Use SourcePath for reading flake.{nix,lock}

Flakes still reside in the Nix store (so there shouldn't be any change
in behaviour), but they are now accessed via the rootFS
accessor. Since rootFS implements access checks, we no longer have to
worry about flake.{nix,lock} or their parents being symlinks that
escape from the flake.

Extracted from the lazy-trees branch.

src/libcmd/installables.cc

@@ -21,6 +21,7 @@
 #include "url.hh"
 #include "registry.hh"
 #include "build-result.hh"
+#include "fs-input-accessor.hh"
 
 #include <regex>
 #include <queue>
@@ -146,7 +147,7 @@ MixFlakeOptions::MixFlakeOptions()
         .category = category,
         .labels = {"flake-lock-path"},
         .handler = {[&](std::string lockFilePath) {
-            lockFlags.referenceLockFilePath = lockFilePath;
+            lockFlags.referenceLockFilePath = getUnfilteredRootPath(CanonPath(absPath(lockFilePath)));
         }},
         .completer = completePath
     });

src/libexpr/flake/flake.cc

@@ -202,43 +202,28 @@ static std::map<FlakeId, FlakeInput> parseFlakeInputs(
     return inputs;
 }
 
-static Flake getFlake(
+static Flake readFlake(
     EvalState & state,
     const FlakeRef & originalRef,
-    bool allowLookup,
+    const FlakeRef & resolvedRef,
-    FlakeCache & flakeCache,
+    const FlakeRef & lockedRef,
-    InputPath lockRootPath)
+    const SourcePath & rootDir,
+    const InputPath & lockRootPath)
 {
-    auto [storePath, resolvedRef, lockedRef] = fetchOrSubstituteTree(
+    auto flakePath = rootDir / CanonPath(resolvedRef.subdir) / "flake.nix";
-        state, originalRef, allowLookup, flakeCache);
 
-    // We need to guard against symlink attacks, but before we start doing
+    Value vInfo;
-    // filesystem operations we should make sure there's a flake.nix in the
+    state.evalFile(flakePath, vInfo, true);
-    // first place.
-    auto unsafeFlakeDir = state.store->toRealPath(storePath) + "/" + lockedRef.subdir;
-    auto unsafeFlakeFile = unsafeFlakeDir + "/flake.nix";
-    if (!pathExists(unsafeFlakeFile))
-        throw Error("source tree referenced by '%s' does not contain a '%s/flake.nix' file", lockedRef, lockedRef.subdir);
 
-    // Guard against symlink attacks.
+    expectType(state, nAttrs, vInfo, state.positions.add(Pos::Origin(rootDir), 1, 1));
-    auto flakeDir = canonPath(unsafeFlakeDir, true);
-    auto flakeFile = canonPath(flakeDir + "/flake.nix", true);
-    if (!isInDir(flakeFile, state.store->toRealPath(storePath)))
-        throw Error("'flake.nix' file of flake '%s' escapes from '%s'",
-            lockedRef, state.store->printStorePath(storePath));
 
     Flake flake {
         .originalRef = originalRef,
         .resolvedRef = resolvedRef,
         .lockedRef = lockedRef,
-        .storePath = storePath,
+        .path = flakePath,
     };
 
-    Value vInfo;
-    state.evalFile(state.rootPath(CanonPath(flakeFile)), vInfo, true); // FIXME: symlink attack
-
-    expectType(state, nAttrs, vInfo, state.positions.add({state.rootPath(CanonPath(flakeFile))}, 1, 1));
-
     if (auto description = vInfo.attrs->get(state.sDescription)) {
         expectType(state, nString, *description->value, description->pos);
         flake.description = description->value->c_str();
@@ -247,7 +232,7 @@ static Flake getFlake(
     auto sInputs = state.symbols.create("inputs");
 
     if (auto inputs = vInfo.attrs->get(sInputs))
-        flake.inputs = parseFlakeInputs(state, inputs->value, inputs->pos, flakeDir, lockRootPath);
+        flake.inputs = parseFlakeInputs(state, inputs->value, inputs->pos, flakePath.parent().path.abs(), lockRootPath); // FIXME
 
     auto sOutputs = state.symbols.create("outputs");
 
