nixos/stub-ld: init module

lib/systems/default.nix

@@ -89,6 +89,13 @@ rec {
         # is why we use the more obscure "bfd" and not "binutils" for this
         # choice.
         else                                     "bfd";
+      # The standard lib directory name that non-nixpkgs binaries distributed
+      # for this platform normally assume.
+      libDir = if final.isLinux then
+        if final.isx86_64 || final.isMips64 || final.isPower64
+        then "lib64"
+        else "lib"
+      else null;
       extensions = lib.optionalAttrs final.hasSharedLibraries {
         sharedLibrary =
           if      final.isDarwin  then ".dylib"

nixos/doc/manual/release-notes/rl-2405.section.md

@@ -10,6 +10,10 @@ In addition to numerous new and upgraded packages, this release has the followin
 
 - `screen`'s module has been cleaned, and will now require you to set `programs.screen.enable` in order to populate `screenrc` and add the program to the environment.
 
+- NixOS now installs a stub ELF loader that prints an informative error message when users attempt to run binaries not made for NixOS.
+   - This can be disabled through the `environment.stub-ld.enable` option.
+   - If you use `programs.nix-ld.enable`, no changes are needed. The stub will be disabled automatically.
+
 ## New Services {#sec-release-24.05-new-services}
 
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->

nixos/modules/config/ldso.nix

@@ -0,0 +1,58 @@
+{ config, lib, pkgs, ... }:
+
+let
+  inherit (lib) last splitString mkOption types mdDoc optionals;
+
+  libDir = pkgs.stdenv.hostPlatform.libDir;
+  ldsoBasename = last (splitString "/" pkgs.stdenv.cc.bintools.dynamicLinker);
+
+  pkgs32 = pkgs.pkgsi686Linux;
+  libDir32 = pkgs32.stdenv.hostPlatform.libDir;
+  ldsoBasename32 = last (splitString "/" pkgs32.stdenv.cc.bintools.dynamicLinker);
+in {
+  options = {
+    environment.ldso = mkOption {
+      type = types.nullOr types.path;
+      default = null;
+      description = mdDoc ''
+        The executable to link into the normal FHS location of the ELF loader.
+      '';
+    };
+
+    environment.ldso32 = mkOption {
+      type = types.nullOr types.path;
+      default = null;
+      description = mdDoc ''
+        The executable to link into the normal FHS location of the 32-bit ELF loader.
+
+        This currently only works on x86_64 architectures.
+      '';
+    };
+  };
+
+  config = {
+    assertions = [
+      { assertion = isNull config.environment.ldso32 || pkgs.stdenv.isx86_64;
+        message = "Option environment.ldso32 currently only works on x86_64.";
+      }
+    ];
+
+    systemd.tmpfiles.rules = (
+      if isNull config.environment.ldso then [
+        "r /${libDir}/${ldsoBasename} - - - - -"
+      ] else [
+        "d /${libDir} 0755 root root - -"
+        "L+ /${libDir}/${ldsoBasename} - - - - ${config.environment.ldso}"
+      ]
+    ) ++ optionals pkgs.stdenv.isx86_64 (
+      if isNull config.environment.ldso32 then [
+        "r /${libDir32}/${ldsoBasename32} - - - - -"
+      ] else [
+        "d /${libDir32} 0755 root root - -"
+        "L+ /${libDir32}/${ldsoBasename32} - - - - ${config.environment.ldso32}"
+      ]
+    );
+  };
+
+  meta.maintainers = with lib.maintainers; [ tejing ];
+}

