path: root/third_party/nix/src/nix/run.cc

#include <queue>

#include <absl/strings/str_split.h>
#include <sys/mount.h>

#include "libmain/common-args.hh"
#include "libmain/shared.hh"
#include "libstore/derivations.hh"
#include "libstore/fs-accessor.hh"
#include "libstore/local-store.hh"
#include "libstore/store-api.hh"
#include "libutil/affinity.hh"
#include "libutil/finally.hh"
#include "nix/command.hh"

using namespace nix;

std::string chrootHelperName = "__run_in_chroot";

struct CmdRun final : InstallablesCommand {
  std::vector<std::string> command = {"bash"};
  StringSet keep, unset;
  bool ignoreEnvironment = false;

  CmdRun() {
    mkFlag()
        .longName("command")
        .shortName('c')
        .description("command and arguments to be executed; defaults to 'bash'")
        .labels({"command", "args"})
        .arity(ArityAny)
        .handler([&](const std::vector<std::string>& ss) {
          if (ss.empty()) {
            throw UsageError("--command requires at least one argument");
          }
          command = ss;
        });

    mkFlag()
        .longName("ignore-environment")
        .shortName('i')
        .description(
            "clear the entire environment (except those specified with --keep)")
        .set(&ignoreEnvironment, true);

    mkFlag()
        .longName("keep")
        .shortName('k')
        .description("keep specified environment variable")
        .arity(1)
        .labels({"name"})
        .handler([&](std::vector<std::string> ss) { keep.insert(ss.front()); });

    mkFlag()
        .longName("unset")
        .shortName('u')
        .description("unset specified environment variable")
        .arity(1)
        .labels({"name"})
        .handler(
            [&](std::vector<std::string> ss) { unset.insert(ss.front()); });
  }

  std::string name() override { return "run"; }

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

  Examples examples() override {
    return {
        Example{"To start a shell providing GNU Hello from NixOS 17.03:",
                "nix run -f channel:nixos-17.03 hello"},
        Example{"To start a shell providing youtube-dl from your 'nixpkgs' "
                "channel:",
                "nix run nixpkgs.youtube-dl"},
        Example{"To run GNU Hello:",
                "nix run nixpkgs.hello -c hello --greeting 'Hi everybody!'"},
        Example{"To run GNU Hello in a chroot store:",
                "nix run --store ~/my-nix nixpkgs.hello -c hello"},
    };
  }

  void run(ref<Store> store) override {
    auto outPaths = toStorePaths(store, Build, installables);

    auto accessor = store->getFSAccessor();

    if (ignoreEnvironment) {
      if (!unset.empty()) {
        throw UsageError(
            "--unset does not make sense with --ignore-environment");
      }

      std::map<std::string, std::string> kept;
      for (auto& var : keep) {
        auto s = getenv(var.c_str());
        if (s != nullptr) {
          kept[var] = s;
        }
      }

      clearEnv();

      for (auto& var : kept) {
        setenv(var.first.c_str(), var.second.c_str(), 1);
      }

    } else {
      if (!keep.empty()) {
        throw UsageError(
            "--keep does not make sense without --ignore-environment");
      }

      for (auto& var : unset) {
        unsetenv(var.c_str());
      }
    }

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

    Strings unixPath = absl::StrSplit(getEnv("PATH").value_or(""),
                                      absl::ByChar(':'), absl::SkipEmpty());

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

      { unixPath.push_front(path + "/bin"); }

      auto propPath = path + "/nix-support/propagated-user-env-packages";
      if (accessor->stat(propPath).type == FSAccessor::tRegular) {
        for (auto p :
             absl::StrSplit(readFile(propPath), absl::ByAnyChar(" \t\n\r"),
                            absl::SkipEmpty())) {
          todo.push(std::string(p));
        }
      }
    }

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

    std::string cmd = *command.begin();
    Strings args;
    for (auto& arg : command) {
      args.push_back(arg);
    }

    restoreSignals();

    restoreAffinity();

    /* 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<LocalStore>();

    if (store2 && store->storeDir != store2->realStoreDir) {
      Strings helperArgs = {chrootHelperName, store->storeDir,
                            store2->realStoreDir, cmd};
      for (auto& arg : args) {
        helperArgs.push_back(arg);
      }

      execv(readLink("/proc/self/exe").c_str(),
            stringsToCharPtrs(helperArgs).data());

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

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

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

static RegisterCommand r1(make_ref<CmdRun>());

void chrootHelper(int argc, char** argv) {
  int p = 1;
  std::string storeDir = argv[p++];
  std::string realStoreDir = 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,
              nullptr) == -1) {
      throw SysError("mounting '%s' on '%s'", realStoreDir, storeDir);
    }

    for (const auto& entry : readDirectory("/")) {
      auto src = "/" + entry.name;
      auto st = lstat(src);
      if (!S_ISDIR(st.st_mode)) {
        continue;
      }
      Path dst = tmpDir + "/" + entry.name;
      if (pathExists(dst)) {
        continue;
      }
      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, nullptr) ==
          -1) {
        throw SysError("mounting '%s' on '%s'", src, dst);
      }
    }

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

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

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

  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
}