Operating Systems 2022F: Assignment 1
This Assignment is still being developed.
Questions
- [2] There are three functions defined in 3000menu.c. Which lines in 3000menu.s implement each of those functions? How do you know those are the correct lines?
- [1] Why is environ referenced but not defined in 3000menu.s?
- [2] What lines in 3000menu.s define the menu array of character arrays? What lines define the constant string arrays it points to?
- [2] Which lines of 3000menu.s correspond to lines 72-76 of 3000menu.c, inside of main()? What do each of these lines do?
- [2] If we replace line 27 of 3000menu.c with "pid = 0;" how does the behavior of the program change? Why?
- [2] In line 30 of 3000menu.c, we replace one of the string pointers in menu with NULL.
- [1] What is the purpose of this line?
- [1] This doesn't change the menu that is displayed to the user. Why not?
- [2] How does the 3000menu.c's behavior change if you delete line 34, the call to wait? Why?
- [4] Replace line 29 in 3000menu.s (the assembly code version) with the following two lines:
mov $57,%eax
syscall
Create a new binary with the command gcc -O -z lazy 3000menu.s -o 3000menu-q2. It should run the same as before.- [1] How does the library call behavior of 3000menu-q2 differ from 3000menu? Why?
- [1] How does the system call behavior of 3000menu-q2 differ from 3000menu? Why?
- [2] Could you modify 3000menu.c so that it compiled to essentially identical assembly as 3000menu-q2? Why or why not? (You only need to consider standard C functionality.)
Code
/* 3000menu.c */
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdlib.h>
extern char **environ;
char *menu[] = {
"/usr/bin/ls",
"/usr/bin/whoami",
"/usr/bin/top",
"QUIT",
NULL
};
int QUIT = 3;
void run_program(int choice)
{
pid_t pid;
int status;
printf("Running %s\n", menu[choice]);
pid = fork();
if (pid == 0) {
menu[choice+1] = NULL;
execve(menu[choice], menu + choice, environ);
exit(-1);
} else {
pid = wait(&status);
}
return;
}
int choose_program(void)
{
int i = 0;
int choice;
char *input = NULL;
size_t result, n = 0;
printf("\nChoose a program to run:\n\n");
while (menu[i] != NULL) {
printf("%d. %s\n", i + 1, menu[i]);
i++;
}
printf("\nYour choice? ");
result = getline(&input, &n, stdin);
choice = atoi(input);
if ((choice > 0) && (choice <= i)) {
return choice - 1;
} else {
return -1;
}
}
int main()
{
int program;
while (1) {
program = choose_program();
if (program == QUIT) {
return 0;
}
if (program != -1) {
run_program(program);
}
}
}
.file "3000menu.c"
.text
.section .rodata.str1.1,"aMS",@progbits,1
.LC0:
.string "Running %s\n"
.text
.globl run_program
.type run_program, @function
run_program:
.LFB51:
.cfi_startproc
endbr64
pushq %rbx
.cfi_def_cfa_offset 16
.cfi_offset 3, -16
subq $16, %rsp
.cfi_def_cfa_offset 32
movl %edi, %ebx
movq %fs:40, %rax
movq %rax, 8(%rsp)
xorl %eax, %eax
movslq %edi, %rdx
leaq menu(%rip), %rax
movq (%rax,%rdx,8), %rdx
leaq .LC0(%rip), %rsi
movl $1, %edi
movl $0, %eax
call __printf_chk@PLT
call fork@PLT
testl %eax, %eax
