fix: verify QEMU registration instead of trusting the installer's exi…

README.md

@@ -115,6 +115,14 @@ The Cartesi CLI drives Docker for most of its commands (`cartesi build`, `cartes
 registration (`setup-qemu`) also runs through Docker. GitHub-hosted Linux runners already provide
 Docker, so this action never installs it there.
 
+### QEMU registration is verified, not just run
+
+The `tonistiigi/binfmt` installer exits `0` even when it fails to register anything — it only logs
+the error. Rather than trust that exit code, the action checks
+`/proc/sys/fs/binfmt_misc/qemu-riscv64` directly afterwards and fails the step with an explanatory
+message if it's missing, instead of reporting success while `docker buildx`/`cartesi build` are
+left unable to run RISC-V images.
+
 ## Security
 
 - The action only ever downloads from a fixed set of hardcoded sources: the `cartesi/machine-emulator`,

src/qemu.ts

@@ -1,8 +1,25 @@
+import { access } from "node:fs/promises";
 import * as core from "@actions/core";
 import * as exec from "@actions/exec";
 import { BINFMT_IMAGE, QEMU_ARCH } from "./constants.js";
 import type { Platform } from "./platform.js";
 
+/**
+ * Where the kernel exposes a registered handler once `--install` succeeds.
+ * Checked directly after running the installer, because that tool's process
+ * exits 0 even when it fails to register anything (it only logs the error).
+ */
+const BINFMT_ENTRY = `/proc/sys/fs/binfmt_misc/qemu-${QEMU_ARCH}`;
+
+async function binfmtRegistered(): Promise<boolean> {
+    try {
+        await access(BINFMT_ENTRY);
+        return true;
+    } catch {
+        return false;
+    }
+}
+
 /**
  * Registers the QEMU emulator for `QEMU_ARCH` with the kernel's binfmt_misc
  * via the same `tonistiigi/binfmt` image `docker/setup-qemu-action` uses,
@@ -29,4 +46,13 @@ export async function setupQemu(platform: Platform): Promise<void> {
         "--install",
         QEMU_ARCH,
     ]);
+
+    if (!(await binfmtRegistered())) {
+        throw new Error(
+            `the ${QEMU_ARCH} QEMU emulator was not registered: ${BINFMT_ENTRY} does not exist ` +
+                "after running the installer. This runner's kernel may not support binfmt_misc " +
+                "registration from a container. Set 'setup-qemu: false' to skip this step.",
+        );
+    }
+    core.info(`${QEMU_ARCH} QEMU emulator registered`);
 }