lix/src/nix/run.cc

#include "run.hh"
#include "command.hh"
#include "common-args.hh"
#include "shared.hh"
#include "store-api.hh"
#include "derivations.hh"
#include "local-store.hh"
#include "finally.hh"
#include "fs-accessor.hh"
#include "progress-bar.hh"
#include "eval.hh"
#include "build/personality.hh"

#if __linux__
#include <sys/mount.h>
#endif

#include <queue>

using namespace nix;

std::string chrootHelperName = "__run_in_chroot";

namespace nix {

void runProgramInStore(ref<Store> store,
    const std::string & program,
    const Strings & args,
    std::optional<std::string_view> system)
{
    stopProgressBar();

    restoreProcessContext();

    /* If this is a diverted store (i.e. its "logical" location
       (typically /nix/store) differs from its "physical" location
       (e.g. /home/eelco/nix/store), then run the command in a
       chroot. For non-root users, this requires running it in new
       mount and user namespaces. Unfortunately,
       unshare(CLONE_NEWUSER) doesn't work in a multithreaded program
       (which "nix" is), so we exec() a single-threaded helper program
       (chrootHelper() below) to do the work. */
    auto store2 = store.dynamic_pointer_cast<LocalFSStore>();

    if (!store2)
        throw Error("store '%s' is not a local store so it does not support command execution", store->getUri());

    if (store->storeDir != store2->getRealStoreDir()) {
        Strings helperArgs = { chrootHelperName, store->storeDir, store2->getRealStoreDir(), std::string(system.value_or("")), program };
        for (auto & arg : args) helperArgs.push_back(arg);

        execv(getSelfExe().value_or("nix").c_str(), stringsToCharPtrs(helperArgs).data());

        throw SysError("could not execute chroot helper");
    }

    if (system)
        setPersonality(*system);

    execvp(program.c_str(), stringsToCharPtrs(args).data());

    throw SysError("unable to execute '%s'", program);
}

}

struct CmdShell : InstallablesCommand, MixEnvironment
{

    using InstallablesCommand::run;

    std::vector<std::string> command = { getEnv("SHELL").value_or("bash") };

    CmdShell()
    {
        addFlag({
            .longName = "command",
            .shortName = 'c',
            .description = "Command and arguments to be executed, defaulting to `$SHELL`",
            .labels = {"command", "args"},
            .handler = {[&](std::vector<std::string> ss) {
                if (ss.empty()) throw UsageError("--command requires at least one argument");
                command = ss;
            }}
        });
    }

    std::string description() override
    {
        return "run a shell in which the specified packages are available";
    }

    std::string doc() override
    {
        return
          #include "shell.md"
          ;
    }

    void run(ref<Store> store, Installables && installables) override
    {
        auto outPaths = Installable::toStorePaths(getEvalStore(), store, Realise::Outputs, OperateOn::Output, installables);

        auto accessor = store->getFSAccessor();

        std::unordered_set<StorePath> done;
        std::queue<StorePath> todo;
        for (auto & path : outPaths) todo.push(path);

        setEnviron();

        auto unixPath = tokenizeString<Strings>(getEnv("PATH").value_or(""), ":");

        while (!todo.empty()) {
            auto path = todo.front();
            todo.pop();
            if (!done.insert(path).second) continue;

            if (true)
                unixPath.push_front(store->printStorePath(path) + "/bin");

            auto propPath = store->printStorePath(path) + "/nix-support/propagated-user-env-packages";
            if (accessor->stat(propPath).type == FSAccessor::tRegular) {
                for (auto & p : tokenizeString<Paths>(readFile(propPath)))
                    todo.push(store->parseStorePath(p));
            }
        }

        setenv("PATH", concatStringsSep(":", unixPath).c_str(), 1);

        Strings args;
        for (auto & arg : command) args.push_back(arg);

        runProgramInStore(store, *command.begin(), args);
    }
};

static auto rCmdShell = registerCommand<CmdShell>("shell");

struct CmdRun : InstallableCommand
{
    using InstallableCommand::run;

    std::vector<std::string> args;

    CmdRun()
    {
        expectArgs({
            .label = "args",
            .handler = {&args},
            .completer = completePath
        });
    }

    std::string description() override
    {
        return "run a Nix application";
    }

    std::string doc() override
    {
        return
          #include "run.md"
          ;
    }

