Add pure evaluation mode

In this mode, the following restrictions apply:

* The builtins currentTime, currentSystem and storePath throw an
  error.

* $NIX_PATH and -I are ignored.

* fetchGit and fetchMercurial require a revision hash.

* fetchurl and fetchTarball require a sha256 attribute.

* No file system access is allowed outside of the paths returned by
  fetch{Git,Mercurial,url,Tarball}. Thus 'nix build -f ./foo.nix' is
  not allowed.

Thus, the evaluation result is completely reproducible from the
command line arguments. E.g.

  nix build --pure-eval '(
    let
      nix = fetchGit { url = https://github.com/NixOS/nixpkgs.git; rev = "9c927de4b179a6dd210dd88d34bda8af4b575680"; };
      nixpkgs = fetchGit { url = https://github.com/NixOS/nixpkgs.git; ref = "release-17.09"; rev = "66b4de79e3841530e6d9c6baf98702aa1f7124e4"; };
    in (import (nix + "/release.nix") { inherit nix nixpkgs; }).build.x86_64-linux
  )'

The goal is to enable completely reproducible and traceable
evaluation. For example, a NixOS configuration could be fully
described by a single Git commit hash. 'nixos-rebuild' would do
something like

  nix build --pure-eval '(
    (import (fetchGit { url = file:///my-nixos-config; rev = "..."; })).system
  ')

where the Git repository /my-nixos-config would use further fetchGit
calls or Git externals to fetch Nixpkgs and whatever other
dependencies it has. Either way, the commit hash would uniquely
identify the NixOS configuration and allow it to reproduced.

5 files changed, 94 insertions, 44 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 087a95dde..f8685e010 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -300,16 +300,25 @@ EvalState::EvalState(const Strings & _searchPath, ref<Store> store)
 {
     countCalls = getEnv("NIX_COUNT_CALLS", "0") != "0";
 
-    restricted = settings.restrictEval;
-
     assert(gcInitialised);
 
     /* Initialise the Nix expression search path. */
-    Strings paths = parseNixPath(getEnv("NIX_PATH", ""));
-    for (auto & i : _searchPath) addToSearchPath(i);
-    for (auto & i : paths) addToSearchPath(i);
+    if (!settings.pureEval) {
+        Strings paths = parseNixPath(getEnv("NIX_PATH", ""));
+        for (auto & i : _searchPath) addToSearchPath(i);
+        for (auto & i : paths) addToSearchPath(i);
+    }
     addToSearchPath("nix=" + settings.nixDataDir + "/nix/corepkgs");
 
+    if (settings.restrictEval || settings.pureEval) {
+        allowedPaths = PathSet();
+        for (auto & i : searchPath) {
+            auto r = resolveSearchPathElem(i);
+            if (!r.first) continue;
+            allowedPaths->insert(r.second);
+        }
+    }
+
     clearValue(vEmptySet);
     vEmptySet.type = tAttrs;
     vEmptySet.attrs = allocBindings(0);
@@ -326,38 +335,39 @@ EvalState::~EvalState()
 
 Path EvalState::checkSourcePath(const Path & path_)
 {
-    if (!restricted) return path_;
+    if (!allowedPaths) return path_;
+
+    auto doThrow = [&]() [[noreturn]] {
+        throw RestrictedPathError("access to path '%1%' is forbidden in restricted mode", path_);
+    };
+
+    bool found = false;
+
+    for (auto & i : *allowedPaths) {
+        if (isDirOrInDir(path_, i)) {
+            found = true;
+            break;
+        }
+    }
+
+    if (!found) doThrow();
 
     /* Resolve symlinks. */
     debug(format("checking access to '%s'") % path_);
     Path path = canonPath(path_, true);
 
-    for (auto & i : searchPath) {
-        auto r = resolveSearchPathElem(i);
-        if (!r.first) continue;
-        if (path == r.second || isInDir(path, r.second))
+    for (auto & i : *allowedPaths) {
+        if (isDirOrInDir(path, i))
             return path;
     }
 
-    /* To support import-from-derivation, allow access to anything in
-       the store. FIXME: only allow access to paths that have been
-       constructed by this evaluation. */
-    if (store->isInStore(path)) return path;
-
-#if 0
-    /* Hack to support the chroot dependencies of corepkgs (see
-       corepkgs/config.nix.in). */
-    if (path == settings.nixPrefix && isStorePath(settings.nixPrefix))
-        return path;
-#endif
-
-    throw RestrictedPathError(format("access to path '%1%' is forbidden in restricted mode") % path_);
+    doThrow();
 }
 
 
 void EvalState::checkURI(const std::string & uri)
 {
-    if (!restricted) return;
+    if (!settings.restrictEval) return;
 
