llext: add support for ELF init/fini arrays

Load the .preinit_array, .init_array and .fini_array sections in ELF
files. These sections are arrays of function pointers that are filled by
the compiler with the addresses of functions that need to be called at
startup or termination by the loader, such as C++ constructors and
destructors.

Signed-off-by: Luca Burelli <[email protected]>

Author: pillo79

include/zephyr/llext/elf.h

@@ -206,6 +206,9 @@ struct elf64_shdr {
 #define SHT_NOBITS 0x8          /**< Program data with no file image */
 #define SHT_REL 0x9             /**< Relocation entries without addends */
 #define SHT_DYNSYM 0xB          /**< Dynamic linking symbol table */
+#define SHT_INIT_ARRAY 0xe      /**< Array of pointers to init functions */
+#define SHT_FINI_ARRAY 0xf      /**< Array of pointers to termination functions */
+#define SHT_PREINIT_ARRAY 0x10  /**< Array of pointers to early init functions */
 
 /** ELF section flags */
 #define SHF_WRITE 0x1           /**< Section is writable */

include/zephyr/llext/llext.h

@@ -50,6 +50,9 @@ enum llext_mem {
 	LLEXT_MEM_SYMTAB,       /**< Symbol table */
 	LLEXT_MEM_STRTAB,       /**< Symbol name strings */
 	LLEXT_MEM_SHSTRTAB,     /**< Section name strings */
+	LLEXT_MEM_PREINIT,      /**< Array of early setup functions */
+	LLEXT_MEM_INIT,         /**< Array of setup functions */
+	LLEXT_MEM_FINI,         /**< Array of cleanup functions */
 
 	LLEXT_MEM_COUNT,        /**< Number of regions managed by LLEXT */
 };

subsys/llext/llext_load.c

@@ -208,6 +208,12 @@ static int llext_map_sections(struct llext_loader *ldr, struct llext *ext)
 
 		name = llext_string(ldr, ext, LLEXT_MEM_SHSTRTAB, shdr->sh_name);
 
+		if (ldr->sect_map[i].mem_idx != LLEXT_MEM_COUNT) {
+			LOG_DBG("section %d name %s already mapped to region %d",
+				i, name, ldr->sect_map[i].mem_idx);
+			continue;
+		}
+
 		/* Identify the section type by its flags */
 		enum llext_mem mem_idx;
 
@@ -224,6 +230,15 @@ static int llext_map_sections(struct llext_loader *ldr, struct llext *ext)
 				mem_idx = LLEXT_MEM_RODATA;
 			}
 			break;
+		case SHT_PREINIT_ARRAY:
+			mem_idx = LLEXT_MEM_PREINIT;
+			break;
+		case SHT_INIT_ARRAY:
+			mem_idx = LLEXT_MEM_INIT;
+			break;
+		case SHT_FINI_ARRAY:
+			mem_idx = LLEXT_MEM_FINI;
+			break;
 		default:
 			mem_idx = LLEXT_MEM_COUNT;
 			break;
@@ -241,6 +256,19 @@ static int llext_map_sections(struct llext_loader *ldr, struct llext *ext)
 			continue;
 		}
 
+		switch (mem_idx) {
+		case LLEXT_MEM_PREINIT:
+		case LLEXT_MEM_INIT:
+		case LLEXT_MEM_FINI:
+			if (shdr->sh_entsize != sizeof(void *) ||
+			    shdr->sh_size % shdr->sh_entsize != 0) {
+				LOG_ERR("Invalid %s array in section %d", name, i);
+				return -ENOEXEC;
+			}
+		default:
+			break;
+		}
+
 		LOG_DBG("section %d name %s maps to region %d", i, name, mem_idx);
 
 		ldr->sect_map[i].mem_idx = mem_idx;
@@ -261,14 +289,26 @@ static int llext_map_sections(struct llext_loader *ldr, struct llext *ext)
 				return -ENOEXEC;
 			}
 
-			if (mem_idx == LLEXT_MEM_BSS) {
+			/* Check if this region type is extendable */
+			switch (mem_idx) {
+			case LLEXT_MEM_BSS:
 				/* SHT_NOBITS sections cannot be merged properly:
 				 * as they use no space in the file, the logic
 				 * below does not work; they must be treated as
 				 * independent entities.
 				 */
 				LOG_ERR("Multiple SHT_NOBITS sections are not supported");
 				return -ENOTSUP;
+			case LLEXT_MEM_PREINIT:
+			case LLEXT_MEM_INIT:
+			case LLEXT_MEM_FINI:
+				/* These regions are not extendable and must be
+				 * referenced at most once in the ELF file.
+				 */
+				LOG_ERR("Region %d redefined", mem_idx);
+				return -ENOEXEC;
+			default:
+				break;
 			}
 
 			if (ldr->hdr.e_type == ET_DYN) {