Offset the permission of a directory and all its content recursively.

uid_offset.cpp

#include <iostream>
#include <filesystem>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

namespace fs = std::filesystem;

void chown_with_offset(const fs::path &target_path, long offset, bool symbolic = false)
{
    struct stat fileStat;
    if (lstat(target_path.c_str(), &fileStat) != 0)
    {
        std::cerr << "Error: Unable to get status of " << target_path << std::endl;
        return;
    }

    long new_uid = fileStat.st_uid + offset;
    long new_gid = fileStat.st_gid + offset;

    // clip new id to zero if it is negative
    if (new_uid < 0)
        new_uid = 0;
    if (new_gid < 0)
        new_gid = 0;

    // change owner and group of the file or symbolic link
    if (symbolic)
    {
        if (lchown(target_path.c_str(), new_uid, new_gid) != 0)
        {
            std::cerr << "Error: Unable to change owner and group of " << target_path << std::endl;
            return;
        }
    }
    else
    {
        if (chown(target_path.c_str(), new_uid, new_gid) != 0)
        {
            std::cerr << "Error: Unable to change owner and group of " << target_path << std::endl;
            return;
        }
    }
}

void applyUidGidOffset(const fs::path &dirname, long offset)
{
    chown_with_offset(dirname, offset);

    // loop through all files and directories in the directory
    for (auto &entry : fs::recursive_directory_iterator(dirname, fs::directory_options::none))
    {
        chown_with_offset(entry.path(), offset, fs::is_symlink(entry.symlink_status()));
    }

    std::cout << "UID and GID offset applied successfully." << std::endl;
}

int main(int argc, char *argv[])
{
    if (argc != 3)
    {
        std::cerr << "Usage: " << argv[0] << " <directory> <offset>" << std::endl;
        return 1;
    }

    fs::path dirname = argv[1];
    long offset = std::stol(argv[2]);

    applyUidGidOffset(dirname, offset);

    return 0;
}

compile: g++ -std=c++17 -o uid_offset uid_offset.cpp
run: sudo ./uid_offset /var/lib/machines/test -11451419