     /* 'uri' should be equal to a prefix, or in a subdirectory of a
        prefix. Thus, the prefix https://github.co does not permit
@@ -396,7 +406,7 @@ void EvalState::addConstant(const string & name, Value & v)
 }
 
 
-void EvalState::addPrimOp(const string & name,
+Value * EvalState::addPrimOp(const string & name,
     unsigned int arity, PrimOpFun primOp)
 {
     Value * v = allocValue();
@@ -407,6 +417,7 @@ void EvalState::addPrimOp(const string & name,
     staticBaseEnv.vars[symbols.create(name)] = baseEnvDispl;
     baseEnv.values[baseEnvDispl++] = v;
     baseEnv.values[0]->attrs->push_back(Attr(sym, v));
+    return v;
 }
 
 
@@ -659,8 +670,10 @@ Value * ExprPath::maybeThunk(EvalState & state, Env & env)
 }
 
 
-void EvalState::evalFile(const Path & path, Value & v)
+void EvalState::evalFile(const Path & path_, Value & v)
 {
+    auto path = checkSourcePath(path_);
+
     FileEvalCache::iterator i;
     if ((i = fileEvalCache.find(path)) != fileEvalCache.end()) {
         v = i->second;
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index cc971ae80..9e3d30d95 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -76,9 +76,9 @@ public:
        already exist there. */
     RepairFlag repair;
 
-    /* If set, don't allow access to files outside of the Nix search
-       path or to environment variables. */
-    bool restricted;
+    /* The allowed filesystem paths in restricted or pure evaluation
+       mode. */
+    std::experimental::optional<PathSet> allowedPaths;
 
     Value vEmptySet;
 
@@ -212,7 +212,7 @@ private:
 
     void addConstant(const string & name, Value & v);
 
-    void addPrimOp(const string & name,
+    Value * addPrimOp(const string & name,
         unsigned int arity, PrimOpFun primOp);
 
 public:
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 98fe2199e..0ec035b86 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -439,7 +439,7 @@ static void prim_tryEval(EvalState & state, const Pos & pos, Value * * args, Val
 static void prim_getEnv(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     string name = state.forceStringNoCtx(*args[0], pos);
-    mkString(v, state.restricted ? "" : getEnv(name));
+    mkString(v, settings.restrictEval || settings.pureEval ? "" : getEnv(name));
 }
 
 
@@ -1929,7 +1929,14 @@ void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
 
     state.checkURI(url);
 
+    if (settings.pureEval && !expectedHash)
+        throw Error("in pure evaluation mode, '%s' requires a 'sha256' argument", who);
+
     Path res = getDownloader()->downloadCached(state.store, url, unpack, name, expectedHash);
+
+    if (state.allowedPaths)
+        state.allowedPaths->insert(res);
+
     mkString(v, res, PathSet({res}));
 }
 
@@ -1981,11 +1988,28 @@ void EvalState::createBaseEnv()
     mkNull(v);
     addConstant("null", v);
 
-    mkInt(v, time(0));
-    addConstant("__currentTime", v);
+    auto vThrow = addPrimOp("throw", 1, prim_throw);
 
-    mkString(v, settings.thisSystem);
-    addConstant("__currentSystem", v);
+    auto addPurityError = [&](const std::string & name) {
+        Value * v2 = allocValue();
+        mkString(*v2, fmt("'%s' is not allowed in pure evaluation mode", name));
+        mkApp(v, *vThrow, *v2);
+        addConstant(name, v);
+    };
+
+    if (settings.pureEval)
+        addPurityError("__currentTime");
+    else {
+        mkInt(v, time(0));
+        addConstant("__currentTime", v);
+    }
+
+    if (settings.pureEval)
+        addPurityError("__currentSystem");
+    else {
+        mkString(v, settings.thisSystem);
+        addConstant("__currentSystem", v);
+    }
 
     mkString(v, nixVersion);
     addConstant("__nixVersion", v);
@@ -2001,10 +2025,10 @@ void EvalState::createBaseEnv()
     addConstant("__langVersion", v);
 
