Linux kernel will support $ORIGIN, sort of

Summary

For some reason, during TacoSprint 2026 I decided to see if we could tackle relocatable binaries in Nix. I enjoy these lofty goals to push Nix and the surrounding ecosystem forward. I am bold if not stupid. I left the last earlier post with one potential idea of how to get there: We could patch the Linux kernel so that $ORIGIN is supported in PT_INTERP and the shebang. I waded through the complexity of sending patches over email (turns out I actually enjoy this workflow!), and sent a proposal to the Linux kernel mailing list. My first attempt here proposed simply adding direct support for $ORIGIN in the Virtual File System (VFS) subsystem. I waited nervously. I was expecting the result from what I had come to read about online; someone non-politely telling me to F$#CK OFF because there is something I missed, misunderstood or did not consider. 🤬 The result was completely different. 😲 Christian Brauner, the maintainer for VFS responded to me in good faith, asking for the rationale for the change and eventually proposing some ways in which such a support could make it into the subsystem. Note It definitely helped having someone like John Ericson chime in and advocate why having a non-fixed interpreter (PT_INTERP) is useful to Nix and other use-cases (i.e. Buck & Bazel). He offered that potentially we could leverage eBPF as a programmable way to select an interpreter through binfmt_misc. Whoa! 🤯 I wanted to merely allow $ORIGIN but a programmable selection could let us do anything! The idea must have really intrigued him because soon-after, on his vacation, Christian offered the first draft of such a solution. We went back and forth a little over the mailing list and the end result is a patch series that will make its way into -next branch in the near future. If you don’t know what eBPF is or binfmt_misc, WTF did we just collaborate on? Let’s take a look! I won’t do eBPF justice, and there are plenty of articles online about it as it’s quite in-vogue at the moment. tl;d...