llext: warn when devices aren't exported and extension tries to use them

Kconfig option LLEXT_EXPORT_DEVICES can be enabled to make all device
objects of an image available to LLEXTs, but it is disabled by default.
If an extension tries to import devices when the base image was not built
with this option, the dynamic linking equivalent of the much beloved error
"Undefined symbol __device_dts_ord_XXX" is logged, but this can be quite
cryptic to understand since user may be unaware of LLEXT_EXPORT_DEVICES.

Detect such invalid imports and print a special message that directs users
towards the appropriate Kconfig option, which should reduce the confusion.

Signed-off-by: Mathieu Choplain <[email protected]>

Author: Mathieu Choplain

subsys/llext/llext_link.c

@@ -192,6 +192,24 @@ int llext_lookup_symbol(struct llext_loader *ldr, struct llext *ext, uintptr_t *
 			LOG_ERR("Undefined symbol with no entry in "
 				"symbol table %s, offset %zd, link section %d",
 				name, (size_t)rel->r_offset, shdr->sh_link);
+
+			if (!IS_ENABLED(CONFIG_LLEXT_EXPORT_DEVICES)) {
+				/**
+				 * Attempting to import device objects from LLEXT but forgetting to
+				 * enable the corresponding Kconfig option will result in cryptic
+				 * dynamic linking errors. Try to detect this situation by checking
+				 * if the symbol's name starts with the prefix used to name device
+				 * objects, and print a special warning directing users towards the
+				 * missing Kconfig option in such circumstances.
+				 */
+				const char *const dev_prefix = STRINGIFY(DEVICE_NAME_GET(EMPTY));
+				const int prefix_len = strlen(dev_prefix);
+
+				if (strncmp(name, dev_prefix, prefix_len) == 0) {
+					LOG_WRN("(Device objects are not available for import "
+						"because CONFIG_LLEXT_EXPORT_DEVICES is not enabled)");
+				}
+			}
 			return -ENODATA;
 		}