Operating Systems 2022F: Assignment 2
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This assignment is still being developed.
If you submission:
- Doesn't pass the validator,
- Is not named "comp3000-assign2-myname" where myname is your MyCarletonOne username, or
- Does not contain your Student ID number in the designated part of the template
Then your submission will automatically get a grade of zero, because it won't work with the submission scripts. You have been warned!
Code
/* 3000shell2.c */
/* variant of 3000shell.c from here:
https://homeostasis.scs.carleton.ca/~soma/os-2019f/code/3000shell.c
*/
/* This version is under GPLv3, copyright 2022, Anil Somayaji */
/* You really shouldn't be incorporating parts of this in any other code,
it is meant for teaching, not production */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <ctype.h>
#include <fcntl.h>
#include <signal.h>
#include <pwd.h>
#define BUFFER_SIZE 1<<16
#define ARR_SIZE 1<<16
#define COMM_SIZE 32
const char *proc_prefix = "/proc";
void parse_args(char *buffer, char** args,
size_t args_size, size_t *nargs)
{
char *buf_args[args_size]; /* You need C99 */
char **cp, *wbuf;
size_t i, j;
wbuf=buffer;
buf_args[0]=buffer;
args[0] =buffer;
for(cp=buf_args; (*cp=strsep(&wbuf, " \n\t")) != NULL ;){
if ((*cp != NULL) && (++cp >= &buf_args[args_size]))
break;
}
for (j=i=0; buf_args[i]!=NULL; i++){
if (strlen(buf_args[i]) > 0)
args[j++]=buf_args[i];
}
*nargs=j;
args[j]=NULL;
}
void become(char *username)
{
int result;
char *shell_argv[3];
extern char **environ;
struct passwd *pw_entry;
pw_entry = getpwnam(username);
if (pw_entry == NULL) {
fprintf(stderr, "Could not find user %s.\n", username);
exit(-2);
}
result = setgid(pw_entry->pw_gid);
if (result != 0) {
fprintf(stderr, "Failed to change to gid %d\n",
pw_entry->pw_gid);
exit(-3);
}
result = setuid(pw_entry->pw_uid);
if (result != 0) {
fprintf(stderr, "Failed to change to uid %d\n",
pw_entry->pw_uid);
exit(-4);
}
}
void find_binary(char *name, char *path, char *fn, int fn_size) {
char *n, *p;
int r, stat_return;
struct stat file_status;
if (name[0] == '.' || name[0] == '/') {
strncpy(fn, name, fn_size);
return;
}
p = path;
while (*p != '\0') {
r = 0;
while (*p != '\0' && *p != ':' && r < fn_size - 1) {
fn[r] = *p;
r++;
p++;
}
fn[r] = '/';
r++;
n = name;
while (*n != '\0' && r < fn_size) {
fn[r] = *n;
n++;
r++;
}
fn[r] = '\0';
stat_return = stat(fn, &file_status);
if (stat_return == 0) {
return;
}
if (*p != '\0') {
p++;
}
}
}
void setup_comm_fn(char *pidstr, char *comm_fn)
{
char *c;
strcpy(comm_fn, proc_prefix);
c = comm_fn + strlen(comm_fn);
*c = '/';
c++;
strcpy(c, pidstr);
c = c + strlen(pidstr);
strcpy(c, "/comm");
}
void signal_handler(int the_signal)
{
int pid, status;
if (the_signal == SIGHUP) {
fprintf(stderr, "Received SIGHUP.\n");
return;
}
if (the_signal != SIGCHLD) {
fprintf(stderr, "Child handler called for signal %d?!\n",
the_signal);
return;
}
pid = wait(&status);
if (pid == -1) {
/* nothing to wait for */
return;
}
if (WIFEXITED(status)) {
fprintf(stderr, "\nProcess %d exited with status %d.\n",
pid, WEXITSTATUS(status));
} else {
fprintf(stderr, "\nProcess %d aborted.\n", pid);
}
}
void run_program(char *args[], int background, char *stdout_fn,
char *path, char *envp[], char *new_user)
{
pid_t pid;
int fd, *ret_status = NULL;
char bin_fn[BUFFER_SIZE];
pid = fork();
if (pid) {
if (background) {
fprintf(stderr,
"Process %d running in the background.\n",
pid);
} else {
pid = wait(ret_status);
}
} else {
if (new_user) {
become(new_user);
} else {
setegid(getgid());
seteuid(getuid());
}
find_binary(args[0], path, bin_fn, BUFFER_SIZE);
if (stdout_fn != NULL) {
fd = creat(stdout_fn, 0666);
dup2(fd, 1);
close(fd);
}
if (execve(bin_fn, args, envp)) {
puts(strerror(errno));
exit(127);
}
}
}
void pinfo(void)
{
printf("uid=%d, euid=%d, gid=%d, egid=%d\n",
getuid(), geteuid(), getgid(), getegid());
}
void prompt_loop(char *username, char *path, char *envp[])
{
char buffer[BUFFER_SIZE];
char *all_args[ARR_SIZE];
char **args;
int background;
size_t nargs;
char *s;
int i, j;
char *stdout_fn, *new_user;
while(1){
printf("%s $ ", username);
s = fgets(buffer, BUFFER_SIZE, stdin);
if (s == NULL) {
/* we reached EOF */
printf("\n");
exit(0);
}
parse_args(buffer, all_args, ARR_SIZE, &nargs);
if (nargs==0) continue;
args = all_args;
if (!strcmp(args[0], "exit")) {
exit(0);
}
if (!strcmp(args[0], "pinfo")) {
pinfo();
continue;
}
new_user = NULL;
if (!strcmp(args[0], "become")) {
if (nargs > 2) {
new_user = args[1];
args = args + 2;
nargs = nargs - 2;
} else {
fprintf(stderr,
"ERROR: become needs a username "
"and a command\n");
}
}
background = 0;
if (strcmp(args[nargs-1], "&") == 0) {
background = 1;
nargs--;
args[nargs] = NULL;
}
stdout_fn = NULL;
for (i = 1; i < nargs; i++) {
if (args[i][0] == '>') {
stdout_fn = args[i];
stdout_fn++;
printf("Set stdout to %s\n", stdout_fn);
for (j = i; j < nargs - 1; j++) {
args[j] = args[j+1];
}
nargs--;
args[nargs] = NULL;
break;
}
}
run_program(args, background, stdout_fn, path, envp, new_user);
}
}
int main(int argc, char *argv[], char *envp[])
{
struct sigaction signal_handler_struct;
char *username;
char *default_username = "UNKNOWN";
char *path;
char *default_path = "/usr/bin:/bin";
memset (&signal_handler_struct, 0, sizeof(signal_handler_struct));
signal_handler_struct.sa_handler = signal_handler;
signal_handler_struct.sa_flags = SA_RESTART;
if (sigaction(SIGCHLD, &signal_handler_struct, NULL)) {
fprintf(stderr, "Couldn't register SIGCHLD handler.\n");
}
if (sigaction(SIGHUP, &signal_handler_struct, NULL)) {
fprintf(stderr, "Couldn't register SIGHUP handler.\n");
}
username = getenv("USER");
if (!username) {
username = default_username;
}
path = getenv("PATH");
if (!path) {
path = default_path;
}
prompt_loop(username, path, envp);
return 0;
}