@@ -264,7 +249,7 @@ static Flake getFlake(
         }
 
     } else
-        throw Error("flake '%s' lacks attribute 'outputs'", lockedRef);
+        throw Error("flake '%s' lacks attribute 'outputs'", resolvedRef);
 
     auto sNixConfig = state.symbols.create("nixConfig");
 
@@ -313,12 +298,25 @@ static Flake getFlake(
             attr.name != sOutputs &&
             attr.name != sNixConfig)
             throw Error("flake '%s' has an unsupported attribute '%s', at %s",
-                lockedRef, state.symbols[attr.name], state.positions[attr.pos]);
+                resolvedRef, state.symbols[attr.name], state.positions[attr.pos]);
     }
 
     return flake;
 }
 
+static Flake getFlake(
+    EvalState & state,
+    const FlakeRef & originalRef,
+    bool allowLookup,
+    FlakeCache & flakeCache,
+    InputPath lockRootPath)
+{
+    auto [storePath, resolvedRef, lockedRef] = fetchOrSubstituteTree(
+        state, originalRef, allowLookup, flakeCache);
+
+    return readFlake(state, originalRef, resolvedRef, lockedRef, state.rootPath(state.store->toRealPath(storePath)), lockRootPath);
+}
+
 Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup, FlakeCache & flakeCache)
 {
     return getFlake(state, originalRef, allowLookup, flakeCache, {});
@@ -330,6 +328,13 @@ Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup
     return getFlake(state, originalRef, allowLookup, flakeCache);
 }
 
+static LockFile readLockFile(const SourcePath & lockFilePath)
+{
+    return lockFilePath.pathExists()
+        ? LockFile(lockFilePath.readFile(), fmt("%s", lockFilePath))
+        : LockFile();
+}
+
 /* Compute an in-memory lock file for the specified top-level flake,
    and optionally write it to file, if the flake is writable. */
 LockedFlake lockFlake(
@@ -355,17 +360,16 @@ LockedFlake lockFlake(
             throw Error("reference lock file was provided, but the `allow-dirty` setting is set to false");
         }
 
-        // FIXME: symlink attack
+        auto oldLockFile = readLockFile(
-        auto oldLockFile = LockFile::read(
             lockFlags.referenceLockFilePath.value_or(
-                state.store->toRealPath(flake.storePath) + "/" + flake.lockedRef.subdir + "/flake.lock"));
+                flake.lockFilePath()));
 
         debug("old lock file: %s", oldLockFile);
 
         std::map<InputPath, FlakeInput> overrides;
         std::set<InputPath> explicitCliOverrides;
         std::set<InputPath> overridesUsed, updatesUsed;
-        std::map<ref<Node>, StorePath> nodePaths;
+        std::map<ref<Node>, SourcePath> nodePaths;
 
         for (auto & i : lockFlags.inputOverrides) {
             overrides.insert_or_assign(i.first, FlakeInput { .ref = i.second });
@@ -538,7 +542,7 @@ LockedFlake lockFlake(
 
                         if (mustRefetch) {
                             auto inputFlake = getFlake(state, oldLock->lockedRef, false, flakeCache, inputPath);
-                            nodePaths.emplace(childNode, inputFlake.storePath);
+                            nodePaths.emplace(childNode, inputFlake.path.parent());
                             computeLocks(inputFlake.inputs, childNode, inputPath, oldLock, lockRootPath, parentPath, false);
                         } else {
                             computeLocks(fakeInputs, childNode, inputPath, oldLock, lockRootPath, parentPath, true);
@@ -587,13 +591,12 @@ LockedFlake lockFlake(
                                flake. Also, unless we already have this flake
                                in the top-level lock file, use this flake's
                                own lock file. */
-                            nodePaths.emplace(childNode, inputFlake.storePath);
+                            nodePaths.emplace(childNode, inputFlake.path.parent());
                             computeLocks(
                                 inputFlake.inputs, childNode, inputPath,
                                 oldLock
                                 ? std::dynamic_pointer_cast<const Node>(oldLock)
-                                : LockFile::read(
+                                : readLockFile(inputFlake.lockFilePath()).root.get_ptr(),
-                                    state.store->toRealPath(inputFlake.storePath) + "/" + inputFlake.lockedRef.subdir + "/flake.lock").root.get_ptr(),
                                 oldLock ? lockRootPath : inputPath,
                                 localPath,
                                 false);
@@ -605,7 +608,7 @@ LockedFlake lockFlake(
 
