llext: add support for exporting symbols from extensions

Extensions should be able to selectively export their global symbols.
Add a LL_EXTENSION_SYMBOL() macro for that. Change the present
.sym_tab to be a temporary symbol table of all global symbols in an
extensions, used only during linking for internal purposes. Add a new
.exp_tab symbol table to store symbols, exported by an extension
permanently.

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

include/zephyr/llext/llext.h

@@ -33,6 +33,7 @@ enum llext_mem {
 	LLEXT_MEM_DATA,
 	LLEXT_MEM_RODATA,
 	LLEXT_MEM_BSS,
+	LLEXT_MEM_EXPORT,
 	LLEXT_MEM_SYMTAB,
 	LLEXT_MEM_STRTAB,
 	LLEXT_MEM_SHSTRTAB,
@@ -60,9 +61,18 @@ struct llext {
 	/** Total size of the llext memory usage */
 	size_t mem_size;
 
-	/** Exported symbols from the llext, may be linked against by other llext */
+	/*
+	 * These are all global symbols in the extension, all of them don't
+	 * have to be exported to other extensions, but this table is needed for
+	 * faster internal linking, e.g. if the extension is built out of
+	 * several files, if any symbols are referenced between files, this
+	 * table will be used to link them.
+	 */
 	struct llext_symtable sym_tab;
 
+	/** Exported symbols from the llext, may be linked against by other llext */
+	struct llext_symtable exp_tab;
+
 	/** Extension use counter, prevents unloading while in use */
 	unsigned int use_count;
 };

include/zephyr/llext/loader.h

@@ -35,6 +35,8 @@ enum llext_section {
 	LLEXT_SECT_REL_RODATA,
 	LLEXT_SECT_REL_BSS,
 
+	LLEXT_SECT_EXPORT,
+
 	LLEXT_SECT_SYMTAB,
 	LLEXT_SECT_STRTAB,
 	LLEXT_SECT_SHSTRTAB,

include/zephyr/llext/symbol.h

@@ -78,6 +78,9 @@ struct llext_symtable {
 		.name = STRINGIFY(x), .addr = &x,				\
 	}
 
+#define LL_EXTENSION_SYMBOL(x) struct llext_symbol __attribute__((section(".exported_sym"), used)) \
+							symbol_##x = {STRINGIFY(x), &x}
+
 /**
  * @brief Export a system call to a table of symbols
  *

subsys/llext/llext.c

@@ -75,7 +75,7 @@ struct llext *llext_by_name(const char *name)
 const void * const llext_find_sym(const struct llext_symtable *sym_table, const char *sym_name)
 {
 	if (sym_table == NULL) {
-		/* Buildin symbol table */
+		/* Built-in symbol table */
 		STRUCT_SECTION_FOREACH(llext_const_symbol, sym) {
 			if (strcmp(sym->name, sym_name) == 0) {
 				return sym->addr;
@@ -214,6 +214,9 @@ static int llext_map_sections(struct llext_loader *ldr, struct llext *ext)
 		} else if (strcmp(name, ".bss") == 0) {
 			sect_idx = LLEXT_SECT_BSS;
 			mem_idx = LLEXT_MEM_BSS;
+		} else if (strcmp(name, ".exported_sym") == 0) {
+			sect_idx = LLEXT_SECT_EXPORT;
+			mem_idx = LLEXT_MEM_EXPORT;
 		} else {
 			LOG_DBG("Not copied section %s", name);
 			continue;
@@ -240,6 +243,9 @@ static enum llext_section llext_sect_from_mem(enum llext_mem m)
 	case LLEXT_MEM_RODATA:
 		s = LLEXT_SECT_RODATA;
 		break;
+	case LLEXT_MEM_EXPORT:
+		s = LLEXT_SECT_EXPORT;
+		break;
 	case LLEXT_MEM_TEXT:
 		s = LLEXT_SECT_TEXT;
 		break;
@@ -401,6 +407,37 @@ static int llext_allocate_symtab(struct llext_loader *ldr, struct llext *ext)
 	return 0;
 }
 
+static int llext_export_symbols(struct llext_loader *ldr, struct llext *ext)
+{
+	elf_shdr_t *shdr = ldr->sects + LLEXT_SECT_EXPORT;
+	struct llext_symbol *sym;
+	unsigned int i;
+
+	if (shdr->sh_size < sizeof(struct llext_symbol)) {
+		/* Not found, no symbols exported */
+		return 0;
+	}
+
+	struct llext_symtable *exp_tab = &ext->exp_tab;
+
+	exp_tab->sym_cnt = shdr->sh_size / sizeof(struct llext_symbol);
+	exp_tab->syms = k_heap_alloc(&llext_heap, exp_tab->sym_cnt * sizeof(struct llext_symbol),
+				     K_NO_WAIT);
+	if (!exp_tab->syms) {
+		return -ENOMEM;
+	}
+
+	for (i = 0, sym = ext->mem[LLEXT_MEM_EXPORT];
+	     i < exp_tab->sym_cnt;
+	     i++, sym++) {
+		exp_tab->syms[i].name = sym->name;
+		exp_tab->syms[i].addr = sym->addr;
+		LOG_DBG("sym %p name %s in %p", sym->addr, sym->name, exp_tab->syms + i);
+	}
+
+	return 0;
+}
+
 static int llext_copy_symbols(struct llext_loader *ldr, struct llext *ext)
 {
 	size_t ent_size = ldr->sects[LLEXT_SECT_SYMTAB].sh_entsize;
@@ -624,6 +661,8 @@ static int llext_link(struct llext_loader *ldr, struct llext *ext, bool do_local
 		} else if (strcmp(name, ".rel.data") == 0 ||
 			   strcmp(name, ".rela.data") == 0) {
 			loc = (uintptr_t)ext->mem[LLEXT_MEM_DATA];
+		} else if (strcmp(name, ".rel.exported_sym") == 0) {
+			loc = (uintptr_t)ext->mem[LLEXT_MEM_EXPORT];
 		} else if (strcmp(name, ".rela.plt") == 0 ||
 			   strcmp(name, ".rela.dyn") == 0) {
 			llext_link_plt(ldr, ext, &shdr, do_local);
@@ -679,7 +718,8 @@ static int llext_link(struct llext_loader *ldr, struct llext *ext, bool do_local
 						name, rel.r_offset, shdr.sh_link);
 					return -ENODATA;
 				}
-			} else if (ELF_ST_TYPE(sym.st_info) == STT_SECTION) {
+			} else if (ELF_ST_TYPE(sym.st_info) == STT_SECTION ||
+				   ELF_ST_TYPE(sym.st_info) == STT_FUNC) {
 				/* Current relocation location holds an offset into the section */
 				link_addr = (uintptr_t)ext->mem[ldr->sect_map[sym.st_shndx]]
 					+ sym.st_value
@@ -719,6 +759,7 @@ static int do_llext_load(struct llext_loader *ldr, struct llext *ext,
 
