fingerprintd/interfaces/Fingerprintd-Actions.cppm

// SPDX-License-Identifier: GPL-3.0-only
// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®

// lint-disable-file fixed-width-types
/*
Fingerprintd:Actions — what a finger means, beyond "it was you".

The trustlet reports WHICH finger matched, and until now the daemon only used
that to answer yes. This module holds the table that gives each finger a
meaning: run a command as root, tell the user's session, or answer no-match
while doing one of those anyway -- which is the duress case, where the phone
should look like it simply did not recognise the finger.

Two rules shape the whole design.

ROOT DOES NOT LAUNCH APPS. The daemon runs as root with no session bus, no
Wayland display and no user environment, so it cannot meaningfully start a
user's application, and trying would either fail or run the user's software as
root. So a session action is not a command here at all: the daemon emits a
signal naming the finger, and an agent in the user's own session decides what
that means from the user's own configuration. The only commands in this file
are ones root is supposed to run.

WHICH MAKES THIS FILE A ROOT-EXECUTION SURFACE. Anything that can write it
gets root at the next press of a finger. The parser therefore refuses a file
that is not owned by root or that anyone else can write, and refuses it
WHOLESALE rather than skipping the offending line -- a half-applied security
policy is worse than none. Ownership is checked by the shell, which has the
stat; this module states the rule and holds the verdict.

Format, one rule per line, four fields:

    <finger>  <where>  <verdict>  <command...>

    finger    an fprintd finger name, e.g. right-index-finger
    where     system  -- root runs the command
              session -- the user's agent is told; no command is run here
    verdict   match    -- the client is told the finger matched (normal)
              no-match -- the client is told it did not, whatever really
                          happened. The duress case.
    command   required for system, and must be absent for session

'#' comments and blank lines are ignored. A finger with no rule behaves
exactly as before, which is what makes the feature absent until configured.
*/

export module Fingerprintd:Actions;
import std;
import :Store;

export namespace fingerprintd::actions {

    enum class Where { System, Session };
    enum class Verdict { Match, NoMatch };

    struct Rule {
        store::Finger finger{};
        Where where = Where::System;
        Verdict verdict = Verdict::Match;
        std::string command;             // empty for Session
    };

    // Why a file was rejected. Rejection is total: no rule from a file that
    // failed to parse is ever applied.
    enum class Error {
        None,
        NotWritableOnlyByRoot,   // the shell's stat says someone else can write it
        UnknownFinger,
        UnknownWhere,
        UnknownVerdict,
        MissingCommand,          // system without a command
        UnexpectedCommand,       // session with one
        DuplicateFinger,         // two rules for one finger: ambiguous, not merged
        RelativeCommand,         // a command that is not an absolute path
    };

    inline constexpr std::string_view Describe(Error e) {
        switch (e) {
        case Error::None:                  return "ok";
        case Error::NotWritableOnlyByRoot: return "the file must be owned by root and writable by no one else";
        case Error::UnknownFinger:         return "not an fprintd finger name";
        case Error::UnknownWhere:          return "expected 'system' or 'session'";
        case Error::UnknownVerdict:        return "expected 'match' or 'no-match'";
        case Error::MissingCommand:        return "a system rule needs a command";
        case Error::UnexpectedCommand:     return "a session rule runs no command here; the user's agent decides";
        case Error::DuplicateFinger:       return "two rules for the same finger";
        case Error::RelativeCommand:       return "the command must be an absolute path";
        }
        return "unknown";
    }

    struct Parsed {
        std::vector<Rule> rules;
        Error error = Error::None;
        int line = 0;                    // 1-based, 0 when the error is the file itself
        bool Ok() const { return error == Error::None; }
    };

    // Split on runs of spaces and tabs, keeping the tail intact from `upto`
    // fields onward so a command may contain spaces.
    inline std::vector<std::string_view> Fields(std::string_view line, std::size_t upto) {
        std::vector<std::string_view> out;
        std::size_t i = 0;
        while (i < line.size()) {
            while (i < line.size() && (line[i] == ' ' || line[i] == '\t')) i++;
            if (i >= line.size()) break;
            if (out.size() == upto) { out.push_back(line.substr(i)); break; }
            std::size_t j = i;
            while (j < line.size() && line[j] != ' ' && line[j] != '\t') j++;
            out.push_back(line.substr(i, j - i));
            i = j;
        }
        return out;
    }

    // `rootOnlyWritable` is the shell's answer about the file's mode and
    // owner. Passing false rejects the file without looking at a single rule:
    // a config root will execute is not worth parsing if someone else can
    // rewrite it between the parse and the press.
    inline Parsed Parse(std::string_view text, bool rootOnlyWritable) {
        Parsed p;
        // Every rejection discards the rules gathered so far. A file that
        // stops being valid halfway is not "valid up to there": applying the
        // prefix would leave a policy nobody wrote, and the missing half
        // could be the one that mattered.
        auto reject = [&p](Error e, int line) -> Parsed& {
            p.error = e;
            p.line = line;
            p.rules.clear();
            return p;
        };
        if (!rootOnlyWritable) return reject(Error::NotWritableOnlyByRoot, 0);

        int lineNo = 0;
        for (const auto part : std::views::split(text, '\n')) {
            lineNo++;
            std::string_view line(part.begin(), part.end());
            if (!line.empty() && line.back() == '\r') line.remove_suffix(1);
            // Trim leading blanks so a comment may be indented.
            std::size_t s = line.find_first_not_of(" \t");
            if (s == std::string_view::npos) continue;
            line.remove_prefix(s);
            if (line.front() == '#') continue;

            auto f = Fields(line, 3);
            if (f.size() < 3) return reject(Error::UnknownWhere, lineNo);

            Rule r;
            auto fin = store::FingerFromName(std::string(f[0]));
            if (!fin) return reject(Error::UnknownFinger, lineNo);
            r.finger = *fin;

            if (f[1] == "system")       r.where = Where::System;
            else if (f[1] == "session") r.where = Where::Session;
            else return reject(Error::UnknownWhere, lineNo);

            if (f[2] == "match")         r.verdict = Verdict::Match;
            else if (f[2] == "no-match") r.verdict = Verdict::NoMatch;
            else return reject(Error::UnknownVerdict, lineNo);

            if (f.size() > 3) {
                std::string_view cmd = f[3];
                while (!cmd.empty() && (cmd.back() == ' ' || cmd.back() == '\t')) cmd.remove_suffix(1);
                r.command = std::string(cmd);
            }

            if (r.where == Where::System && r.command.empty())
                return reject(Error::MissingCommand, lineNo);
            if (r.where == Where::Session && !r.command.empty())
                return reject(Error::UnexpectedCommand, lineNo);
            // An absolute path only. Resolving a bare name through PATH would
            // make what root executes depend on an environment this daemon
            // does not control.
            if (r.where == Where::System && !r.command.starts_with('/'))
                return reject(Error::RelativeCommand, lineNo);
            for (const Rule& e : p.rules) {
                if (e.finger == r.finger) return reject(Error::DuplicateFinger, lineNo);
            }
            p.rules.push_back(std::move(r));
        }
        return p;
    }

    inline const Rule* Find(const std::vector<Rule>& rules, store::Finger f) {
        for (const Rule& r : rules)
            if (r.finger == f) return &r;
        return nullptr;
    }
}