llext: add support for relocatable objects on Xtensa

Some toolchains cannot create shared objects for Xtensa, with them we
have to use relocatable objects. Add support for them to llext.

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

arch/xtensa/core/elf.c

@@ -27,19 +27,34 @@ LOG_MODULE_DECLARE(llext);
  * supporting modules must implement this.
  */
 void arch_elf_relocate_local(struct llext_loader *ldr, struct llext *ext,
-			     elf_rela_t *rel, size_t got_offset)
+			     const elf_rela_t *rel, const elf_sym_t *sym, size_t got_offset)
 {
 	uint8_t *text = ext->mem[LLEXT_MEM_TEXT];
 	int type = ELF32_R_TYPE(rel->r_info);
+	elf_word *got_entry = (elf_word *)(text + got_offset);
+	uintptr_t sh_addr;
 
-	if (type == R_XTENSA_RELATIVE) {
+	if (ELF_ST_TYPE(sym->st_info) == STT_SECTION) {
-		elf_word ptr_offset = *(elf_word *)(text + got_offset);
+		elf_shdr_t *shdr = llext_peek(ldr, ldr->hdr.e_shoff +
-
+					      sym->st_shndx * ldr->hdr.e_shentsize);
-		LOG_DBG("relocation type %u offset %#x value %#x",
+		sh_addr = shdr->sh_addr;
-			type, got_offset, ptr_offset);
+	} else {
-
+		sh_addr = ldr->sects[LLEXT_MEM_TEXT].sh_addr;
-		/* Relocate a local symbol: Xtensa specific */
-		*(elf_word *)(text + got_offset) = (elf_word)(text + ptr_offset -
-							      ldr->sects[LLEXT_MEM_TEXT].sh_addr);
 	}
+
+	switch (type) {
+	case R_XTENSA_RELATIVE:
+		/* Relocate a local symbol: Xtensa specific */
+		*got_entry += (uintptr_t)text - sh_addr;
+		break;
+	case R_XTENSA_32:
+		*got_entry += sh_addr;
+		break;
+	default:
+		LOG_DBG("unsupported relocation type %u", type);
+
+		return;
+	}
+
+	LOG_DBG("relocation to %#x type %u at %p", *got_entry, type, (void *)got_entry);
 }

include/zephyr/llext/llext.h

@@ -222,10 +222,11 @@ ssize_t llext_find_section(struct llext_loader *loader, const char *search_name)
  * @param[in] loader Extension loader data and context
  * @param[in] ext Extension to call function in
  * @param[in] rel Relocation data provided by elf
+ * @param[in] sym Corresponding symbol table entry
  * @param[in] got_offset Offset within a relocation table
  */
 void arch_elf_relocate_local(struct llext_loader *loader, struct llext *ext,
-			     elf_rela_t *rel, size_t got_offset);
+			     const elf_rela_t *rel, const elf_sym_t *sym, size_t got_offset);
 
 /**
  * @}

include/zephyr/llext/loader.h

@@ -68,6 +68,9 @@ struct llext_loader {
 	 */
 	void *(*peek)(struct llext_loader *ldr, size_t pos);
 
+	/** Total calculated .data size for relocatable extensions */
+	size_t prog_data_size;
+
 	/** @cond ignore */
 	elf_ehdr_t hdr;
 	elf_shdr_t sects[LLEXT_MEM_COUNT];

subsys/llext/llext.c

@@ -32,7 +32,7 @@ static sys_slist_t _llext_list = SYS_SLIST_STATIC_INIT(&_llext_list);
 
 static struct k_mutex llext_lock = Z_MUTEX_INITIALIZER(llext_lock);
 
-ssize_t llext_find_section(struct llext_loader *ldr, const char *search_name)
+static elf_shdr_t *llext_section_by_name(struct llext_loader *ldr, const char *search_name)
 {
 	elf_shdr_t *shdr;
 	unsigned int i;
@@ -44,7 +44,7 @@ ssize_t llext_find_section(struct llext_loader *ldr, const char *search_name)
 		shdr = llext_peek(ldr, pos);
 		if (!shdr) {
 			/* The peek() method isn't supported */
-			return -EOPNOTSUPP;
+			return NULL;
 		}
 
 		const char *name = llext_peek(ldr,
@@ -52,11 +52,18 @@ ssize_t llext_find_section(struct llext_loader *ldr, const char *search_name)
 					      shdr->sh_name);
 
 		if (!strcmp(name, search_name)) {
-			return shdr->sh_offset;
+			return shdr;
 		}
 	}
 
