GCC: (Ubuntu 4.4
0010 2e332d34 7562756e 74753529 20342e34 .3-4ubuntu5) 4.4
0020 2e3300 .3.

20. entry pointer
%ld -o hello_ld hello.o -lc
ld: warning: cannot find entry symbol _start; defaulting to 0000000008048074
%./hello_ld
bash: ./hello_ld: No such file or directory
%gcc -nostdlib -o hello-nostdlib hello.c
/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 00000000080480b8
/tmp/ccmKwryA.o: In function `main':
hello.c:(.text+0x11): undefined reference to `puts'
collect2: ld returned 1 exit status
So what does ld forget here?

21. difference between hello and hello-ld
%nm hello
1 080493d8 d _DYNAMIC %nm hello-ld
2 080494a4 d _GLOBAL_OFFSET_TABLE_ 1 080491e4 d _DYNAMIC
3 080483c4 R _IO_stdin_used 2 08049284 d _GLOBAL_OFFSET_TABLE_
4 080494c0 A __bss_start 3 08049294 A __bss_start
5 080494bc D __data_start 4 U __printf_chk@@GLIBC_2.3.4
6 w __gmon_start__ 5 08049294 A _edata
7 0804837a T __i686.get_pc_thunk.bx 6 08049294 A _end
8 080493d6 d __init_array_end 7 U _start
9 080493d6 d __init_array_start 8 080481ac T main
10 08048310 T __libc_csu_fini
11 08048320 T __libc_csu_init
12 U __libc_start_main@@GLIBC_2.0
13 U __printf_chk@@GLIBC_2.3.4
14 080494c0 A _edata
15 080494c0 A _end
16 080483a8 T _fini
17 080483c0 R _fp_hw
18 08048278 T _init
19 080482e0 T _start
20 080494bc W data_start
21 08048380 T main

22. %strace ./hello
1 execve("./hello", ["./hello"], [/* 43 vars */]) = 0
2 brk(0) = 0x848a000
3 uname({sys="Linux", node="lele-laptop", ...}) = 0
4 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
5 mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7757000
6 access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
7 open("/etc/ld.so.cache", O_RDONLY) =3
8 fstat64(3, {st_mode=S_IFREG|0644, st_size=170053, ...}) = 0
9 mmap2(NULL, 170053, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb772d000
10 close(3) =0
11 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
12 open("/lib/tls/i686/cmov/libc.so.6", O_RDONLY) = 3
13 read(3, "177ELF11100000000030301000000m10004000"..., 512) = 512
14 fstat64(3, {st_mode=S_IFREG|0755, st_size=1405508, ...}) = 0
15 mmap2(NULL, 1415592, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xb84000
16 mprotect(0xcd7000, 4096, PROT_NONE) = 0
17 mmap2(0xcd8000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x153) =
0xcd8000
18 mmap2(0xcdb000, 10664, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) =
0xcdb000
19 close(3) =0
20 mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb772c000
21 set_thread_area({entry_number:-1 -> 6, base_addr:0xb772c6c0, limit:1048575, seg_32bit:1, contents:0, read_exec_only:0,
limit_in_pages:1, seg_not_present:0, useable:1}) = 0
22 mprotect(0xcd8000, 8192, PROT_READ) = 0
23 mprotect(0xea6000, 4096, PROT_READ) = 0
24 munmap(0xb772d000, 170053) =0
25 fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
26 mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7756000
27 write(1, "Hello world!n", 13) = 13
28 exit_group(0) =?

23. Layout of Hello World

24. Internal Symbols
printf("(end of text)etext:%pn",&etext);
printf("(end of data)edata:%pn",&edata);
printf("(end of segments)end:%pn",&end);
printf("(__executable_start):%pn",&__executable_start);
(end of text)etext:0x80486b8
(end of data)edata:0x804a028
(end of segments)end:0x804c060
(__executable_start):0x8048000

25. Child and Parent(wait4)
%strace -e trace=process -f sh -c "hello" > /dev/null
1 execve("/bin/sh", ["sh", "-c", "hello"], [/* 43 vars */]) = 0
2 clone(Process 6067 attached
3 child_stack=0,
flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,
child_tidptr=0xb781a938) = 6067
4 [pid 6066] wait4(-1, Process 6066 suspended
5 <unfinished ...>
6 [pid 6067] execve("/usr/bin/hello", ["hello"], [/* 43 vars */]) = 0
7 [pid 6067] exit_group(0) =?
8 Process 6066 resumed
9 Process 6067 detached
10 <... wait4 resumed> [{WIFEXITED(s) && WEXITSTATUS(s) ==
0}], 0, NULL) = 6067
11 --- SIGCHLD (Child exited) @ 0 (0) ---
12 exit_group(0)

26. Can we do it without c library?
%more hello-nostd.c
void _start() {
/* exit system call */
asm("movl $1,%eax;"
"xorl %ebx,%ebx;"
"int $0x80"
);
}
int main(int argc, char *argv[])
{
char* str="Hello world!n";
return 0;
}
%gcc -nostdlib hello-nostd.c -o hello-nostd
asm(call main): http://blog.ksplice.com/2010/03/libc-free-world/

27. Freak out
1. gcc -v
2. ld --verbose
3. objdump -d (man objdump)

28. Web References
1. A Whirlwind Tutorial on Creating Really Teensy ELF Executables for Linux
2. Hello from a libc-free world!(part one and part two)
3. bash: ./hello-ld: No such file or directory
4. Linkers and Loaders (Linux Journal)
5. Structure and Interpretation of Computer Programs
6. Linux Standard Base (LSB)

29. Books
1. Loader and Linker John R. Levine - 2000
2. 程序员的自我修养 --链接、装载与库 俞甲子/石凡/潘爱
民 2009
3. Computer Systems: A Programmer's Perspective Randal
E.Bryant / David O'Hallaron 2003