lix/src/nix-daemon/nix-daemon.cc

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#include "shared.hh"
#include "local-store.hh"
#include "remote-store.hh"
#include "util.hh"
#include "serialise.hh"
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#include "archive.hh"
#include "globals.hh"
#include "derivations.hh"
#include "finally.hh"
#include "../nix/legacy.hh"
#include "daemon.hh"
#include <algorithm>
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#include <climits>
#include <cstring>
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#include <unistd.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <errno.h>
#include <pwd.h>
#include <grp.h>
#include <fcntl.h>
#if __APPLE__ || __FreeBSD__
#include <sys/ucred.h>
#endif
using namespace nix;
using namespace nix::daemon;
#ifndef __linux__
#define SPLICE_F_MOVE 0
static ssize_t splice(int fd_in, void *off_in, int fd_out, void *off_out, size_t len, unsigned int flags)
{
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// We ignore most parameters, we just have them for conformance with the linux syscall
std::vector<char> buf(8192);
auto read_count = read(fd_in, buf.data(), buf.size());
if (read_count == -1)
return read_count;
auto write_count = decltype(read_count)(0);
while (write_count < read_count) {
auto res = write(fd_out, buf.data() + write_count, read_count - write_count);
if (res == -1)
return res;
write_count += res;
}
return read_count;
}
#endif
static void sigChldHandler(int sigNo)
{
// Ensure we don't modify errno of whatever we've interrupted
auto saved_errno = errno;
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// Reap all dead children.
while (waitpid(-1, 0, WNOHANG) > 0) ;
errno = saved_errno;
}
static void setSigChldAction(bool autoReap)
{
struct sigaction act, oact;
act.sa_handler = autoReap ? sigChldHandler : SIG_DFL;
sigfillset(&act.sa_mask);
act.sa_flags = 0;
if (sigaction(SIGCHLD, &act, &oact))
throw SysError("setting SIGCHLD handler");
}
bool matchUser(const string & user, const string & group, const Strings & users)
{
if (find(users.begin(), users.end(), "*") != users.end())
return true;
if (find(users.begin(), users.end(), user) != users.end())
return true;
for (auto & i : users)
if (string(i, 0, 1) == "@") {
if (group == string(i, 1)) return true;
struct group * gr = getgrnam(i.c_str() + 1);
if (!gr) continue;
for (char * * mem = gr->gr_mem; *mem; mem++)
if (user == string(*mem)) return true;
}
return false;
}
struct PeerInfo
{
bool pidKnown;
pid_t pid;
bool uidKnown;
uid_t uid;
bool gidKnown;
gid_t gid;
};
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// Get the identity of the caller, if possible.
static PeerInfo getPeerInfo(int remote)
{
PeerInfo peer = { false, 0, false, 0, false, 0 };
#if defined(SO_PEERCRED)
ucred cred;
socklen_t credLen = sizeof(cred);
if (getsockopt(remote, SOL_SOCKET, SO_PEERCRED, &cred, &credLen) == -1)
throw SysError("getting peer credentials");
peer = { true, cred.pid, true, cred.uid, true, cred.gid };
#elif defined(LOCAL_PEERCRED)
#if !defined(SOL_LOCAL)
#define SOL_LOCAL 0
#endif
xucred cred;
socklen_t credLen = sizeof(cred);
if (getsockopt(remote, SOL_LOCAL, LOCAL_PEERCRED, &cred, &credLen) == -1)
throw SysError("getting peer credentials");
peer = { false, 0, true, cred.cr_uid, false, 0 };
#endif
return peer;
}
#define SD_LISTEN_FDS_START 3
static ref<Store> openUncachedStore()
{
Store::Params params; // FIXME: get params from somewhere
// Disable caching since the client already does that.
params["path-info-cache-size"] = "0";
return openStore(settings.storeUri, params);
}
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static void daemonLoop(char * * argv)
{
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if (chdir("/") == -1)
throw SysError("cannot change current directory");
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// Get rid of children automatically; don't let them become zombies.
setSigChldAction(true);
AutoCloseFD fdSocket;
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// Handle socket-based activation by systemd.