    Strings getDefaultFlakeAttrPaths() override
    {
        Strings res{
            "apps." + settings.thisSystem.get() + ".default",
            "defaultApp." + settings.thisSystem.get(),
        };
        for (auto & s : SourceExprCommand::getDefaultFlakeAttrPaths())
            res.push_back(s);
        return res;
    }

    Strings getDefaultFlakeAttrPathPrefixes() override
    {
        Strings res{"apps." + settings.thisSystem.get() + "."};
        for (auto & s : SourceExprCommand::getDefaultFlakeAttrPathPrefixes())
            res.push_back(s);
        return res;
    }

    void run(ref<Store> store, ref<Installable> installable) override
    {
        auto state = getEvalState();

        lockFlags.applyNixConfig = true;
        auto app = installable->toApp(*state).resolve(getEvalStore(), store);

        Strings allArgs{app.program};
        for (auto & i : args) allArgs.push_back(i);

        runProgramInStore(store, app.program, allArgs);
    }
};

static auto rCmdRun = registerCommand<CmdRun>("run");

void chrootHelper(int argc, char * * argv)
{
    int p = 1;
    std::string storeDir = argv[p++];
    std::string realStoreDir = argv[p++];
    std::string system = argv[p++];
    std::string cmd = argv[p++];
    Strings args;
    while (p < argc)
        args.push_back(argv[p++]);

#if __linux__
    uid_t uid = getuid();
    uid_t gid = getgid();

    if (unshare(CLONE_NEWUSER | CLONE_NEWNS) == -1)
        /* Try with just CLONE_NEWNS in case user namespaces are
           specifically disabled. */
        if (unshare(CLONE_NEWNS) == -1)
            throw SysError("setting up a private mount namespace");

    /* Bind-mount realStoreDir on /nix/store. If the latter mount
       point doesn't already exists, we have to create a chroot
       environment containing the mount point and bind mounts for the
       children of /. Would be nice if we could use overlayfs here,
       but that doesn't work in a user namespace yet (Ubuntu has a
       patch for this:
       https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1478578). */
    if (!pathExists(storeDir)) {
        // FIXME: Use overlayfs?

        Path tmpDir = createTempDir();

        createDirs(tmpDir + storeDir);

        if (mount(realStoreDir.c_str(), (tmpDir + storeDir).c_str(), "", MS_BIND, 0) == -1)
            throw SysError("mounting '%s' on '%s'", realStoreDir, storeDir);

        for (auto entry : readDirectory("/")) {
            auto src = "/" + entry.name;
            Path dst = tmpDir + "/" + entry.name;
            if (pathExists(dst)) continue;
            auto st = lstat(src);
            if (S_ISDIR(st.st_mode)) {
                if (mkdir(dst.c_str(), 0700) == -1)
                    throw SysError("creating directory '%s'", dst);
                if (mount(src.c_str(), dst.c_str(), "", MS_BIND | MS_REC, 0) == -1)
                    throw SysError("mounting '%s' on '%s'", src, dst);
            } else if (S_ISLNK(st.st_mode))
                createSymlink(readLink(src), dst);
        }

        char * cwd = getcwd(0, 0);
        if (!cwd) throw SysError("getting current directory");
        Finally freeCwd([&]() { free(cwd); });

        if (chroot(tmpDir.c_str()) == -1)
            throw SysError("chrooting into '%s'", tmpDir);

        if (chdir(cwd) == -1)
            throw SysError("chdir to '%s' in chroot", cwd);
    } else
        if (mount(realStoreDir.c_str(), storeDir.c_str(), "", MS_BIND, 0) == -1)
            throw SysError("mounting '%s' on '%s'", realStoreDir, storeDir);

    writeFile("/proc/self/setgroups", "deny");
    writeFile("/proc/self/uid_map", fmt("%d %d %d", uid, uid, 1));
    writeFile("/proc/self/gid_map", fmt("%d %d %d", gid, gid, 1));

    if (system != "")
        setPersonality(system);

    execvp(cmd.c_str(), stringsToCharPtrs(args).data());

    throw SysError("unable to exec '%s'", cmd);

#else
    throw Error("mounting the Nix store on '%s' is not supported on this platform", storeDir);
#endif
}