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CS 5600 Computer Systems CS 5600 Computer Systems Lecture 4: Programs, Processes, and Threads • Programs • Processes • Context Switching • Protected Mode Execution • Inter-process Communication • Threads 2 Running Dynamic Code • One basic function of an OS is to execute and manage code dynamically, e.g.: – A command issued at a command line terminal – An icon double clicked from the desktop – Jobs/tasks run as part of a batch system (MapReduce) • A process is the basic unit of a program in execution 3 Programs and Processes Process The running instantiation of a program, stored in RAM Program An executable file in long-term One-to-many storage relationship between program and processes 4 How to Run a Program? • When you double-click on an .exe, how does the OS turn the file on disk into a process? • What information must the .exe file contain in order to run as a program? 5 Program Formats • Programs obey specific file formats – CP/M and DOS: COM executables (*.com) – DOS: MZ executables (*.exe) • Named after Mark Zbikowski, a DOS developer – Windows Portable Executable (PE, PE32+) (*.exe) • Modified version of Unix COFF executable format • PE files start with an MZ header. – Mac OSX: Mach object file format (Mach-O) – Unix/Linux: Executable and Linkable Format (ELF) • designed to be flexible and extensible • all you need to know to load and start execution regardless of architecture 6 ABI - Application Binary Interface • interface between 2 programs at the binary (machine code) level – informally, similar to API but on bits and bytes • Calling conventions – where are args and results stored • Binary format info to be passed from one program to another • Compiler and OS take care of this – binaries created from different compiler-OS pair will not always run on your machine! 7 test.c #include <stdio.h> int big_big_array[10 * 1024 * 1024]; char *a_string = "Hello, World!"; int a_var_with_value = 100; int main(void) { big_big_array[0] = 100; printf("%s\n", a_string); a_var_with_value += 20; printf("main is : %p\n", &main); return 0; } 8 ELF File Format • ELF Header – Contains compatibility info – Entry point of the executable code • Program header table – Lists all the segments in the file – Used to load and execute the program • Section header table – Used by the linker 9 ELF Header Format typedef struct { unsigned char e_ident[EI_NIDENT]; Elf32_Half e_type; Elf32_Half e_machine; ISA of executable code Elf32_Word e_version; Elf32_Addr e_entry; • OffsetEntry of point program of Elf32_Off e_phoff; executableheaders code Elf32_Off e_shoff; Offset• What of section should headersEIP be Elf32_Word e_flags; set to initially? Elf32_Half e_ehsize; Elf32_Half e_phentsize; Elf32_Half e_phnum; # of program headers Elf32_Half e_shentsize; Elf32_Half e_shnum; # of section headers Elf32_Half e_shstrndx; } Elf32_Ehdr; 10 ELF Header Example $ gcc –g –o test test.c $ readelf --header test 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: EXEC (Executable file) Machine: Advanced Micro Devices X86-64 Version: 0x1 Entry point address: 0x400460 Start of program headers: 64 (bytes into file) Start of section headers: 5216 (bytes into file) Flags: 0x0 Size of this header: 64 (bytes) Size of program headers: 56 (bytes) Number of program headers: 9 Size of section headers: 64 (bytes) Number of section headers: 36 Section header string table index: 33 11 Investigating the Entry Point int main(void) { … printf("main is : %p\n", &main); return 0; } $ gcc -g -o test test.c $ readelf --headers ./test | grep Entry point' Entry point address: 0x400460 $ ./test Hello World! main is : 0x400544 12 Entry point != &main $ ./test • Most compilers insert extra Hello World! code into compiled main is : 0x400544 programs $ readelf --headers ./test | grep Entry point' Entry point address: 0x400460 • This code typically runs $ objdump --disassemble –M intel ./test before and after main() … 0000000000400460 <_start>: 400460: 31 ed xor ebp,ebp 400462: 49 89 d1 mov r9,rdx 400465: 5e pop rsi 400466: 48 89 e2 mov rdx,rsp 400469: 48 83 e4 f0 and rsp,0xfffffffffffffff0 40046d: 50 push rax 40046e: 54 push rsp 40046f: 49 c7 c0 20 06 40 00 mov r8,0x400620 400476: 48 c7 c1 90 05 40 00 mov rcx,0x400590 40047d: 48 c7 c7 44 05 40 00 mov rdi,0x400544 400484: e8 c7 ff ff ff call 400450 <__libc_start_main@plt> … 13 Sections and Segments Multiple sections • Sections are the various in one segments pieces of code and data Segments that get linked together by the compiler • Each segment contains one or more sections – Each segment contains sections that are related • E.g. all code sections – Segments are the basic units for the loader 14 Common Sections • Sections are the various pieces of code and data that compose a program • Key sections: – .text – Executable code – .bss – Global variables initialized to zero – .data, .rodata – Initialized data and strings – .strtab – Names of functions and variables – .symtab – Debug symbols 15 Section! Example String variable .data Empty 10 MB array ! .bss int big_big_array[10*1024*1024]; char *a_string = "Hello, World!"; int a_var_with_value = 0x100; int main(void) { Initialized global variable ! .data big_big_array[0] = 100; printf("%s\n", a_string); a_var_with_value += 20; … } String constant ! .rodata Code ! .text 16 $ readelf --headers ./test … Section to Segment mapping: Segment Sections... 