auto listenFds = getEnv("LISTEN_FDS");
if (listenFds) {
if (getEnv("LISTEN_PID") != std::to_string(getpid()) || listenFds != "1")
throw Error("unexpected systemd environment variables");
fdSocket = SD_LISTEN_FDS_START;
closeOnExec(fdSocket.get());
}
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// Otherwise, create and bind to a Unix domain socket.
else {
createDirs(dirOf(settings.nixDaemonSocketFile));
fdSocket = createUnixDomainSocket(settings.nixDaemonSocketFile, 0666);
}
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// Loop accepting connections.
while (1) {
try {
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// Accept a connection.
struct sockaddr_un remoteAddr;
socklen_t remoteAddrLen = sizeof(remoteAddr);
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AutoCloseFD remote = accept(fdSocket.get(),
(struct sockaddr *) &remoteAddr, &remoteAddrLen);
checkInterrupt();
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if (!remote) {
if (errno == EINTR) continue;
throw SysError("accepting connection");
}
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closeOnExec(remote.get());
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Recursive Nix support This allows Nix builders to call Nix to build derivations, with some limitations. Example: let nixpkgs = fetchTarball channel:nixos-18.03; in with import <nixpkgs> {}; runCommand "foo" { buildInputs = [ nix jq ]; NIX_PATH = "nixpkgs=${nixpkgs}"; } '' hello=$(nix-build -E '(import <nixpkgs> {}).hello.overrideDerivation (args: { name = "hello-3.5"; })') $hello/bin/hello mkdir -p $out/bin ln -s $hello/bin/hello $out/bin/hello nix path-info -r --json $hello | jq . '' This derivation makes a recursive Nix call to build GNU Hello and symlinks it from its $out, i.e. # ll ./result/bin/ lrwxrwxrwx 1 root root 63 Jan 1 1970 hello -> /nix/store/s0awxrs71gickhaqdwxl506hzccb30y5-hello-3.5/bin/hello # nix-store -qR ./result /nix/store/hwwqshlmazzjzj7yhrkyjydxamvvkfd3-glibc-2.26-131 /nix/store/s0awxrs71gickhaqdwxl506hzccb30y5-hello-3.5 /nix/store/sgmvvyw8vhfqdqb619bxkcpfn9lvd8ss-foo This is implemented as follows: * Before running the outer builder, Nix creates a Unix domain socket '.nix-socket' in the builder's temporary directory and sets $NIX_REMOTE to point to it. It starts a thread to process connections to this socket. (Thus you don't need to have nix-daemon running.) * The daemon thread uses a wrapper store (RestrictedStore) to keep track of paths added through recursive Nix calls, to implement some restrictions (see below), and to do some censorship (e.g. for purity, queryPathInfo() won't return impure information such as signatures and timestamps). * After the build finishes, the output paths are scanned for references to the paths added through recursive Nix calls (in addition to the inputs closure). Thus, in the example above, $out has a reference to $hello. The main restriction on recursive Nix calls is that they cannot do arbitrary substitutions. For example, doing nix-store -r /nix/store/kmwd1hq55akdb9sc7l3finr175dajlby-hello-2.10 is forbidden unless /nix/store/kmwd... is in the inputs closure or previously built by a recursive Nix call. This is to prevent irreproducible derivations that have hidden dependencies on substituters or the current store contents. Building a derivation is fine, however, and Nix will use substitutes if available. In other words, the builder has to present proof that it knows how to build a desired store path from scratch by constructing a derivation graph for that path. Probably we should also disallow instantiating/building fixed-output derivations (specifically, those that access the network, but currently we have no way to mark fixed-output derivations that don't access the network). Otherwise sandboxed derivations can bypass sandbox restrictions and access the network. When sandboxing is enabled, we make paths appear in the sandbox of the builder by entering the mount namespace of the builder and bind-mounting each path. This is tricky because we do a pivot_root() in the builder to change the root directory of its mount namespace, and thus the host /nix/store is not visible in the mount namespace of the builder. To get around this, just before doing pivot_root(), we branch a second mount namespace that shares its /nix/store mountpoint with the parent. Recursive Nix currently doesn't work on macOS in sandboxed mode (because we can't change the sandbox policy of a running build) and in non-root mode (because setns() barfs).