nixos/modules/config/stub-ld.nix

@@ -0,0 +1,56 @@
+{ config, lib, pkgs, ... }:
+
+let
+  inherit (lib) optionalString mkOption types mdDoc mkIf mkDefault;
+
+  cfg = config.environment.stub-ld;
+
+  message = ''
+    NixOS cannot run dynamically linked executables intended for generic
+    linux environments out of the box. For more information, see:
+    https://nix.dev/permalink/stub-ld
+  '';
+
+  stub-ld-for = pkgsArg: messageArg: pkgsArg.pkgsStatic.runCommandCC "stub-ld" {
+    nativeBuildInputs = [ pkgsArg.unixtools.xxd ];
+    inherit messageArg;
+  } ''
+    printf "%s" "$messageArg" | xxd -i -n message >main.c
+    cat <<EOF >>main.c
+    #include <stdio.h>
+    int main(int argc, char * argv[]) {
+      fprintf(stderr, "Could not start dynamically linked executable: %s\n", argv[0]);
+      fwrite(message, sizeof(unsigned char), message_len, stderr);
+      return 127; // matches behavior of bash and zsh without a loader. fish uses 139
+    }
+    EOF
+    $CC -Os main.c -o $out
+  '';
+
+  pkgs32 = pkgs.pkgsi686Linux;
+
+  stub-ld = stub-ld-for pkgs message;
+  stub-ld32 = stub-ld-for pkgs32 message;
+in {
+  options = {
+    environment.stub-ld = {
+      enable = mkOption {
+        type = types.bool;
+        default = true;
+        example = false;
+        description = mdDoc ''
+          Install a stub ELF loader to print an informative error message
+          in the event that a user attempts to run an ELF binary not
+          compiled for NixOS.
+        '';
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    environment.ldso = mkDefault stub-ld;
+    environment.ldso32 = mkIf pkgs.stdenv.isx86_64 (mkDefault stub-ld32);
+  };
+
+  meta.maintainers = with lib.maintainers; [ tejing ];
+}

nixos/modules/module-list.nix

@@ -12,6 +12,7 @@
   ./config/iproute2.nix
   ./config/krb5/default.nix
   ./config/ldap.nix
+  ./config/ldso.nix
   ./config/locale.nix
   ./config/malloc.nix
   ./config/mysql.nix
@@ -28,6 +29,7 @@
   ./config/resolvconf.nix
   ./config/shells-environment.nix
   ./config/stevenblack.nix
+  ./config/stub-ld.nix
   ./config/swap.nix
   ./config/sysctl.nix
   ./config/system-environment.nix

nixos/modules/profiles/minimal.nix

@@ -21,6 +21,8 @@ with lib;
   # Perl is a default package.
   environment.defaultPackages = mkDefault [ ];
 
+  environment.stub-ld.enable = false;
+
   # The lessopen package pulls in Perl.
   programs.less.lessopen = mkDefault null;
 

nixos/modules/programs/nix-ld.nix

@@ -47,7 +47,7 @@ in
   };
 
   config = lib.mkIf config.programs.nix-ld.enable {
-    systemd.tmpfiles.packages = [ cfg.package ];
+    environment.ldso = "${cfg.package}/libexec/nix-ld";
 
     environment.systemPackages = [ nix-ld-libraries ];
 

nixos/tests/all-tests.nix

@@ -787,6 +787,7 @@ in {
   step-ca = handleTestOn ["x86_64-linux"] ./step-ca.nix {};
   stratis = handleTest ./stratis {};
   strongswan-swanctl = handleTest ./strongswan-swanctl.nix {};
+  stub-ld = handleTestOn [ "x86_64-linux" "aarch64-linux" ] ./stub-ld.nix {};
   stunnel = handleTest ./stunnel.nix {};
   sudo = handleTest ./sudo.nix {};
   sudo-rs = handleTest ./sudo-rs.nix {};

