4 files changed, 23 insertions, 13 deletions

diff --git a/src/libexpr/eval-settings.cc b/src/libexpr/eval-settings.cc
index 105fd3e9d..0bdf1b9a5 100644
--- a/src/libexpr/eval-settings.cc
+++ b/src/libexpr/eval-settings.cc
@@ -63,11 +63,9 @@ Strings EvalSettings::getDefaultNixPath()
         }
     };
 
-    if (!evalSettings.restrictEval && !evalSettings.pureEval) {
-        add(getNixDefExpr() + "/channels");
-        add(rootChannelsDir() + "/nixpkgs", "nixpkgs");
-        add(rootChannelsDir());
-    }
+    add(getNixDefExpr() + "/channels");
+    add(rootChannelsDir() + "/nixpkgs", "nixpkgs");
+    add(rootChannelsDir());
 
     return res;
 }
diff --git a/src/libexpr/eval-settings.hh b/src/libexpr/eval-settings.hh
index cd73d195f..4673c509b 100644
--- a/src/libexpr/eval-settings.hh
+++ b/src/libexpr/eval-settings.hh
@@ -75,8 +75,17 @@ struct EvalSettings : Config
         R"(
           Pure evaluation mode ensures that the result of Nix expressions is fully determined by explicitly declared inputs, and not influenced by external state:
 
-          - Restrict file system and network access to files specified by cryptographic hash
-          - Disable [`builtins.currentSystem`](@docroot@/language/builtin-constants.md#builtins-currentSystem) and [`builtins.currentTime`](@docroot@/language/builtin-constants.md#builtins-currentTime)
+          - File system and network access is restricted to accesses to immutable data only:
+            - Path literals relative to the home directory like `~/lix` are rejected at parse time.
+            - Access to absolute paths that did not result from Nix language evaluation is rejected when such paths are given as parameters to builtins like, for example, [`builtins.readFile`](@docroot@/language/builtins.md#builtins-readFile).
+
+              Access is nonetheless allowed to (absolute) paths in the Nix store that are returned by builtins like [`builtins.filterSource`](@docroot@/language/builtins.md#builtins-filterSource), [`builtins.fetchTarball`](@docroot@/language/builtins.md#builtins-fetchTarball) and similar.
+            - Impure fetches such as not specifying a commit ID for `builtins.fetchGit` or not specifying a hash for `builtins.fetchTarball` are rejected.
+            - In flakes, access to relative paths outside of the root of the flake's source tree (often, a git repository) is rejected.
+          - The evaluator ignores `NIX_PATH`, `-I` and the `nix-path` setting. Thus, [`builtins.nixPath`](@docroot@/language/builtin-constants.md#builtins-nixPath) is an empty list.
+          - The builtins [`builtins.currentSystem`](@docroot@/language/builtin-constants.md#builtins-currentSystem) and [`builtins.currentTime`](@docroot@/language/builtin-constants.md#builtins-currentTime) are absent from `builtins`.
+          - [`builtins.getEnv`](@docroot@/language/builtin-constants.md#builtins-currentSystem) always returns empty string for any variable.
+          - [`builtins.storePath`](@docroot@/language/builtins.md#builtins-storePath) throws an error (Lix may change this, tracking issue: <https://git.lix.systems/lix-project/lix/issues/402>)
         )"
         };
 
@@ -98,6 +107,7 @@ struct EvalSettings : Config
           allowed to access `https://github.com/NixOS/patchelf.git`.
         )"};
 
+
     Setting<bool> traceFunctionCalls{this, false, "trace-function-calls",
         R"(
           If set to `true`, the Nix evaluator will trace every function call.
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 25d98b23b..4b3593f09 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -418,7 +418,7 @@ EvalState::EvalState(
     }
 
     if (evalSettings.restrictEval || evalSettings.pureEval) {
-        allowedPaths = PathSet();
+        allowedPaths = std::optional(PathSet());
 
         for (auto & i : searchPath.elements) {
             auto r = resolveSearchPathPath(i.path);
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 4b7e61dfe..20851da70 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -923,14 +923,15 @@ static RegisterPrimOp primop_getEnv({
     .args = {"s"},
     .doc = R"(
       `getEnv` returns the value of the environment variable *s*, or an
-      empty string if the variable doesn’t exist. This function should be
+      empty string if the variable doesn't exist. This function should be
       used with care, as it can introduce all sorts of nasty environment
       dependencies in your Nix expression.
 
-      `getEnv` is used in Nix Packages to locate the file
-      `~/.nixpkgs/config.nix`, which contains user-local settings for Nix
-      Packages. (That is, it does a `getEnv "HOME"` to locate the user’s
-      home directory.)
+      `getEnv` is used in nixpkgs for evil impurities such as locating the file
+      `~/.config/nixpkgs/config.nix` which contains user-local settings for nixpkgs.
+      (That is, it does a `getEnv "HOME"` to locate the user's home directory.)
+
+      When in [pure evaluation mode](@docroot@/command-ref/conf-file.md#conf-pure-eval), this function always returns an empty string.
     )",
     .fun = prim_getEnv,
 });
@@ -1506,6 +1507,7 @@ static RegisterPrimOp primop_storePath({
       in a new path (e.g. `/nix/store/ld01dnzc…-source-source`).
 
       Not available in [pure evaluation mode](@docroot@/command-ref/conf-file.md#conf-pure-eval).
+      Lix may change this, tracking issue: <https://git.lix.systems/lix-project/lix/issues/402>
 
       See also [`builtins.fetchClosure`](#builtins-fetchClosure).
     )",