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TrustedFlag trusted = NotTrusted;
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PeerInfo peer = getPeerInfo(remote.get());
struct passwd * pw = peer.uidKnown ? getpwuid(peer.uid) : 0;
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string user = pw ? pw->pw_name : std::to_string(peer.uid);
struct group * gr = peer.gidKnown ? getgrgid(peer.gid) : 0;
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string group = gr ? gr->gr_name : std::to_string(peer.gid);
Strings trustedUsers = settings.trustedUsers;
Strings allowedUsers = settings.allowedUsers;
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if (matchUser(user, group, trustedUsers))
Recursive Nix support This allows Nix builders to call Nix to build derivations, with some limitations. Example: let nixpkgs = fetchTarball channel:nixos-18.03; in with import <nixpkgs> {}; runCommand "foo" { buildInputs = [ nix jq ]; NIX_PATH = "nixpkgs=${nixpkgs}"; } '' hello=$(nix-build -E '(import <nixpkgs> {}).hello.overrideDerivation (args: { name = "hello-3.5"; })') $hello/bin/hello mkdir -p $out/bin ln -s $hello/bin/hello $out/bin/hello nix path-info -r --json $hello | jq . '' This derivation makes a recursive Nix call to build GNU Hello and symlinks it from its $out, i.e. # ll ./result/bin/ lrwxrwxrwx 1 root root 63 Jan 1 1970 hello -> /nix/store/s0awxrs71gickhaqdwxl506hzccb30y5-hello-3.5/bin/hello # nix-store -qR ./result /nix/store/hwwqshlmazzjzj7yhrkyjydxamvvkfd3-glibc-2.26-131 /nix/store/s0awxrs71gickhaqdwxl506hzccb30y5-hello-3.5 /nix/store/sgmvvyw8vhfqdqb619bxkcpfn9lvd8ss-foo This is implemented as follows: * Before running the outer builder, Nix creates a Unix domain socket '.nix-socket' in the builder's temporary directory and sets $NIX_REMOTE to point to it. It starts a thread to process connections to this socket. (Thus you don't need to have nix-daemon running.) * The daemon thread uses a wrapper store (RestrictedStore) to keep track of paths added through recursive Nix calls, to implement some restrictions (see below), and to do some censorship (e.g. for purity, queryPathInfo() won't return impure information such as signatures and timestamps). * After the build finishes, the output paths are scanned for references to the paths added through recursive Nix calls (in addition to the inputs closure). Thus, in the example above, $out has a reference to $hello. The main restriction on recursive Nix calls is that they cannot do arbitrary substitutions. For example, doing nix-store -r /nix/store/kmwd1hq55akdb9sc7l3finr175dajlby-hello-2.10 is forbidden unless /nix/store/kmwd... is in the inputs closure or previously built by a recursive Nix call. This is to prevent irreproducible derivations that have hidden dependencies on substituters or the current store contents. Building a derivation is fine, however, and Nix will use substitutes if available. In other words, the builder has to present proof that it knows how to build a desired store path from scratch by constructing a derivation graph for that path. Probably we should also disallow instantiating/building fixed-output derivations (specifically, those that access the network, but currently we have no way to mark fixed-output derivations that don't access the network). Otherwise sandboxed derivations can bypass sandbox restrictions and access the network. When sandboxing is enabled, we make paths appear in the sandbox of the builder by entering the mount namespace of the builder and bind-mounting each path. This is tricky because we do a pivot_root() in the builder to change the root directory of its mount namespace, and thus the host /nix/store is not visible in the mount namespace of the builder. To get around this, just before doing pivot_root(), we branch a second mount namespace that shares its /nix/store mountpoint with the parent. Recursive Nix currently doesn't work on macOS in sandboxed mode (because we can't change the sandbox policy of a running build) and in non-root mode (because setns() barfs).
