llext: fix read-only extension image

When using the LLEXT buffer loader we now avoid copying extensions
from storage to allocated memory by pointing directly into the stored
image. We then also perform linking and relocation in that memory,
which modifies its contents. However, this is impossible if that
storage is read-only. Add a Kconfig flag to distinguish between
writable and read-only storage types. Also use that flag to decide,
whether the extension image in test_llext_simple.c should be defined
as const or not.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

subsys/llext/Kconfig

@@ -27,6 +27,12 @@ config LLEXT_SHELL_MAX_SIZE
 	help
 	  When loading llext with shell it is stored in a temporary buffer of this size
 
+config LLEXT_STORAGE_WRITABLE
+	bool "llext storage is writable"
+	help
+	  Select if LLEXT storage is writable, i.e. if extensions are stored in
+	  RAM and can be modified in place
+
 module = LLEXT
 module-str = llext
 source "subsys/logging/Kconfig.template.log_config"

subsys/llext/llext.c

@@ -303,7 +303,8 @@ static int llext_copy_section(struct llext_loader *ldr, struct llext *ext,
 		return 0;
 	}
 
-	if (ldr->sects[sect_idx].sh_type != SHT_NOBITS) {
+	if (ldr->sects[sect_idx].sh_type != SHT_NOBITS &&
+	    IS_ENABLED(CONFIG_LLEXT_STORAGE_WRITABLE)) {
 		ext->mem[mem_idx] = llext_peek(ldr, ldr->sects[sect_idx].sh_offset);
 		if (ext->mem[mem_idx]) {
 			ext->mem_on_heap[mem_idx] = false;

tests/subsys/llext/src/test_llext_simple.c

@@ -10,7 +10,10 @@
 #include <zephyr/llext/buf_loader.h>
 
 #if defined(CONFIG_ARM) /* ARMV7 */ || defined(CONFIG_XTENSA)
-const static uint8_t hello_world_elf[] __aligned(4) = {
+#ifndef CONFIG_LLEXT_STORAGE_WRITABLE
+const
+#endif
+static uint8_t hello_world_elf[] __aligned(4) = {
 #include "hello_world.inc"
 };
 #endif

tests/subsys/llext/testcase.yaml

@@ -11,6 +11,8 @@ tests:
       - nuvoton_pfm_m487 # See #63167
   llext.simple.xtensa:
     arch_allow: xtensa
+    extra_configs:
+      - CONFIG_LLEXT_STORAGE_WRITABLE=y
     # Broken platforms
     platform_exclude:
       - qemu_xtensa_mmu # ELF sections are read-only, and without peek() .text copy isn't executable