 	memset(ldr->sects, 0, sizeof(ldr->sects));
 	ldr->sect_cnt = 0;
+	ext->sym_tab.sym_cnt = 0;
 
 	size_t sect_map_sz = ldr->hdr.e_shnum * sizeof(uint32_t);
 
@@ -788,6 +829,12 @@ static int do_llext_load(struct llext_loader *ldr, struct llext *ext,
 		goto out;
 	}
 
+	ret = llext_export_symbols(ldr, ext);
+	if (ret != 0) {
+		LOG_ERR("Failed to export, ret %d", ret);
+		goto out;
+	}
+
 out:
 	k_heap_free(&llext_heap, ldr->sect_map);
 
@@ -798,12 +845,16 @@ out:
 				k_heap_free(&llext_heap, ext->mem[mem_idx]);
 			}
 		}
-		k_heap_free(&llext_heap, ext->sym_tab.syms);
+		k_heap_free(&llext_heap, ext->exp_tab.syms);
 	} else {
 		LOG_DBG("loaded module, .text at %p, .rodata at %p", ext->mem[LLEXT_MEM_TEXT],
 			ext->mem[LLEXT_MEM_RODATA]);
 	}
 
+	ext->sym_tab.sym_cnt = 0;
+	k_heap_free(&llext_heap, ext->sym_tab.syms);
+	ext->sym_tab.syms = NULL;
+
 	return ret;
 }
 
@@ -912,7 +963,7 @@ int llext_unload(struct llext **ext)
 		}
 	}
 
-	k_heap_free(&llext_heap, tmp->sym_tab.syms);
+	k_heap_free(&llext_heap, tmp->exp_tab.syms);
 	k_heap_free(&llext_heap, tmp);
 
 	return 0;
@@ -922,7 +973,7 @@ int llext_call_fn(struct llext *ext, const char *sym_name)
 {
 	void (*fn)(void);
 
-	fn = llext_find_sym(&ext->sym_tab, sym_name);
+	fn = llext_find_sym(&ext->exp_tab, sym_name);
 	if (fn == NULL) {
 		return -EINVAL;
 	}

tests/subsys/llext/hello_world/CMakeLists.txt

@@ -11,6 +11,8 @@ if(CONFIG_ARM)
 
     add_custom_command(OUTPUT ${PROJECT_BINARY_DIR}/hello_world.llext
 	COMMAND ${CMAKE_C_COMPILER} ${CMAKE_C_FLAGS} -c
+		-I ${PROJECT_SOURCE_DIR}/../../../../include
+		-imacros${PROJECT_BINARY_DIR}/../zephyr/include/generated/autoconf.h
 		-o ${PROJECT_BINARY_DIR}/hello_world.llext
 		${PROJECT_SOURCE_DIR}/hello_world.c
     )
@@ -19,6 +21,8 @@ elseif(CONFIG_XTENSA)
 
     add_custom_command(OUTPUT ${PROJECT_BINARY_DIR}/hello_world.llext
 	COMMAND ${CMAKE_C_COMPILER} ${CMAKE_C_FLAGS}
+		-I ${PROJECT_SOURCE_DIR}/../../../../include
+		-imacros${PROJECT_BINARY_DIR}/../zephyr/include/generated/autoconf.h
 		-o ${PROJECT_BINARY_DIR}/hello_world.pre.llext
 		${PROJECT_SOURCE_DIR}/hello_world.c
 	COMMAND ${CROSS_COMPILE}strip -R .xt.*

tests/subsys/llext/hello_world/hello_world.c

@@ -12,13 +12,15 @@
  */
 
 #include <stdint.h>
+#include <zephyr/llext/symbol.h>
 
 extern void printk(char *fmt, ...);
 
 static const uint32_t number = 42;
 
-extern void hello_world(void)
+void hello_world(void)
 {
 	printk("hello world\n");
 	printk("A number is %lu\n", number);
 }
+LL_EXTENSION_SYMBOL(hello_world);

tests/subsys/llext/src/test_llext_simple.c

@@ -38,7 +38,7 @@ ZTEST(llext, test_llext_simple)
 
 	zassert_ok(res, "load should succeed");
 
-	const void * const hello_world_fn = llext_find_sym(&ext->sym_tab, "hello_world");
+	const void * const hello_world_fn = llext_find_sym(&ext->exp_tab, "hello_world");
 
 	zassert_not_null(hello_world_fn, "hello_world should be an exported symbol");