add fallback mechanism if environment variables are unset

README.md

@@ -25,8 +25,8 @@ Also read this [blog post](https://blog.thalheim.io/2022/12/31/nix-ld-a-clean-so
 to get the explaination in full detail. A summary is below:
 
 Precompiled binaries that were not created for NixOS usually have a so-called
-link-loader hardcoded into  them. On Linux/x86_64 this is for example
-`/lib64/ld-linux-x86-64.so.2`.  for glibc. NixOS, on the other hand, usually has
+link-loader hardcoded into them. On Linux/x86_64 this is for example
+`/lib64/ld-linux-x86-64.so.2`. for glibc. NixOS, on the other hand, usually has
 its dynamic linker in the glibc package in the Nix store and therefore cannot
 run these binaries. Nix-ld provides a shim layer for these types of binaries. It
 is installed in the same location where other Linux distributions install their
@@ -42,7 +42,7 @@ libraries that the executable needs to run.
 nix-ld is part of nixpkgs since NixOS 22.05. There one can enable it with the following
 nixos setting:
 
-``` nix
+```nix
 {
   programs.nix-ld.enable = true;
 }
@@ -62,10 +62,14 @@ $ sudo nix-channel --update
   imports = [
     <nix-ld/modules/nix-ld.nix>
   ];
+  # The module in this repository defines a new module under (programs.nix-ld.dev) instead of (programs.nix-ld) 
+  # to not collide with the nixpkgs version.
+  programs.nix-ld.dev.enable = true;
 }
 ```
 
-### With nix flake 
+
+### With nix flake
 
 Add the following lines to `/etc/nixos/flake.nix`. Replace `myhostname` with the
 actual hostname of your system.
@@ -77,24 +81,27 @@ actual hostname of your system.
   inputs.nix-ld.url = "github:Mic92/nix-ld";
   # this line assume that you also have nixpkgs as an input
   inputs.nix-ld.inputs.nixpkgs.follows = "nixpkgs";
-  
+
   outputs = { nix-ld, nixpkgs, ... }: {
     # replace `myhostname` with your actual hostname
     nixosConfigurations.myhostname = nixpkgs.lib.nixosSystem {
       system = "x86_64-linux";
       modules = [
         # ... add this line to the rest of your configuration modules
         nix-ld.nixosModules.nix-ld
+
+        # The module in this repository defines a new module under (programs.nix-ld.dev) instead of (programs.nix-ld) 
+        # to not collide with the nixpkgs version.
+        { programs.nix-ld.dev.enable = true; }
       ];
     };
   };
 }
 ```
 
-
 ## Usage
 
-After setting up the nix-ld symlink as described above, one needs to  set
+After setting up the nix-ld symlink as described above, one needs to set
 `NIX_LD` and `NIX_LD_LIBRARY_PATH` to run executables. For example, this can
 be done with a `shell.nix` in a nix-shell like this:
 
@@ -123,6 +130,21 @@ variables set.
 To figure out what libraries a program needs, you can use `ldd` on the binary or
 set the `LD_DEBUG=libs` environment variable.
 
+## Default Configuration for nix-ld
+
+In some scenarios, certain build systems or programs might ignore environment variables,
+which could disrupt the functioning of nix-ld.
+
+To counteract this, nix-ld implements a fallback mechanism. 
+If the `NIX_LD` environment variable is not set, 
+nix-ld will verify the existence of `/run/current-system/sw/share/nix-ld/lib/ld.so`. 
+If this file exists, it will be used, alongside `/run/current-system/sw/share/nix-ld/lib`.
+
+This behavior essentially defaults back to the NixOS configuration for nix-ld. 
+In terms of library paths, it will default to using the paths specified in `programs.nix-ld.libraries`. 
+This ensures that nix-ld can function effectively, even when its configuration 
+is not explicitly defined through the `NIX_LD` environment variable.
+
 ## Known Issues
 
 ### LD_LIBRARY_PATH is inherited by child processes
@@ -154,14 +176,14 @@ with nix-ld will break the system.
 ### My python/nodejs/ruby/$interpreter libraries do not find the libraries configured by nix-ld
 
 Nix-ld is only used by unpatched executables that use the link loader at `/lib`
-or `/lib64`.  If you use for example python from nixpkgs than it will not pick
-up `NIX_LD_LIBRARY_PATH` and `NIX_LD`  since these types of binaries are
+or `/lib64`. If you use for example python from nixpkgs than it will not pick
+up `NIX_LD_LIBRARY_PATH` and `NIX_LD` since these types of binaries are
 configured to use a glibc from the nix store. If you encounter these cases i.e.
 when you are trying to use python packages installed in a virtualenv than you
 need to set `LD_LIBRARY_PATH` directly. You can also create yourself a wrapper
 like this:
 
