セキュリティミニキャンプオンライン2021「Linuxシステムプログラミング入門：コンテナ技術を支える名前空間」 演習課題 #4

pretask2.c

#define _GNU_SOURCE
#include <sched.h>
#include <signal.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

#define OUTPUT_BUFFER_SIZE 1024

int execute_binary(const char* const binary, char* const buffer, const size_t bufsize);

int main(){
    char buf[OUTPUT_BUFFER_SIZE] = {0};
    int r = execute_binary("/bin/ls", buf, OUTPUT_BUFFER_SIZE);
    if(r >= 0){
        buf[r] = '\0';
        printf("read %d bytes\n%s\n", r, buf);
    } else {
        printf("error %d\n", r);
    }
}

int child_proc(void* arg) {
    char *path = (char *)arg;
    char *argv[] = {path, NULL}; // argv
    char *env[] = {NULL}; // envp
    
    /* execve */
    execve(path, argv, env);
    
    exit(EXIT_FAILURE);
}


int execute_binary(const char* const binary, char* const buffer, const size_t bufsize) {
    void *stack; // stack for child process
    pid_t child_pid; // child pid
    int status; // child process status 
 
    int tmp_fd; // temp file descriptors(STDOUT_FILENO)
    int pipe_fd[2]; // file descriptors (pipe_fd[0]: read, pipe_fd[1]: write)
    
    ssize_t sz; // read size

    /* make pipe */
    if(pipe(pipe_fd) == -1) {
        puts("pipe failed!\n");
        return -1;
    }

    /* make stack */
    stack = mmap(NULL, 1024*1024, PROT_READ|PROT_WRITE,
    MAP_PRIVATE|MAP_ANONYMOUS|MAP_GROWSDOWN|MAP_STACK,-1,0);
    if(stack == MAP_FAILED) {
        puts("mmap failed!\n");
        return -1;
    }

    /* switch fd */
    tmp_fd = dup(STDOUT_FILENO);
    dup2(pipe_fd[1], STDOUT_FILENO);

    /* exec binary */
    child_pid = clone(child_proc, stack+1024*1024, SIGCHLD, binary);

    /* restore STDOUT_FILENO fd */
    dup2(tmp_fd, STDOUT_FILENO);
    if(child_pid == -1) {
        puts("clone failed!\n");
        return -1;
    }
    waitpid(child_pid, &status, 0);
    
    /* check status */
    if(status != 0) {
        puts("execve failed!\n");
        return -1;
    }

    /* read */
    sz = read(pipe_fd[0], buffer, bufsize);
    if(sz < 0) {
        puts("read failed!\n");
        return -1;
    }

    return sz;
}