                             auto childNode = make_ref<LockedNode>(lockedRef, ref, false);
 
-                            nodePaths.emplace(childNode, storePath);
+                            nodePaths.emplace(childNode, state.rootPath(state.store->toRealPath(storePath)));
 
                             node->inputs.insert_or_assign(id, childNode);
                         }
@@ -619,9 +622,9 @@ LockedFlake lockFlake(
         };
 
         // Bring in the current ref for relative path resolution if we have it
-        auto parentPath = canonPath(state.store->toRealPath(flake.storePath) + "/" + flake.lockedRef.subdir, true);
+        auto parentPath = flake.path.parent().path.abs();
 
-        nodePaths.emplace(newLockFile.root, flake.storePath);
+        nodePaths.emplace(newLockFile.root, flake.path.parent());
 
         computeLocks(
             flake.inputs,
@@ -746,13 +749,15 @@ void callFlake(EvalState & state,
 
     auto overrides = state.buildBindings(lockedFlake.nodePaths.size());
 
-    for (auto & [node, storePath] : lockedFlake.nodePaths) {
+    for (auto & [node, sourcePath] : lockedFlake.nodePaths) {
         auto override = state.buildBindings(2);
 
         auto & vSourceInfo = override.alloc(state.symbols.create("sourceInfo"));
 
         auto lockedNode = node.dynamic_pointer_cast<const LockedNode>();
 
+        auto [storePath, subdir] = state.store->toStorePath(sourcePath.path.abs());
+
         emitTreeAttrs(
             state,
             storePath,
@@ -766,7 +771,7 @@ void callFlake(EvalState & state,
 
         override
             .alloc(state.symbols.create("dir"))
-            .mkString(lockedNode ? lockedNode->lockedRef.subdir : lockedFlake.flake.lockedRef.subdir);
+            .mkString(CanonPath(subdir).rel());
 
         overrides.alloc(state.symbols.create(key->second)).mkAttrs(override);
     }
@@ -928,7 +933,7 @@ Fingerprint LockedFlake::getFingerprint() const
     // flake.sourceInfo.storePath for the fingerprint.
     return hashString(HashAlgorithm::SHA256,
         fmt("%s;%s;%d;%d;%s",
-            flake.storePath.to_string(),
+            flake.path.to_string(),
             flake.lockedRef.subdir,
             flake.lockedRef.input.getRevCount().value_or(0),
             flake.lockedRef.input.getLastModified().value_or(0),

src/libexpr/flake/flake.hh

@@ -77,18 +77,27 @@ struct Flake
      * the specific local store result of invoking the fetcher
      */
     FlakeRef lockedRef;
+    /**
+     * The path of `flake.nix`.
+     */
+    SourcePath path;
     /**
      * pretend that 'lockedRef' is dirty
      */
     bool forceDirty = false;
     std::optional<std::string> description;
-    StorePath storePath;
     FlakeInputs inputs;
     /**
      * 'nixConfig' attribute
      */
     ConfigFile config;
+
     ~Flake();
+
+    SourcePath lockFilePath()
+    {
+        return path.parent() / "flake.lock";
+    }
 };
 
 Flake getFlake(EvalState & state, const FlakeRef & flakeRef, bool allowLookup);
@@ -104,11 +113,11 @@ struct LockedFlake
     LockFile lockFile;
 
     /**
-     * Store paths of nodes that have been fetched in
+     * Source tree accessors for nodes that have been fetched in
      * lockFlake(); in particular, the root node and the overriden
      * inputs.
      */
-    std::map<ref<Node>, StorePath> nodePaths;
+    std::map<ref<Node>, SourcePath> nodePaths;
 