-	return -ENOENT;
+	return NULL;
+}
+
+ssize_t llext_find_section(struct llext_loader *ldr, const char *search_name)
+{
+	elf_shdr_t *shdr = llext_section_by_name(ldr, search_name);
+
+	return shdr ? shdr->sh_offset : -ENOENT;
 }
 
 /*
@@ -214,9 +221,12 @@ static int llext_map_sections(struct llext_loader *ldr, struct llext *ext)
 {
 	int i, ret;
 	size_t pos;
-	elf_shdr_t shdr;
+	elf_shdr_t shdr, rodata = {.sh_addr = ~0},
+		high_shdr = {.sh_offset = 0}, low_shdr = {.sh_offset = ~0};
 	const char *name;
 
+	ldr->sects[LLEXT_MEM_RODATA].sh_size = 0;
+
 	for (i = 0, pos = ldr->hdr.e_shoff;
 	     i < ldr->hdr.e_shnum;
 	     i++, pos += ldr->hdr.e_shentsize) {
@@ -230,12 +240,38 @@ static int llext_map_sections(struct llext_loader *ldr, struct llext *ext)
 			return ret;
 		}
 
+		/* Identify the lowest and the highest data sections */
+		if (!(shdr.sh_flags & SHF_EXECINSTR) &&
+		    shdr.sh_type == SHT_PROGBITS) {
+			if (shdr.sh_offset > high_shdr.sh_offset) {
+				high_shdr = shdr;
+			}
+			if (shdr.sh_offset < low_shdr.sh_offset) {
+				low_shdr = shdr;
+			}
+		}
+
 		name = llext_string(ldr, ext, LLEXT_MEM_SHSTRTAB, shdr.sh_name);
 
 		LOG_DBG("section %d name %s", i, name);
 
 		enum llext_mem mem_idx;
 
+		/*
+		 * .rodata section is optional. If there isn't one, use the
+		 * first read-only data section
+		 */
+		if (shdr.sh_addr && !(shdr.sh_flags & (SHF_WRITE | SHF_EXECINSTR)) &&
+		    shdr.sh_addr < rodata.sh_addr) {
+			rodata = shdr;
+			LOG_DBG("rodata: select %#zx name %s", (size_t)shdr.sh_addr, name);
+		}
+
+		/*
+		 * Keep in mind, that when using relocatable (partially linked)
+		 * objects, ELF segments aren't created, so ldr->sect_map[] and
+		 * ldr->sects[] don't contain all the sections
+		 */
 		if (strcmp(name, ".text") == 0) {
 			mem_idx = LLEXT_MEM_TEXT;
 		} else if (strcmp(name, ".data") == 0) {
@@ -255,6 +291,13 @@ static int llext_map_sections(struct llext_loader *ldr, struct llext *ext)
 		ldr->sect_map[i] = mem_idx;
 	}
 
+	ldr->prog_data_size = high_shdr.sh_size + high_shdr.sh_offset - low_shdr.sh_offset;
+
+	/* No verbatim .rodata, use an automatically selected one */
+	if (!ldr->sects[LLEXT_MEM_RODATA].sh_size) {
+		ldr->sects[LLEXT_MEM_RODATA] = rodata;
+	}
+
 	return 0;
 }
 
@@ -556,12 +599,12 @@ static size_t llext_file_offset(struct llext_loader *ldr, size_t offset)
 }
 