-``` nix
+```nix
 (pkgs.writeShellScriptBin "python" ''
   export LD_LIBRARY_PATH=$NIX_LD_LIBRARY_PATH
   exec ${pkgs.python3}/bin/python "$@"

modules/nix-ld.nix

@@ -1,9 +1,75 @@
-{pkgs, lib, ...}:
+{ pkgs, lib, config, ... }:
+let
+  cfg = config.programs.nix-ld.dev;
+
+  nix-ld-libraries = pkgs.buildEnv {
+    name = "lb-library-path";
+    pathsToLink = [ "/lib" ];
+    paths = map lib.getLib cfg.libraries;
+    # TODO make glibc here configurable?
+    postBuild = ''
+      ln -s "$(cat '${pkgs.stdenv.cc}/nix-support/dynamic-linker')" $out/share/nix-ld/lib/ld.so
+    '';
+    extraPrefix = "/share/nix-ld";
+    ignoreCollisions = true;
+  };
+
+  # We currently take all libraries from systemd and nix as the default.
+  # Is there a better list?
+  baseLibraries = with pkgs; [
+    zlib
+    zstd
+    stdenv.cc.cc
+    curl
+    openssl
+    attr
+    libssh
+    bzip2
+    libxml2
+    acl
+    libsodium
+    util-linux
+    xz
+    systemd
+  ];
+in
 {
-  config = {
-    programs.nix-ld.enable = true;
-  } // lib.optionalAttrs (lib.versionAtLeast (lib.versions.majorMinor lib.version) "23.05") {
-    # 22.11 users won't actually use nix-ld from this repo but at least it does not break their configuration.
-    programs.nix-ld.package = pkgs.callPackage ../nix-ld.nix {};
+  meta.maintainers = [ lib.maintainers.mic92 ];
+
+
+  options.programs.nix-ld.dev = {
+    enable = lib.mkEnableOption (lib.mdDoc ''nix-ld, Documentation: <https://github.com/Mic92/nix-ld>'');
+    package = lib.mkOption {
+      type = lib.types.package;
+      description = lib.mdDoc "The package to be used for nix-ld.";
+      default = pkgs.callPackage ../default.nix {};
+    };
+    libraries = lib.mkOption {
+      type = lib.types.listOf lib.types.package;
+      description = lib.mdDoc "Libraries that automatically become available to all programs. The default set includes common libraries.";
+      default = baseLibraries;
+      defaultText = lib.literalExpression "baseLibraries derived from systemd and nix dependencies.";
+    };
+  };
+
+  config = lib.mkIf config.programs.nix-ld.dev.enable {
+    assertions = [{ 
+      assertion = !config.programs.nix-ld.enable;
+      message = ''
+        nix-ld.dev cannot be enabled at the same time as nix-ld.
+      '';
+    }];
+
+    systemd.tmpfiles.packages = [ cfg.package ];
+
+    environment.systemPackages = [ nix-ld-libraries ];
+
+    environment.pathsToLink = [ "/share/nix-ld" ];
+
+    environment.variables = {
+      NIX_LD = "/run/current-system/sw/share/nix-ld/lib/ld.so";
+      NIX_LD_LIBRARY_PATH = "/run/current-system/sw/share/nix-ld/lib";
+    };
   };
 }
+

nixos-example.nix

@@ -1,6 +1,6 @@
 {pkgs, ...}: {
   imports = [./modules/nix-ld.nix];
-  programs.nix-ld.enable = true;
+  programs.nix-ld.dev.enable = true;
   environment.systemPackages = [
     (pkgs.runCommand "patched-hello" {} ''
       install -D -m755 ${pkgs.hello}/bin/hello $out/bin/hello

nixos-test.nix

@@ -11,8 +11,11 @@ in {
       testScript = ''
         start_all()
         machine.succeed("hello")
+        machine.succeed("ls -la /run/current-system/sw/share/nix-ld/lib/ld.so >&2")
         machine.succeed("$(< ${nix-ld}/nix-support/ldpath) --version")
-        machine.succeed("$(< ${nix-ld}/nix-support/ldpath) $(which hello)")
+
+        # test fallback if NIX_LD is not set
+        machine.succeed("unset NIX_LD; unset NIX_LD_LIBRARY_PATH; $(< ${nix-ld}/nix-support/ldpath) $(which hello)")
       '';
     } {
       inherit pkgs;

src/nix-ld.c

@@ -27,6 +27,9 @@ typedef Elf64_Phdr Phdr;
 #error unsupported word width
 #endif
 
+#define DEFAULT_NIX_LD "/run/current-system/sw/share/nix-ld/lib/ld.so"
+#define DEFAULT_NIX_LD_LIBRARY_PATH "/run/current-system/sw/share/nix-ld/lib"
+
 typedef struct {
   void *addr;
   size_t size;
@@ -382,6 +385,11 @@ static void* get_at_base(size_t *auxv) {
   return NULL;
 }
 
+#define R_OK 4
+int access(const char *pathname, int mode) {
+  return my_syscall2(__NR_access, (long)pathname, mode);
+}
+
 int main(int argc, char** argv, char** envp) {
   size_t *auxv;
   for (auxv = (size_t *)envp; *auxv; auxv++) {
@@ -391,8 +399,15 @@ int main(int argc, char** argv, char** envp) {
   struct ld_ctx ctx = init_ld_ctx(argc, argv, envp, auxv);
 
   if (!ctx.nix_ld) {
-    log_error(&ctx, "You are trying to run an unpatched binary on nixos, but you have not configured NIX_LD or NIX_LD_" NIX_SYSTEM ". See https://github.com/Mic92/nix-ld for more details");
-    return 1;
+    // fallback to default ld.so
+    if (access(DEFAULT_NIX_LD, R_OK) == 0) {
+      ctx.nix_ld = DEFAULT_NIX_LD;
+      // if no NIX_LD is set we also don't trust NIX_LD_LIBRARY_PATH since it may point to a different libc
+      ctx.nix_lib_path_prefix = DEFAULT_NIX_LD_LIBRARY_PATH;
+    } else {
+      log_error(&ctx, "You are trying to run an unpatched binary on nixos, but you have not configured NIX_LD or NIX_LD_" NIX_SYSTEM ". See https://github.com/Mic92/nix-ld for more details");
+      return 1;
+    }
   }
 
   if (!ctx.page_size) {