     Fingerprint getFingerprint() const;
 };
@@ -165,7 +174,7 @@ struct LockFlags
     /**
      * The path to a lock file to read instead of the `flake.lock` file in the top-level flake
      */
-    std::optional<std::string> referenceLockFilePath;
+    std::optional<SourcePath> referenceLockFilePath;
 
     /**
      * The path to a lock file to write to instead of the `flake.lock` file in the top-level flake

src/libexpr/flake/lockfile.cc

@@ -84,8 +84,10 @@ std::shared_ptr<Node> LockFile::findInput(const InputPath & path)
     return doFind(root, path, visited);
 }
 
-LockFile::LockFile(const nlohmann::json & json, const Path & path)
+LockFile::LockFile(std::string_view contents, std::string_view path)
 {
+    auto json = nlohmann::json::parse(contents);
+
     auto version = json.value("version", 0);
     if (version < 5 || version > 7)
         throw Error("lock file '%s' has unsupported version %d", path, version);
@@ -203,12 +205,6 @@ std::pair<std::string, LockFile::KeyMap> LockFile::to_string() const
     return {json.dump(2), std::move(nodeKeys)};
 }
 
-LockFile LockFile::read(const Path & path)
-{
-    if (!pathExists(path)) return LockFile();
-    return LockFile(nlohmann::json::parse(readFile(path)), path);
-}
-
 std::ostream & operator <<(std::ostream & stream, const LockFile & lockFile)
 {
     stream << lockFile.toJSON().first.dump(2);

src/libexpr/flake/lockfile.hh

@@ -55,7 +55,7 @@ struct LockFile
     ref<Node> root = make_ref<Node>();
 
     LockFile() {};
-    LockFile(const nlohmann::json & json, const Path & path);
+    LockFile(std::string_view contents, std::string_view path);
 
     typedef std::map<ref<const Node>, std::string> KeyMap;
 
@@ -63,8 +63,6 @@ struct LockFile
 
     std::pair<std::string, KeyMap> to_string() const;
 
-    static LockFile read(const Path & path);
-
     /**
      * Check whether this lock file has any unlocked inputs. If so,
      * return one.

src/nix/flake.cc

@@ -205,6 +205,9 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
         auto lockedFlake = lockFlake();
         auto & flake = lockedFlake.flake;
 
+        // Currently, all flakes are in the Nix store via the rootFS accessor.
+        auto storePath = store->printStorePath(store->toStorePath(flake.path.path.abs()).first);
+
         if (json) {
             nlohmann::json j;
             if (flake.description)
@@ -223,7 +226,7 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
                 j["revCount"] = *revCount;
             if (auto lastModified = flake.lockedRef.input.getLastModified())
                 j["lastModified"] = *lastModified;
-            j["path"] = store->printStorePath(flake.storePath);
+            j["path"] = storePath;
             j["locks"] = lockedFlake.lockFile.toJSON().first;
             logger->cout("%s", j.dump());
         } else {
@@ -239,7 +242,7 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
                     *flake.description);
             logger->cout(
                 ANSI_BOLD "Path:" ANSI_NORMAL "          %s",
-                store->printStorePath(flake.storePath));
+                storePath);
             if (auto rev = flake.lockedRef.input.getRev())
                 logger->cout(
                     ANSI_BOLD "Revision:" ANSI_NORMAL "      %s",
@@ -1031,7 +1034,9 @@ struct CmdFlakeArchive : FlakeCommand, MixJSON, MixDryRun
 
         StorePathSet sources;
 
-        sources.insert(flake.flake.storePath);
+        auto storePath = store->toStorePath(flake.flake.path.path.abs()).first;
+
+        sources.insert(storePath);
 
         // FIXME: use graph output, handle cycles.
         std::function<nlohmann::json(const Node & node)> traverse;
@@ -1060,7 +1065,7 @@ struct CmdFlakeArchive : FlakeCommand, MixJSON, MixDryRun
 
         if (json) {
             nlohmann::json jsonRoot = {
-                {"path", store->printStorePath(flake.flake.storePath)},
+                {"path", store->printStorePath(storePath)},
                 {"inputs", traverse(*flake.lockFile.root)},
             };
             logger->cout("%s", jsonRoot);