
Runs programs – command line

Not the only program that runs other programs
 make
 gcc
 and more …

How this happens ? Who is responsible for that?
 Operating System
 System call
 exec family - execvp, execl, execv, …
2

These functions
 receive the path of the program executable file
 receive a list of the program arguments
 loads the program into memory, and run it …

Last lecture, fork( ) and exec( )
 why these function are used together ?
3

Unix has several different types of executable files

You are already familiar with some of them:
 Binary files
 Bash script
 more ?

Are these files executed similarly by the OS ?
4

Binary file





Scripts





Compiled – intermediate binary
Loading data and code to memory
Moving control to the entry point
Running the machine-code
No compilation
We don’t have the binary code nor the memory content
A specified program parses and runs the commands
Commands are interpreted line-by-line
How do we specify which interpreter runs the script ?
5

The first line - #!/bin/sh

exec( ) uses this to determine the type of the executable

shebang is encountered  it is a script file

The first line contains the path of the relevant program too
 What can we do with it ?
6

No sanity checks for the shebang line

Every program/utility could appear there
#!/bin/cat
Hello world
#!/bin/rm
ls -l
7
8

ELF defines a format of executable binary files

There are three main types
 Relocatable
▪ Created by compilers or assemblers. Need to be processed by the linker before
running
 Executable
▪ Have all relocation done and all symbol resolved except perhaps shared library
symbols that must be resolved at run time
 Shared Object
▪ Shared library containing both symbol information for the linker and directly
runnable code for run time
9

Compilers, assemblers, and linkers treat the file as a set of
logical sections

The system loader treats the file as a set of segments

Sections are intended
for further processing
by a linker

Segments are intended
to be mapped into
memory
10
Program and section
header table offsets
section .text
global _start ;
_start:
mov
mov
mov
mov
int
;tell linker entry point
edx,len
ecx,msg
ebx,1
eax,4
0x80
mov eax,1
int 0x80
;message length
;message to write
;file descriptor (stdout)
;system call number (sys_write)
;call kernel
;system call number (sys_exit)
;call kernel
section .data
msg db 'Hello world!',0xa
len equ $ - msg
;our dear string
;length of our dear string
12
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:
Intel 80386
Version:
0x1
Entry point address:
0x8048080
Start of program headers:
52 (bytes into file)
Start of section headers:
256 (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:
7
Section header string table index:
4
13

A relocatable or shared object file is a collection of sections

Each section contains a single type of information, such as
program code, read-only data, read/write data, symbols

Every symbol’s address is defined relative to a section
 For example, the entry point is relative to the code section
14
[Nr]
[ 0]
[ 1]
[ 2]
[ 3]
[ 4]
[ 5]
[ 6]
Name
NULL
.text
.data
.comment
.shstrtab
.symtab
.strtab
Type
00000000
PROGBITS
PROGBITS
PROGBITS
STRTAB
SYMTAB
STRTAB
Addr
000000
08048080
080490a0
00000000
00000000
00000000
00000000
Off
000000
000080
0000a0
0000ae
0000cd
000218
0002c8
Size
000000
00001d
00000e
00001f
000030
0000b0
000032
15

An object file symbol table holds information needed to locate
and relocate a program’s symbolic definition and references
Num:
0:
1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
Value
00000000
00000000
00000000
00000000
00000000
0000000e
0000000e
08048080
080490ae
080490ae
080490b0
Size
0
0
0
0
0
0
0
0
0
0
0
Type
NOTYPE
FILE
SECTION
SECTION
NOTYPE
NOTYPE
NOTYPE
NOTYPE
NOTYPE
NOTYPE
NOTYPE
Bind
LOCAL
LOCAL
LOCAL
LOCAL
LOCAL
LOCAL
LOCAL
GLOBAL
GLOBAL
GLOBAL
GLOBAL
Vis
DEFAULT
DEFAULT
DEFAULT
DEFAULT
DEFAULT
DEFAULT
DEFAULT
DEFAULT
DEFAULT
DEFAULT
DEFAULT
Ndx Name
UND
ABS hello.asm
1
2
2 msg
ABS len
ABS len
1 _start
ABS __bss_start
ABS _edata
ABS _end
16
17
18