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trusted = Trusted;
if ((!trusted && !matchUser(user, group, allowedUsers)) || group == settings.buildUsersGroup)
throw Error("user '%1%' is not allowed to connect to the Nix daemon", user);
printInfo(format((string) "accepted connection from pid %1%, user %2%" + (trusted ? " (trusted)" : ""))
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% (peer.pidKnown ? std::to_string(peer.pid) : "<unknown>")
% (peer.uidKnown ? user : "<unknown>"));
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// Fork a child to handle the connection.
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ProcessOptions options;
options.errorPrefix = "unexpected Nix daemon error: ";
options.dieWithParent = false;
options.runExitHandlers = true;
options.allowVfork = false;
startProcess([&]() {
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fdSocket = -1;
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// Background the daemon.
if (setsid() == -1)
throw SysError("creating a new session");
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// Restore normal handling of SIGCHLD.
setSigChldAction(false);
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// For debugging, stuff the pid into argv[1].
if (peer.pidKnown && argv[1]) {
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string processName = std::to_string(peer.pid);
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strncpy(argv[1], processName.c_str(), strlen(argv[1]));
}
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// Handle the connection.
FdSource from(remote.get());
FdSink to(remote.get());
processConnection(openUncachedStore(), from, to, trusted, NotRecursive, [&](Store & store) {
#if 0
/* Prevent users from doing something very dangerous. */
if (geteuid() == 0 &&
querySetting("build-users-group", "") == "")
throw Error("if you run 'nix-daemon' as root, then you MUST set 'build-users-group'!");
#endif
store.createUser(user, peer.uid);
});
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exit(0);
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}, options);
} catch (Interrupted & e) {
return;
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} catch (Error & error) {
ErrorInfo ei = error.info();
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ei.hint = std::optional(hintfmt("error processing connection: %1%",
(error.info().hint.has_value() ? error.info().hint->str() : "")));
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logError(ei);
}
}
}
static int _main(int argc, char * * argv)
{
{
auto stdio = false;
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parseCmdLine(argc, argv, [&](Strings::iterator & arg, const Strings::iterator & end) {
if (*arg == "--daemon")
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; // ignored for backwards compatibility
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else if (*arg == "--help")
showManPage("nix-daemon");
else if (*arg == "--version")
printVersion("nix-daemon");
else if (*arg == "--stdio")
stdio = true;
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else return false;
return true;
});
initPlugins();
if (stdio) {
if (auto store = openUncachedStore().dynamic_pointer_cast<RemoteStore>()) {
auto conn = store->openConnectionWrapper();
int from = conn->from.fd;
int to = conn->to.fd;
auto nfds = std::max(from, to) + 1;
while (true) {
fd_set fds;
FD_ZERO(&fds);
FD_SET(from, &fds);
FD_SET(STDIN_FILENO, &fds);
if (select(nfds, &fds, nullptr, nullptr, nullptr) == -1)
throw SysError("waiting for data from client or server");
if (FD_ISSET(from, &fds)) {
auto res = splice(from, nullptr, STDOUT_FILENO, nullptr, SSIZE_MAX, SPLICE_F_MOVE);
if (res == -1)
throw SysError("splicing data from daemon socket to stdout");
else if (res == 0)
throw EndOfFile("unexpected EOF from daemon socket");
}
if (FD_ISSET(STDIN_FILENO, &fds)) {
auto res = splice(STDIN_FILENO, nullptr, to, nullptr, SSIZE_MAX, SPLICE_F_MOVE);
if (res == -1)
throw SysError("splicing data from stdin to daemon socket");
else if (res == 0)
return 0;
}
}
} else {
FdSource from(STDIN_FILENO);
FdSink to(STDOUT_FILENO);
/* Auth hook is empty because in this mode we blindly trust the
standard streams. Limitting access to thoses is explicitly
not `nix-daemon`'s responsibility. */
processConnection(openUncachedStore(), from, to, Trusted, NotRecursive, [&](Store & _){});
}
} else {
daemonLoop(argv);
}
return 0;
}
}
static RegisterLegacyCommand s1("nix-daemon", _main);