validate app presence more robustly, allow debugger flashing e.g with jlink without flashing the setting page

AGENTS.md

@@ -87,6 +87,10 @@ Each board lives in `src/boards/{board_name}/` with:
 - `DEFAULT_TO_OTA_DFU` — Default to BLE OTA instead of serial DFU
 - `DEBUG` — Enable RTT debugging, larger bootloader region
 
+## PR Review Checklist
+
+- **VID/PID assignment**: When a PR adds or modifies a board, verify `USB_DESC_VID`/`USB_DESC_UF2_PID` in `board.h`. New boards should use unique values. The Adafruit VID `0x239A` is reserved for Adafruit-allocated boards; third-party boards must use their own VID. Some legacy boards intentionally share a VID/PID with a reference board (see `// Shared VID/PID with ...` comments) — these are pending cleanup, not a template for new boards.
+
 ## CI
 
 GitHub Actions (`.github/workflows/githubci.yml`) builds all boards in parallel using a matrix generated from `src/boards/` directory names. On release, artifacts (zip, hex, uf2) are uploaded as release assets.

lib/sdk11/components/libraries/bootloader_dfu/bootloader.c

@@ -149,23 +149,22 @@ static void wait_for_events(void)
 
 bool bootloader_app_is_valid(void)
 {
-  bool success = false;
   uint32_t const app_addr = DFU_BANK_0_REGION_START;
-
-  bootloader_settings_t const *p_bootloader_settings;
-  bootloader_util_settings_get(&p_bootloader_settings);
+  uint32_t const sp_addr = *((uint32_t*)app_addr);
+  uint32_t const reset_addr = *((uint32_t*)(app_addr + 4));
 
   enum { EMPTY_FLASH = 0xFFFFFFFFUL };
 
-  // Application is invalid if first 2 words are all 0xFFFFFFF
-  if ( *((uint32_t *)app_addr    ) == EMPTY_FLASH &&
-       *((uint32_t *)(app_addr+4)) == EMPTY_FLASH )
+  if (sp_addr == EMPTY_FLASH && reset_addr == EMPTY_FLASH)
   {
     return false;
   }
 
-  // The application in CODE region 1 is flagged as valid during update.
-  if ( p_bootloader_settings->bank_0 == BANK_VALID_APP )
+  bootloader_settings_t const * p_bootloader_settings;
+  bootloader_util_settings_get(&p_bootloader_settings);
+
+  // Primary path: a prior DFU flagged bank 0 as valid.
+  if (p_bootloader_settings->bank_0 == BANK_VALID_APP)
   {
     uint16_t image_crc = 0;
 
@@ -177,10 +176,26 @@ bool bootloader_app_is_valid(void)
                                 NULL);
     }
 
-    success = (image_crc == p_bootloader_settings->bank_0_crc);
+    return (image_crc == p_bootloader_settings->bank_0_crc);
   }
 
-  return success;
+  // Fallback for debugger-flashed apps: settings page is erased (bank_0 = 0xFFFF)
+  // but an app may still have been written to bank 0 directly. Accept it if its
+  // Cortex-M vector table is plausible: initial SP within RAM, reset vector
+  // within app flash region with the Thumb bit set. The SP upper bound is
+  // inclusive because the empty-descending stack initializes one past RAM end.
+  if (p_bootloader_settings->bank_0 == 0xFFFF)
+  {
+    uint32_t const ram_start = 0x20000000UL;
+    uint32_t const ram_end = ram_start + (NRF_FICR->INFO.RAM << 10u);
+    bool const sp_valid = (sp_addr >= ram_start) && (sp_addr <= ram_end) && ((sp_addr & 3U) == 0);
+    bool const reset_in_app = (reset_addr & 1U) &&
+      (reset_addr >= app_addr) &&
+      (reset_addr < BOOTLOADER_REGION_START);
+    return sp_valid && reset_in_app;
+  }
+
+  return false;
 }