 __weak void arch_elf_relocate_local(struct llext_loader *ldr, struct llext *ext,
-				    elf_rela_t *rel, size_t got_offset)
+				    const elf_rela_t *rel, const elf_sym_t *sym, size_t got_offset)
 {
 }
 
 static void llext_link_plt(struct llext_loader *ldr, struct llext *ext,
-			   elf_shdr_t *shdr, bool do_local)
+			   elf_shdr_t *shdr, bool do_local, elf_shdr_t *tgt)
 {
 	unsigned int sh_cnt = shdr->sh_size / shdr->sh_entsize;
 	/*
@@ -613,25 +656,36 @@ static void llext_link_plt(struct llext_loader *ldr, struct llext *ext,
 		}
 
 		uint32_t stt = ELF_ST_TYPE(sym_tbl.st_info);
-		uint32_t stb = ELF_ST_BIND(sym_tbl.st_info);
 
-		if (stt != STT_FUNC && (stt != STT_NOTYPE || sym_tbl.st_shndx != SHN_UNDEF)) {
+		if (stt != STT_FUNC &&
+		    stt != STT_SECTION &&
+		    (stt != STT_NOTYPE || sym_tbl.st_shndx != SHN_UNDEF)) {
 			continue;
 		}
 
 		const char *name = llext_string(ldr, ext, LLEXT_MEM_STRTAB, sym_tbl.st_name);
+
 		/*
 		 * Both r_offset and sh_addr are addresses for which the extension
 		 * has been built.
 		 */
-		size_t got_offset = llext_file_offset(ldr, rela.r_offset) -
+		size_t got_offset;
-			ldr->sects[LLEXT_MEM_TEXT].sh_offset;
 
+		if (tgt) {
+			got_offset = rela.r_offset + tgt->sh_offset -
+				ldr->sects[LLEXT_MEM_TEXT].sh_offset;
+		} else {
+			got_offset = llext_file_offset(ldr, rela.r_offset) -
+				ldr->sects[LLEXT_MEM_TEXT].sh_offset;
+		}
+
+		uint32_t stb = ELF_ST_BIND(sym_tbl.st_info);
 		const void *link_addr;
 
 		switch (stb) {
 		case STB_GLOBAL:
 			link_addr = llext_find_sym(NULL, name);
+
 			if (!link_addr)
 				link_addr = llext_find_sym(&ext->sym_tab, name);
 
@@ -648,9 +702,9 @@ static void llext_link_plt(struct llext_loader *ldr, struct llext *ext,
 			/* Resolve the symbol */
 			*(const void **)(text + got_offset) = link_addr;
 			break;
-		case  STB_LOCAL:
+		case STB_LOCAL:
 			if (do_local) {
-				arch_elf_relocate_local(ldr, ext, &rela, got_offset);
+				arch_elf_relocate_local(ldr, ext, &rela, &sym_tbl, got_offset);
 			}
 		}
 
@@ -697,8 +751,7 @@ static int llext_link(struct llext_loader *ldr, struct llext *ext, bool do_local
 
 		name = llext_string(ldr, ext, LLEXT_MEM_SHSTRTAB, shdr.sh_name);
 
-		if (strcmp(name, ".rel.text") == 0 ||
+		if (strcmp(name, ".rel.text") == 0) {
-		    strcmp(name, ".rela.text") == 0) {
 			loc = (uintptr_t)ext->mem[LLEXT_MEM_TEXT];
 		} else if (strcmp(name, ".rel.bss") == 0 ||
 			   strcmp(name, ".rela.bss") == 0) {
@@ -706,14 +759,19 @@ static int llext_link(struct llext_loader *ldr, struct llext *ext, bool do_local
 		} else if (strcmp(name, ".rel.rodata") == 0 ||
 			   strcmp(name, ".rela.rodata") == 0) {
 			loc = (uintptr_t)ext->mem[LLEXT_MEM_RODATA];
-		} else if (strcmp(name, ".rel.data") == 0 ||
+		} 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);
+			llext_link_plt(ldr, ext, &shdr, do_local, NULL);
+			continue;
+		} else if (strncmp(name, ".rela", 5) == 0 && strlen(name) > 5) {
+			elf_shdr_t *tgt = llext_section_by_name(ldr, name + 5);
+
+			if (tgt)
+				llext_link_plt(ldr, ext, &shdr, do_local, tgt);
 			continue;
 		}