pos: refuse to flash anything that is not a valid LittleFS image

The flasher hex-encoded whatever the /flash endpoint returned, so a
server error body was written to devices as the config partition.
Check the response status, require a 128KB image with the LittleFS
magic, and surface the error above the form instead.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_014gVYGi29FjTeLdNTiutkZx

2 files changed, 40 insertions, 1 deletions

diff --git a/src/routes/(app)/pos/+page.server.ts b/src/routes/(app)/pos/+page.server.ts
index 1df1bb33..1e3e1e99 100644
--- a/src/routes/(app)/pos/+page.server.ts
+++ b/src/routes/(app)/pos/+page.server.ts
@@ -1,6 +1,18 @@
 import { PUBLIC_COINOS_URL } from "$env/static/public";
 import { auth, fd, get } from "$lib/utils";
 import { hex } from "@scure/base";
+import { fail } from "@sveltejs/kit";
+
+const LITTLEFS_SIZE = 0x20000;
+const LITTLEFS_MAGIC = "littlefs";
+
+// A valid image is exactly one partition long and carries the LittleFS
+// superblock magic at byte 8. Anything else (e.g. a JSON error body from
+// the server) must never reach the flasher.
+const isLittleFS = (buf: Uint8Array) =>
+	buf.length === LITTLEFS_SIZE &&
+	new TextDecoder().decode(buf.subarray(8, 8 + LITTLEFS_MAGIC.length)) ===
+		LITTLEFS_MAGIC;
 
 export async function load({ cookies }) {
 	let token;
@@ -25,6 +37,22 @@ export const actions = {
 		});
 
 		const buf = new Uint8Array(await res.arrayBuffer());
+
+		if (!res.ok) {
+			let message = `Config server returned ${res.status}`;
+			try {
+				message = JSON.parse(new TextDecoder().decode(buf)).message || message;
+			} catch (e) {}
+			return fail(res.status >= 400 && res.status < 600 ? res.status : 500, {
+				error: message,
+			});
+		}
+
+		if (!isLittleFS(buf))
+			return fail(500, {
+				error: `Config server returned an invalid image (${buf.length} bytes)`,
+			});
+
 		return { bytes: hex.encode(buf) };
 	},
 };
diff --git a/src/routes/(app)/pos/+page.svelte b/src/routes/(app)/pos/+page.svelte
index 871c355d..36869fba 100644
--- a/src/routes/(app)/pos/+page.svelte
+++ b/src/routes/(app)/pos/+page.svelte
@@ -30,7 +30,11 @@
   let portInfo = $state("");
 
   // --- Existing config path (generated server-side to hex in form.bytes) ---
-  let bytes = $derived(form ? hex.decode(form.bytes) : undefined);
+  let bytes = $derived(form?.bytes ? hex.decode(form.bytes) : undefined);
+  let configError = $state("");
+  $effect(() => {
+    configError = form?.error || "";
+  });
   let littlefsAddress = $state(LITTLEFS_ADDRESS_DEFAULT);
   let configProgress = $state(0);
   let configDone = $state(false);
@@ -68,6 +72,10 @@
 
   let flashConfig = async () => {
     if (!esploader || !bytes) return;
+    if (bytes.length !== 0x20000) {
+      configError = `Refusing to flash: image is ${bytes.length} bytes, expected ${0x20000}`;
+      return;
+    }
     configDone = false;
     configProgress = 0;
 
@@ -175,6 +183,9 @@
           {#if configProgress > 0}<div class="mt-2">Progress: {configProgress}%</div>{/if}
         {/if}
       {:else}
+        {#if configError}
+          <div class="text-red-600 mb-2">{configError}</div>
+        {/if}
         <form method="POST" use:enhance class="space-y-2">
           <input name="ssid" placeholder="Wifi SSID" class="input" />
           <input name="key" placeholder="Wifi password" class="input" />