00 01 .interp 02 .interp .note.ABI-tag .note.gnu.build- id .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .plt .text .fini .rodata .eh_frame_hdr .eh_frame 03 .ctors .dtors .jcr .dynamic .got .got.plt .data .bss 04 .dynamic 05 .note.ABI-tag .note.gnu.build-id 06 .eh_frame_hdr 07 08 .ctors .dtors .jcr .dynamic .got … There are 36 section headers, starting at offset 0x1460: Section Headers: [Nr] Name Type Address Offset Size ES Flags Link Info Align [ 0] NULL 00000000 00000000 00000000 00 0 0 0 [ 1] .interp PROGBITS 00400238 00000238 0000001c 00 A 0 0 1 [ 2] .note.ABI-tag NOTE 00400254 00000254 00000020 00 A 0 0 4 [ 3] .note.gnu.build-I NOTE 00400274 00000274 00000024 00 A 0 0 4 [ 4] .gnu.hash GNU_HASH 00400298 00000298 0000001c 00 A 5 0 8 [ 5] .dynsym DYNSYM 004002b8 000002b8 00000078 18 A 6 1 8 [ 6] .dynstr STRTAB 00400330 00000330 00000044 00 A 0 0 1 [ 7] .gnu.version VERSYM 00400374 00000374 0000000a 02 A 5 0 2 … .text Example Header typedef struct { Elf32_Word p_type; Elf32_Off p_offset; Elf32_Addr p_vaddr; Address to load Elf32_Addr p_paddr; section in Elf32_Word p_filesz; memory Elf32_Word p_memsz; Elf32_Word p_flags; Offset of data in the Elf32_Word p_align; file Data } for the program How many bytes (in $ readelf --sections ./test hex) are in the section ... Section Headers: … Executable [Nr] Name Type Address Offset Size ES Flags Link Info Align [13] .text PROGBITS 00400460 00000460 00000218 00 AX 0 0 16 … .bss Example Header int big_big_array[10*1024*1024]; typedef struct { Offset of data in the Elf32_Word p_type; file Elf32_Off p_offset; Elf32_Addr p_vaddr; Address to load Elf32_Addr p_paddr; section in Elf32_Word p_filesz; memory Elf32_Word p_memsz; Elf32_Word p_flags; Contains Elf32_Word p_align; no data } hex(4*10*1024*1024) = $ readelf --sections ./test 0x2800020 ... Section Headers: Writable … [Nr] Name Type Address Offset Size ES Flags Link Info Align [25] .bss NOBITS 00601040 00001034 02800020 00 WA 0 0 32 [26] .comment PROGBITS 00000000 00001034 000002a 01 MS 0 0 1 … Segments • Each segment contains one or more sections – All of the sections in a segment are related, e.g.: • All sections contain compiled code • Or, all sections contain initialized data • Or, all sections contain debug information • … etc… • Segments are used by the loader to: – Place data and code in memory – Determine memory permissions (read/write/ execute) 20 Segment Header typedef struct { Type of segment Elf32_Word p_type Offset within the ELF Elf32_Off p_offset; file Locationfor the segment to load thedata Elf32_Addr p_vaddr; segment into memory Elf32_Addr p_paddr; Size of the segment Size of the segment in Elf32_Word p_filesz; data on disk Elf32_Word p_memsz; memory Elf32_Word p_flags; Elf32_Word p_align;• Flags describing the } section data • Examples: executable, read-only 21 $ readelf --segments ./test Elf file type is EXEC (Executable file) Entry point 0x400460 There are 9 program headers, starting at offset 64 Executable Program Headers: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flags Align PHDR 0x00000040 0x00400040 0x00400040 0x000001f8 0x000001f8 R E 8 INTERP 0x00000238 0x00400238 0x00400238 0x0000001c 0x0000001c R 1 LOAD 0x00000000 0x00400000 0x00400000 0x0000077c 0x0000077c R E 200000 LOAD 0x00000e28 0x00600e28 0x00600e28 0x0000020c 0x02800238 RW 200000 DYNAMIC 0x00000e50 0x00600e50 0x00600e50 0x00000190 0x00000190 RW 8 NOTE 0x00000254 0x00400254 0x00400254 0x00000044 0x00000044 R 4 GNU_EH_FRAME 0x000006a8 0x004006a8 0x004006a8 0x0000002c 0x0000002c R 4 GNU_STACK 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 RW 8 GNU_RELRO 0x00000e28 0x00600e28 0x00600e28 0x000001d8 0x000001d8 R 1 Section to Segment mapping: Segment Sections... 00 01 .interp 02 .interp .note.ABI-tag .note.gnu.build- id .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .plt .text .fini .rodata .eh_frame_hdr .e h_frame 03 .ctors .dtors .jcr .dynamic .got .got.plt .data .bss 04 .dynamic … What About Static Data? #include <stdio.h> $ strings –t d ./test 568 /lib64/ld-linux- int big_big_array[10 * 1024 * 1024];
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