Nix Harden Needed Setup Hook

This repository contains the Nix equivalent of the idea proposed by Harmen Stoppels in the blog post Stop searching for shared libraries.

The idea of the blog post is that if you set the SONAME of shared library to be the absolute path, it will get propagated as the DT_NEEDED entry for the library or application which links against it. Since in Nix it is always safe to link against the full _/nix/store/ path entry, this ultimately removes the need to have any RPATH or RUNPATH entries.

This setup-hook is used like any other setup-hook and you need only add it as a dependency for it to trigger.

Why do this?

I have shown in another repository Shrinkwrap that the cost of walking RPATH can be problematic for binaries that pull in many dependencies. The cost is even worse if you have your Nix store on something like NFS. This is another attempt at solving the same problem of Shrinkwrap but takes more advantage of systems like Nix that build from the ground up. You only need to set the SONAME correctly once and everything above it profits.

Example

Imagine a library libf that used this setup hook.

stdenv.mkDerivation rec {
  pname = "libf";
  version = "0.1";

  dontUnpack = true;

  buildInputs = [
    nix-harden-needed-hook
  ];

  buildPhase = ''
    $CC -shared -o libf.so -Wl,-soname,libf.so -x c - <<EOF
        #include <stdio.h>
        void f() { puts("hello world"); }
    EOF
  '';

  installPhase = ''
    mkdir -p $out/lib
    mv libf.so $out/lib
  '';
}

The SONAME is modified to point to the absolute path of the file.

❯ patchelf --print-soname /nix/store/zir4jfm86i3037lnsaz5br55iwavvhpz-libf-0.1/lib/libf.so
/nix/store/zir4jfm86i3037lnsaz5br55iwavvhpz-libf-0.1/lib/libf.so

If we then had a library or executable depend on it.

stdenv.mkDerivation rec {
  pname = "app";

  version = "0.1";

  dontUnpack = true;

  buildInputs = [
    libf
  ];

  buildPhase = ''
    # Enable if you'd like to see wrapper debug information
    # NIX_DEBUG=1 
    $CC -o app -lf -x c - <<EOF
        void f();
        int main() { f(); }
    EOF
  '';

  installPhase = ''
    mkdir -p $out/bin
    mv app $out/bin
  '';

}

It would propagate the entry into the DT_NEEDED entry correctly, making the need for RPATH redundant.

❯ patchelf --print-needed /nix/store/6pg9d3lwlmgcmmswv937fcy211vkqxch-app-0.1/bin/app
/nix/store/znxycsxlnx2s9zn6g0s0fl4z57ar7aps-libf-0.1/lib/libf.so
libc.so.6

What's next?

It would be great to upstream this into Nixpkgs such that it is the default way of building with stdenv. The benefit would be all binaries in Nix would load much quicker and managing RPATHs would no longer be necessary.