example4-detached-mount-exec.c

#include "common.h"
#include <stdlib.h>

int main(void) {
    char dummy_lower_layer[255] = "dummy-lower-layer/";
    addcurpath(dummy_lower_layer);
	char lower_layer[255] = "lower-layer/";
	addcurpath(lower_layer);
    char lowerdir_str[512];
    snprintf(lowerdir_str, sizeof(lowerdir_str), "%s:%s", lower_layer, dummy_lower_layer);
    const char * binary_name = "hello-world";

	printpid();
    printf("Creating overlayfs with hello-world binary...\n");

    wait("* Step 1: Will create the overlayfs context");
    int overlay_fd = fsopen("overlay", FSOPEN_CLOEXEC);
    if (overlay_fd < 0) {
        perror("fsopen overlay failed");
        return 1;
    }
	printf("The overlayfs context was successfully created. The fd handle is %d\n", overlay_fd);


	wait("* Step 2: Will configure with the lowerdirs - the true one, and a dummy one: %s", lowerdir_str);
    if (fsconfig(overlay_fd, FSCONFIG_SET_STRING, "lowerdir", lowerdir_str, 0) < 0) {
        perror("fsconfig set lowerdir failed");
        return 1;
    }
	printf("lowerdir set\n");


	wait("* Step 3: Will validate the configuration and create the superblock");
    if (fsconfig(overlay_fd, FSCONFIG_CMD_CREATE, NULL, NULL, 0) < 0) {
        perror("fsconfig create failed");
        return 1;
    }
	printf("Superblock created\n");


    wait("* Step 4: Mount the overlayfs (read-only, no devices, no suid)");
    int mount_fd = fsmount(overlay_fd, FSMOUNT_CLOEXEC, MS_RDONLY | MS_NODEV | MS_NOSUID);
    if (mount_fd < 0) {
        perror("fsmount failed");
        return 1;
    }
    printf("Overlayfs created successfully!\n");


    wait("* Step 5: Will open() the file int he detached filesystem");
	int binary_fd = openat(mount_fd, binary_name, O_RDONLY | O_CLOEXEC);
    if (binary_fd < 0) {
        perror("openat hello-world failed");
        return 1;
    }
    printf("Opened hello-world binary from overlayfs (fd=%d)\n", binary_fd);
    wait("You can now execute it with: /proc/%d/fd/%d\n", getpid(), binary_fd);


    return 0;
}