nixos/tests/stub-ld.nix

@@ -0,0 +1,73 @@
+import ./make-test-python.nix ({ lib, pkgs, ... }: {
+  name = "stub-ld";
+
+  nodes.machine = { lib, ... }:
+    {
+      environment.stub-ld.enable = true;
+
+      specialisation.nostub = {
+        inheritParentConfig = true;
+
+        configuration = { ... }: {
+          environment.stub-ld.enable = lib.mkForce false;
+        };
+      };
+    };
+
+  testScript = let
+    libDir = pkgs.stdenv.hostPlatform.libDir;
+    ldsoBasename = lib.last (lib.splitString "/" pkgs.stdenv.cc.bintools.dynamicLinker);
+
+    check32 = pkgs.stdenv.isx86_64;
+    pkgs32 = pkgs.pkgsi686Linux;
+
+    libDir32 = pkgs32.stdenv.hostPlatform.libDir;
+    ldsoBasename32 = lib.last (lib.splitString "/" pkgs32.stdenv.cc.bintools.dynamicLinker);
+
+    test-exec = builtins.mapAttrs (n: v: pkgs.runCommand "test-exec-${n}" { src = pkgs.fetchurl v; } "mkdir -p $out;cd $out;tar -xzf $src") {
+      x86_64-linux.url = "https://github.com/rustic-rs/rustic/releases/download/v0.6.1/rustic-v0.6.1-x86_64-unknown-linux-gnu.tar.gz";
+      x86_64-linux.hash = "sha256-3zySzx8MKFprMOi++yr2ZGASE0aRfXHQuG3SN+kWUCI=";
+      i686-linux.url = "https://github.com/rustic-rs/rustic/releases/download/v0.6.1/rustic-v0.6.1-i686-unknown-linux-gnu.tar.gz";
+      i686-linux.hash = "sha256-fWNiATFeg0B2pfB5zndlnzGn7Ztl8diVS1rFLEDnSLU=";
+      aarch64-linux.url = "https://github.com/rustic-rs/rustic/releases/download/v0.6.1/rustic-v0.6.1-aarch64-unknown-linux-gnu.tar.gz";
+      aarch64-linux.hash = "sha256-hnldbd2cctQIAhIKoEZLIWY8H3jiFBClkNy2UlyyvAs=";
+    };
+    exec-name = "rustic";
+
+    if32 = pythonStatement: if check32 then pythonStatement else "pass";
+  in
+    ''
+      machine.start()
+      machine.wait_for_unit("multi-user.target")
+
+      with subtest("Check for stub (enabled, initial)"):
+          machine.succeed('test -L /${libDir}/${ldsoBasename}')
+          ${if32 "machine.succeed('test -L /${libDir32}/${ldsoBasename32}')"}
+
+      with subtest("Try FHS executable"):
+          machine.copy_from_host('${test-exec.${pkgs.system}}','test-exec')
+          machine.succeed('if test-exec/${exec-name} 2>outfile; then false; else [ $? -eq 127 ];fi')
+          machine.succeed('grep -qi nixos outfile')
+          ${if32 "machine.copy_from_host('${test-exec.${pkgs32.system}}','test-exec32')"}
+          ${if32 "machine.succeed('if test-exec32/${exec-name} 2>outfile32; then false; else [ $? -eq 127 ];fi')"}
+          ${if32 "machine.succeed('grep -qi nixos outfile32')"}
+
+      with subtest("Disable stub"):
+          machine.succeed("/run/booted-system/specialisation/nostub/bin/switch-to-configuration test")
+
+      with subtest("Check for stub (disabled)"):
+          machine.fail('test -e /${libDir}/${ldsoBasename}')
+          ${if32 "machine.fail('test -e /${libDir32}/${ldsoBasename32}')"}
+
+      with subtest("Create file in stub location (to be overwritten)"):
+          machine.succeed('mkdir -p /${libDir};touch /${libDir}/${ldsoBasename}')
+          ${if32 "machine.succeed('mkdir -p /${libDir32};touch /${libDir32}/${ldsoBasename32}')"}
+
+      with subtest("Re-enable stub"):
+          machine.succeed("/run/booted-system/bin/switch-to-configuration test")
+
+      with subtest("Check for stub (enabled, final)"):
+          machine.succeed('test -L /${libDir}/${ldsoBasename}')
+          ${if32 "machine.succeed('test -L /${libDir32}/${ldsoBasename32}')"}
+    '';
+})

pkgs/os-specific/linux/nix-ld/default.nix

@@ -5,11 +5,7 @@
 , ninja
 , nixosTests
 }:
-let
-  libDir = if builtins.elem stdenv.system [ "x86_64-linux" "mips64-linux" "powerpc64le-linux" ]
-           then "/lib64"
-           else "/lib";
-in
+
 stdenv.mkDerivation rec {
   pname = "nix-ld";
   version = "1.2.2";
@@ -36,7 +32,7 @@ stdenv.mkDerivation rec {
   postInstall = ''
     mkdir -p $out/nix-support
 
-    ldpath=${libDir}/$(basename $(< ${stdenv.cc}/nix-support/dynamic-linker))
+    ldpath=/${stdenv.hostPlatform.libDir}/$(basename $(< ${stdenv.cc}/nix-support/dynamic-linker))
     echo "$ldpath" > $out/nix-support/ldpath
     mkdir -p $out/lib/tmpfiles.d/
     cat > $out/lib/tmpfiles.d/nix-ld.conf <<EOF