llext: optimize allocations for read-only sections

When loading an extension, read-only sections that do not have
relocations and whose data is accessible from the ELF buffer can be
directly mapped as-is in the extension memory. This avoids the need to
allocate and copy unmodified data from the ELF buffer to the LLEXT heap.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>

include/zephyr/llext/elf.h

@@ -211,9 +211,11 @@ struct elf64_shdr {
 #define SHT_PREINIT_ARRAY 0x10  /**< Array of pointers to early init functions */
 
 /** ELF section flags */
-#define SHF_WRITE 0x1           /**< Section is writable */
-#define SHF_ALLOC 0x2           /**< Section is present in memory */
-#define SHF_EXECINSTR 0x4       /**< Section contains executable instructions */
+#define SHF_WRITE 0x1                   /**< Section is writable */
+#define SHF_ALLOC 0x2                   /**< Section is present in memory */
+#define SHF_EXECINSTR 0x4               /**< Section contains executable instructions */
+#define SHF_MASKOS 0x0ff00000           /**< OS specific flags */
+#define SHF_LLEXT_HAS_RELOCS 0x00100000 /**< Section is a target for relocations */
 
 #define SHF_BASIC_TYPE_MASK	(SHF_WRITE | SHF_ALLOC | SHF_EXECINSTR)
 

subsys/llext/llext_load.c

@@ -494,11 +494,20 @@ static int llext_map_sections(struct llext_loader *ldr, struct llext *ext,
 	/*
 	 * Calculate each ELF section's offset inside its memory region. This
 	 * is done as a separate pass so the final regions are already defined.
+	 * Also mark the regions that include relocation targets.
 	 */
 	for (i = 0; i < ext->sect_cnt; ++i) {
 		elf_shdr_t *shdr = ext->sect_hdrs + i;
 		enum llext_mem mem_idx = ldr->sect_map[i].mem_idx;
 
+		if (shdr->sh_type == SHT_REL || shdr->sh_type == SHT_RELA) {
+			enum llext_mem target_region = ldr->sect_map[shdr->sh_info].mem_idx;
+
+			if (target_region != LLEXT_MEM_COUNT) {
+				ldr->sects[target_region].sh_flags |= SHF_LLEXT_HAS_RELOCS;
+			}
+		}
+
 		if (mem_idx != LLEXT_MEM_COUNT) {
 			ldr->sect_map[i].offset = shdr->sh_offset - ldr->sects[mem_idx].sh_offset;
 		}

subsys/llext/llext_mem.c

@@ -95,7 +95,10 @@ static int llext_copy_region(struct llext_loader *ldr, struct llext *ext,
 		}
 	}
 
-	if (ldr->storage == LLEXT_STORAGE_WRITABLE) {
+	if (ldr->storage == LLEXT_STORAGE_WRITABLE ||           /* writable storage         */
+	    (ldr->storage == LLEXT_STORAGE_PERSISTENT &&        /* || persistent storage    */
+	     !(region->sh_flags & SHF_WRITE) &&                 /*    && read-only region   */
+	     !(region->sh_flags & SHF_LLEXT_HAS_RELOCS))) {     /*    && no relocs to apply */
 		/*
 		 * Try to reuse data areas from the ELF buffer, if possible.
 		 * If any of the following tests fail, a normal allocation