From a30b4022fa85320806b8b4d004ea713b68df6566 Mon Sep 17 00:00:00 2001
From: Kamen Mladenov <kamen@syndamia.com>
Date: Thu, 27 Mar 2025 13:25:05 +0200
Subject: feat(zkvms/risc0): Implement elf encoding

Recently, RISC0 have added code to their SDK which adds magic bytes to
the (already compiled) ELF. Since we're taking another route than
intended, we'll need to replicate this step too.

We're doing things in a different fashion, because their SDK normally
compiles the guest, and doesn't allow you to supply a prebuilt ELF.
---
 zkvms/risc0/default.nix                 |  7 ++++++-
 zkvms/risc0/guest/Cargo.toml            |  3 +++
 zkvms/risc0/guest_elf_patch/Cargo.toml  |  9 +++++++++
 zkvms/risc0/guest_elf_patch/src/main.rs | 11 +++++++++++
 4 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 zkvms/risc0/guest_elf_patch/Cargo.toml
 create mode 100644 zkvms/risc0/guest_elf_patch/src/main.rs

diff --git a/zkvms/risc0/default.nix b/zkvms/risc0/default.nix
index 6a4b796..132bce2 100644
--- a/zkvms/risc0/default.nix
+++ b/zkvms/risc0/default.nix
@@ -23,10 +23,15 @@ in zkvmLib.buildPackage craneLib (commonArgs // {
 
   guestTarget = "riscv32im-risc0-zkvm-elf";
 
+  postBuildGuest = ''
+    cd ../guest_elf_patch
+    cargo run --release
+  '';
+
   preBuild = ''
     # Used for verification
     # https://github.com/risc0/risc0/blob/881e512732eca72849b2d0e263a1242aba3158af/risc0/build/src/lib.rs#L192-L195
-    export GUEST_ID="$(${metacraft-labs.risc0}/bin/r0vm --elf ./host/src/guest --id)"
+    export GUEST_ID="$(${metacraft-labs.risc0}/bin/r0vm --elf ./src/guest --id)"
   '';
 
   preRunBinaries = [ metacraft-labs.risc0 ];
diff --git a/zkvms/risc0/guest/Cargo.toml b/zkvms/risc0/guest/Cargo.toml
index 8a12a8c..a5a61f2 100644
--- a/zkvms/risc0/guest/Cargo.toml
+++ b/zkvms/risc0/guest/Cargo.toml
@@ -11,5 +11,8 @@ risc0-zkvm = { path = "/nix/store/6992z4qxba51p1m9vjbjvcwh16gvdp56-risc0-unstabl
 
 wrapper_macro = { version = "0.1.0", path = "../wrapper_macro" }
 
+# Not used, but needed for Nix to handle dependencies
+guest-elf-patch-risc0 = { path = "../guest_elf_patch" }
+
 # The zkp dependency references a chosen guest in guests. It is included
 # (inserted here) by Nix. See zkvmLib.nix
diff --git a/zkvms/risc0/guest_elf_patch/Cargo.toml b/zkvms/risc0/guest_elf_patch/Cargo.toml
new file mode 100644
index 0000000..2008faf
--- /dev/null
+++ b/zkvms/risc0/guest_elf_patch/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "guest-elf-patch-risc0"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+risc0-binfmt = { path = "/nix/store/6992z4qxba51p1m9vjbjvcwh16gvdp56-risc0-unstable-2025-03-12/risc0/binfmt" }
+risc0-zkos-v1compat = { path = "/nix/store/6992z4qxba51p1m9vjbjvcwh16gvdp56-risc0-unstable-2025-03-12/risc0/zkos/v1compat" }
+bytemuck_derive = "<1.9.0"
diff --git a/zkvms/risc0/guest_elf_patch/src/main.rs b/zkvms/risc0/guest_elf_patch/src/main.rs
new file mode 100644
index 0000000..6999ab4
--- /dev/null
+++ b/zkvms/risc0/guest_elf_patch/src/main.rs
@@ -0,0 +1,11 @@
+use risc0_binfmt::ProgramBinary;
+use risc0_zkos_v1compat::V1COMPAT_ELF;
+
+static ELF: &[u8] = include_bytes!("../../host/src/guest");
+
+// https://github.com/risc0/risc0/blob/fee2e19a3c49b3da492403de5e2d011c890e52de/risc0/build/src/lib.rs#L179-L187
+fn main() {
+    let binary = ProgramBinary::new(&ELF, V1COMPAT_ELF);
+    let elf = binary.encode();
+    std::fs::write("../host/src/guest", &elf).expect("couldn't add magic bytes to elf!");
+}
-- 
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