je .L5
leaq 4(%rsp), %rdi
call wait@PLT
movq 8(%rsp), %rax
subq %fs:40, %rax
jne .L6
addq $16, %rsp
.cfi_remember_state
.cfi_def_cfa_offset 16
popq %rbx
.cfi_def_cfa_offset 8
ret
.L5:
.cfi_restore_state
leaq menu(%rip), %rax
leal 1(%rbx), %edx
movslq %edx, %rdx
movq $0, (%rax,%rdx,8)
movslq %ebx, %rbx
leaq (%rax,%rbx,8), %rsi
movq (%rax,%rbx,8), %rdi
movq environ(%rip), %rdx
call execve@PLT
movl $-1, %edi
call exit@PLT
.L6:
call __stack_chk_fail@PLT
.cfi_endproc
.LFE51:
.size run_program, .-run_program
.section .rodata.str1.1
.LC1:
.string "\nChoose a program to run:\n"
.LC2:
.string "%d. %s\n"
.LC3:
.string "\nYour choice? "
.text
.globl choose_program
.type choose_program, @function
choose_program:
.LFB52:
.cfi_startproc
endbr64
pushq %r13
.cfi_def_cfa_offset 16
.cfi_offset 13, -16
pushq %r12
.cfi_def_cfa_offset 24
.cfi_offset 12, -24
pushq %rbp
.cfi_def_cfa_offset 32
.cfi_offset 6, -32
pushq %rbx
.cfi_def_cfa_offset 40
.cfi_offset 3, -40
subq $40, %rsp
.cfi_def_cfa_offset 80
movq %fs:40, %rax
movq %rax, 24(%rsp)
xorl %eax, %eax
movq $0, 8(%rsp)
movq $0, 16(%rsp)
leaq .LC1(%rip), %rdi
call puts@PLT
movq menu(%rip), %rcx
testq %rcx, %rcx
je .L12
movl $1, %ebx
leaq .LC2(%rip), %r13
leaq -8+menu(%rip), %r12
.L9:
movl %ebx, %ebp
movl %ebx, %edx
movq %r13, %rsi
movl $1, %edi
movl $0, %eax
call __printf_chk@PLT
addq $1, %rbx
movq (%r12,%rbx,8), %rcx
testq %rcx, %rcx
jne .L9
.L8:
leaq .LC3(%rip), %rsi
movl $1, %edi
movl $0, %eax
call __printf_chk@PLT
leaq 16(%rsp), %rsi
leaq 8(%rsp), %rdi
movq stdin(%rip), %rdx
call getline@PLT
movl $10, %edx
movl $0, %esi
movq 8(%rsp), %rdi
call strtol@PLT
testl %eax, %eax
jle .L13
cmpl %eax, %ebp
jl .L13
subl $1, %eax
.L7:
movq 24(%rsp), %rdx
subq %fs:40, %rdx
jne .L16
addq $40, %rsp
.cfi_remember_state
.cfi_def_cfa_offset 40
popq %rbx
.cfi_def_cfa_offset 32
popq %rbp
.cfi_def_cfa_offset 24
popq %r12
.cfi_def_cfa_offset 16
popq %r13
.cfi_def_cfa_offset 8
ret
.L12:
.cfi_restore_state
movl $0, %ebp
jmp .L8
.L13:
movl $-1, %eax
jmp .L7
.L16:
call __stack_chk_fail@PLT
.cfi_endproc
.LFE52:
.size choose_program, .-choose_program
.globl main
.type main, @function
main:
.LFB53:
.cfi_startproc
endbr64
subq $8, %rsp
.cfi_def_cfa_offset 16
.L19:
call choose_program
cmpl %eax, QUIT(%rip)
je .L22
cmpl $-1, %eax
je .L19
movl %eax, %edi
call run_program
jmp .L19
.L22:
movl $0, %eax
addq $8, %rsp
.cfi_def_cfa_offset 8
ret
.cfi_endproc
.LFE53:
.size main, .-main
.globl QUIT
.data
.align 4
.type QUIT, @object
.size QUIT, 4
QUIT:
.long 3
.globl menu
.section .rodata.str1.1
.LC4:
.string "/usr/bin/ls"
.LC5:
.string "/usr/bin/whoami"
.LC6:
.string "/usr/bin/top"
.LC7:
.string "QUIT"
.section .data.rel.local,"aw"
.align 32
.type menu, @object
.size menu, 40
menu:
.quad .LC4
.quad .LC5
.quad .LC6
.quad .LC7
.quad 0
.ident "GCC: (Ubuntu 11.2.0-19ubuntu1) 11.2.0"
.section .note.GNU-stack,"",@progbits
.section .note.gnu.property,"a"
.align 8
.long 1f - 0f
.long 4f - 1f
.long 5
0:
.string "GNU"
1:
.align 8
.long 0xc0000002
.long 3f - 2f
2:
.long 0x3
3:
.align 8
4: