fd-perm.cpp

// fd-perm $(id -u nobody)

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fcntl.h>
#include <sys/unistd.h>
#include <sys/wait.h>

int main(int, char *argv[]) {
    int fd = open("1.txt", O_CREAT | O_RDWR, 0660);
    if (fd == -1) {
        perror("open");
        return 1;
    }
    write(fd, "Hello World\n", 12);
    if (fork()) {
        printf("perent: %d\n", getpid());
        wait(nullptr);
        close(fd);
    } else {
        printf("child: %d\n", getpid());
        setuid(strtol(argv[1], nullptr, 10));
        int fd2 = open("1.txt", O_RDONLY);
        if (fd2 == -1) {
            perror("open");
            printf("child process cannot open file by name.\n");
        } else {
            close(fd2);
        }
        char buffer[32] = {0};
        lseek(fd, 0, SEEK_SET);
        int n = read(fd, buffer, sizeof(buffer));
        if (n == -1) {
            perror("read");
        } else {
            printf("file content: %s\n", buffer);
        }
        close(fd);
    }
    return 0;
}