fix: Clarify and validate alignment requirements for flashing

Author: DNedic

include/esp_loader.h

@@ -181,9 +181,9 @@ esp_loader_error_t esp_loader_connect_secure_download_mode(esp_loader_connect_ar
 /**
   * @brief Initiates flash operation
   *
-  * @param offset[in]       Address from which flash operation will be performed.
-  * @param image_size[in]   Size of the whole binary to be loaded into flash.
-  * @param block_size[in]   Size of buffer used in subsequent calls to esp_loader_flash_write.
+  * @param offset[in] Address from which flash operation will be performed. Must be 4 byte aligned.
+  * @param image_size[in] Size of the whole binary to be loaded into flash. Must be 4 byte aligned.
+  * @param block_size[in] Size of buffer used in subsequent calls to esp_loader_flash_write.
   *
   * @note  image_size is size of the whole image, whereas, block_size is chunk of data sent
   *        to the target, each time esp_loader_flash_write function is called.

src/esp_loader.c

@@ -320,6 +320,11 @@ esp_loader_error_t esp_loader_flash_start(uint32_t offset, uint32_t image_size,
 {
     s_flash_write_size = block_size;
 
+    // Both the address and image size must be aligned to 4 bytes
+    if (offset % 4 != 0 || image_size % 4 != 0) {
+        return ESP_LOADER_ERROR_INVALID_PARAM;
+    }
+
     /* Flash size will be known in advance if we're in secure download mode or we already read it*/
     if (s_target_flash_size == 0) {
         if (esp_loader_flash_detect_size(&s_target_flash_size) == ESP_LOADER_SUCCESS) {