     // Miscellaneous
-    addPrimOp("scopedImport", 2, prim_scopedImport);
+    auto vScopedImport = addPrimOp("scopedImport", 2, prim_scopedImport);
     Value * v2 = allocValue();
     mkAttrs(*v2, 0);
-    mkApp(v, *baseEnv.values[baseEnvDispl - 1], *v2);
+    mkApp(v, *vScopedImport, *v2);
     forceValue(v);
     addConstant("import", v);
     if (settings.enableNativeCode) {
@@ -2020,7 +2044,6 @@ void EvalState::createBaseEnv()
     addPrimOp("__isBool", 1, prim_isBool);
     addPrimOp("__genericClosure", 1, prim_genericClosure);
     addPrimOp("abort", 1, prim_abort);
-    addPrimOp("throw", 1, prim_throw);
     addPrimOp("__addErrorContext", 2, prim_addErrorContext);
     addPrimOp("__tryEval", 1, prim_tryEval);
     addPrimOp("__getEnv", 1, prim_getEnv);
@@ -2035,7 +2058,10 @@ void EvalState::createBaseEnv()
 
     // Paths
     addPrimOp("__toPath", 1, prim_toPath);
-    addPrimOp("__storePath", 1, prim_storePath);
+    if (settings.pureEval)
+        addPurityError("__storePath");
+    else
+        addPrimOp("__storePath", 1, prim_storePath);
     addPrimOp("__pathExists", 1, prim_pathExists);
     addPrimOp("baseNameOf", 1, prim_baseNameOf);
     addPrimOp("dirOf", 1, prim_dirOf);
diff --git a/src/libexpr/primops/fetchGit.cc b/src/libexpr/primops/fetchGit.cc
index fb664cffb..2e3e2634d 100644
--- a/src/libexpr/primops/fetchGit.cc
+++ b/src/libexpr/primops/fetchGit.cc
@@ -22,10 +22,15 @@ struct GitInfo
     uint64_t revCount = 0;
 };
 
+std::regex revRegex("^[0-9a-fA-F]{40}$");
+
 GitInfo exportGit(ref<Store> store, const std::string & uri,
     std::experimental::optional<std::string> ref, std::string rev,
     const std::string & name)
 {
+    if (settings.pureEval && rev == "")
+        throw Error("in pure evaluation mode, 'fetchGit' requires a Git revision");
+
     if (!ref && rev == "" && hasPrefix(uri, "/") && pathExists(uri + "/.git")) {
 
         bool clean = true;
@@ -76,11 +81,8 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
 
     if (!ref) ref = "master"s;
 
-    if (rev != "") {
-        std::regex revRegex("^[0-9a-fA-F]{40}$");
-        if (!std::regex_match(rev, revRegex))
-            throw Error("invalid Git revision '%s'", rev);
-    }
+    if (rev != "" && !std::regex_match(rev, revRegex))
+        throw Error("invalid Git revision '%s'", rev);
 
     Path cacheDir = getCacheDir() + "/nix/git";
 
@@ -231,6 +233,9 @@ static void prim_fetchGit(EvalState & state, const Pos & pos, Value * * args, Va
     mkString(*state.allocAttr(v, state.symbols.create("shortRev")), gitInfo.shortRev);
     mkInt(*state.allocAttr(v, state.symbols.create("revCount")), gitInfo.revCount);
     v.attrs->sort();
+
+    if (state.allowedPaths)
+        state.allowedPaths->insert(gitInfo.storePath);
 }
 
 static RegisterPrimOp r("fetchGit", 1, prim_fetchGit);
diff --git a/src/libexpr/primops/fetchMercurial.cc b/src/libexpr/primops/fetchMercurial.cc
index a317476c5..5517d83df 100644
--- a/src/libexpr/primops/fetchMercurial.cc
+++ b/src/libexpr/primops/fetchMercurial.cc
@@ -27,6 +27,9 @@ std::regex commitHashRegex("^[0-9a-fA-F]{40}$");
 HgInfo exportMercurial(ref<Store> store, const std::string & uri,
     std::string rev, const std::string & name)
 {
+    if (settings.pureEval && rev == "")
+        throw Error("in pure evaluation mode, 'fetchMercurial' requires a Mercurial revision");
+
     if (rev == "" && hasPrefix(uri, "/") && pathExists(uri + "/.hg")) {
 
         bool clean = runProgram("hg", true, { "status", "-R", uri, "--modified", "--added", "--removed" }) == "";
@@ -196,6 +199,9 @@ static void prim_fetchMercurial(EvalState & state, const Pos & pos, Value * * ar
     mkString(*state.allocAttr(v, state.symbols.create("shortRev")), std::string(hgInfo.rev, 0, 12));
     mkInt(*state.allocAttr(v, state.symbols.create("revCount")), hgInfo.revCount);
     v.attrs->sort();
+
+    if (state.allowedPaths)
+        state.allowedPaths->insert(hgInfo.storePath);
 }
 
 static RegisterPrimOp r("fetchMercurial", 1, prim_fetchMercurial);