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Unix Shell - Revisited

Unix Shell - Revisited. Runs programs – command line Not the only program that runs other programs m ake gcc and more … How this happens ? Who is responsible for that? Operating System System call exec family - execvp , execl , execv , … . Unix Shell - Revisited. These functions

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Unix Shell - Revisited

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  1. Unix Shell - Revisited • 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. Unix Shell - Revisited • 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. Executable Files • 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. Executable Files – (cont’d) • Binary file • Compiled – intermediate binary • Loading data and code to memory • Moving control to the entry point • Running the machine-code • Scripts • No compilation • We don’t have the binary code nor the memory content • Aspecified program parses and runs the commands • Commands are interpreted line-by-line • How do we specify which interpreter runs the script ?

  5. Executable Files – (cont’d) • 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. Executable Files – (cont’d) • No sanity checks for the shebang line • Every program/utility could appear there #!/bin/catHello world #!/bin/rmls -l

  7. ELF – Executable and Linkable Format

  8. ELF – Executable and Linkable Format • 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. ELF – Dual Nature • 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. ELF Structure Program and section header table offsets

  11. Your Best Example section .text global _start ; _start: ;tell linker entry point movedx,len ;message length movecx,msg ;message to write mov ebx,1 ;file descriptor (stdout) mov eax,4 ;system call number (sys_write) int 0x80 ;call kernel mov eax,1 ;system call number (sys_exit) int 0x80 ;call kernel section .data msgdb 'Hello world!',0xa ;our dear string lenequ $ - msg ;length of our dear string

  12. ELF Header - Hello Example Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00Class: ELF32Data: 2's complement, little endianVersion: 1 (current)OS/ABI: UNIX - System VABI Version: 0Type: EXEC (Executable file)Machine: Intel 80386Version: 0x1Entry point address: 0x8048080Start of program headers: 52 (bytes into file)Start of section headers: 256 (bytes into file)Flags: 0x0Size of this header: 52 (bytes)Size of program headers: 32 (bytes)Number of program headers: 2Size of section headers: 40 (bytes)Number of section headers: 7Section header string table index: 4

  13. Relocatable Files • 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. ELF Sections – Hello Example [Nr] Name Type Addr Off Size [ 0] NULL 00000000 000000 000000 000000 [ 1] .text PROGBITS 08048080000080 00001d[ 2] .data PROGBITS 080490a00000a0 00000e[ 3] .comment PROGBITS 00000000 0000ae 00001f [ 4] .shstrtab STRTAB 00000000 0000cd 000030[ 5] .symtab SYMTAB 00000000 000218 0000b0[ 6] .strtab STRTAB 00000000 0002c8 000032

  15. Special Section - Symbol Table • An object file symbol table holds information needed to locate and relocate a program’s symbolic definition and references Num: Value Size Type Bind Vis NdxName 0: 00000000 0 NOTYPE LOCAL DEFAULT UND 1: 00000000 0 FILE LOCAL DEFAULT ABS hello.asm 2: 00000000 0 SECTION LOCAL DEFAULT 1 3: 00000000 0 SECTION LOCAL DEFAULT 2 4: 00000000 0 NOTYPE LOCAL DEFAULT 2 msg 5: 0000000e 0 NOTYPE LOCAL DEFAULT ABS len 6: 0000000e 0 NOTYPE LOCAL DEFAULT ABS len 7: 08048080 0 NOTYPE GLOBAL DEFAULT 1 _start 8: 080490ae 0 NOTYPE GLOBAL DEFAULT ABS __bss_start 9: 080490ae 0 NOTYPE GLOBAL DEFAULT ABS _edata 10: 080490b0 0 NOTYPE GLOBAL DEFAULT ABS _end

  16. Executable Files

  17. Navigate through the Maze

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