slocker-lite/src/user_spec.cpp

// Copyright (C) 2026 Viorel Munteanu
//
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 2 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License along
// with this program; if not, write to the Free Software Foundation, Inc.,
// 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.

#include "user_spec.h"

#include <algorithm>
#include <cctype>
#include <fstream>
#include <sstream>

#include <spdlog/spdlog.h>

namespace {

std::vector<std::string> split(const std::string& line, char delim) {
    std::vector<std::string> fields;
    std::istringstream stream(line);
    std::string field;
    while (std::getline(stream, field, delim)) {
        fields.push_back(field);
    }
    return fields;
}

bool is_all_digits(const std::string& s) {
    return !s.empty() && std::all_of(s.begin(), s.end(), [](unsigned char c) { return std::isdigit(c); });
}

std::optional<int> parse_int(const std::string& s) {
    try {
        return std::stoi(s);
    } catch (const std::exception&) {
        return std::nullopt;
    }
}

// Looks up `key` by name (field 0) or numeric id (field `id_field`) in a
// colon-separated database file (/etc/passwd or /etc/group). Malformed lines are
// skipped rather than treated as errors.
std::optional<std::vector<std::string>> lookup_entry(const std::filesystem::path& db_file,
                                                       const std::string& key, size_t id_field) {
    std::ifstream in(db_file, std::ios::binary);
    if (!in) {
        return std::nullopt;
    }

    std::string line;
    while (std::getline(in, line)) {
        auto fields = split(line, ':');
        if (fields.size() <= id_field) {
            continue;
        }
        if (fields[0] == key || fields[id_field] == key) {
            return fields;
        }
    }
    return std::nullopt;
}

}  // namespace

std::optional<ResolvedUser> resolve_user_and_group(const std::string& user,
                                                     const std::optional<std::string>& group,
                                                     const std::filesystem::path& image_root) {
    std::filesystem::path passwd_file = image_root / "etc" / "passwd";

    int uid = 0;
    int gid = 0;

    if (is_all_digits(user)) {
        uid = *parse_int(user);
        auto entry = lookup_entry(passwd_file, user, 2);
        if (entry && entry->size() > 3) {
            gid = parse_int((*entry)[3]).value_or(uid);
        } else {
            gid = uid;
        }
    } else {
        auto entry = lookup_entry(passwd_file, user, 2);
        if (!entry || entry->size() <= 3) {
            spdlog::error("could not resolve user '{}' in the image's /etc/passwd", user);
            return std::nullopt;
        }
        auto entry_uid = parse_int((*entry)[2]);
        auto entry_gid = parse_int((*entry)[3]);
        if (!entry_uid || !entry_gid) {
            spdlog::error("malformed /etc/passwd entry for user '{}'", user);
            return std::nullopt;
        }
        uid = *entry_uid;
        gid = *entry_gid;
    }

    if (group) {
        if (is_all_digits(*group)) {
            gid = *parse_int(*group);
        } else {
            std::filesystem::path group_file = image_root / "etc" / "group";
            auto entry = lookup_entry(group_file, *group, 2);
            auto entry_gid = entry ? parse_int((*entry)[2]) : std::nullopt;
            if (!entry_gid) {
                spdlog::error("could not resolve group '{}' in the image's /etc/group", *group);
                return std::nullopt;
            }
            gid = *entry_gid;
        }
    }

    // Looked up by the final resolved uid (field 2), independent of whether `user`
    // was given as a name or a number, so it matches whichever /etc/passwd row
    // actually owns that uid. No matching row -> fall back to "/root" for uid 0
    // (matches useradd-less images' own convention for root) or "/" otherwise.
    std::string home = uid == 0 ? "/root" : "/";
    auto home_entry = lookup_entry(passwd_file, std::to_string(uid), 2);
    if (home_entry && home_entry->size() > 5 && !(*home_entry)[5].empty()) {
        home = (*home_entry)[5];
    }

    return ResolvedUser{uid, gid, home};
}