llext: fix symbol exporting for ET_REL

When building relocatable extensions, exported symbol names might end
up in a separate section (in my case ".rodata.str1.1") for which
storage address don't match section address. To access those names
addresses have to be relalculated.

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

Author: lyakh

subsys/llext/llext_link.c

@@ -48,7 +48,7 @@ __weak void arch_elf_relocate_global(struct llext_loader *ldr, struct llext *ext
  * Find the memory region containing the supplied offset and return the
  * corresponding file offset
  */
-static size_t llext_file_offset(struct llext_loader *ldr, size_t offset)
+ssize_t llext_file_offset(struct llext_loader *ldr, uintptr_t offset)
 {
 	unsigned int i;
 
@@ -59,7 +59,7 @@ static size_t llext_file_offset(struct llext_loader *ldr, size_t offset)
 		}
 	}
 
-	return offset;
+	return -ENOEXEC;
 }
 
 /*
@@ -224,7 +224,15 @@ static void llext_link_plt(struct llext_loader *ldr, struct llext *ext, elf_shdr
 			rel_addr += rela.r_offset + tgt->sh_offset;
 		} else {
 			/* Shared / dynamically linked ELF */
-			rel_addr += llext_file_offset(ldr, rela.r_offset);
+			ssize_t offset = llext_file_offset(ldr, rela.r_offset);
+
+			if (offset < 0) {
+				LOG_ERR("Offset %#zx not found in ELF, trying to continue",
+					(size_t)rela.r_offset);
+				continue;
+			}
+
+			rel_addr += offset;
 		}
 
 		uint32_t stb = ELF_ST_BIND(sym.st_info);

subsys/llext/llext_load.c

@@ -517,7 +517,8 @@ 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)
+static int llext_export_symbols(struct llext_loader *ldr, struct llext *ext,
+				const struct llext_load_param *ldr_parm)
 {
 	struct llext_symtable *exp_tab = &ext->exp_tab;
 	struct llext_symbol *sym;
@@ -545,7 +546,25 @@ static int llext_export_symbols(struct llext_loader *ldr, struct llext *ext)
 	}
 
 	for (i = 0; i < exp_tab->sym_cnt; i++, sym++) {
-		exp_tab->syms[i].name = sym->name;
+		/*
+		 * Offsets in objects, built for pre-defined addresses have to
+		 * be translated to memory locations for symbol name access
+		 * during dependency resolution.
+		 */
+		const char *name = NULL;
+
+		if (ldr_parm && ldr_parm->pre_located) {
+			ssize_t name_offset = llext_file_offset(ldr, (uintptr_t)sym->name);
+
+			if (name_offset > 0) {
+				name = llext_peek(ldr, name_offset);
+			}
+		}
+		if (!name) {
+			name = sym->name;
+		}
+
+		exp_tab->syms[i].name = name;
 		exp_tab->syms[i].addr = sym->addr;
 		LOG_DBG("sym %p name %s", sym->addr, sym->name);
 	}
@@ -730,7 +749,7 @@ int do_llext_load(struct llext_loader *ldr, struct llext *ext,
 		}
 	}
 
-	ret = llext_export_symbols(ldr, ext);
+	ret = llext_export_symbols(ldr, ext, ldr_parm);
 	if (ret != 0) {
 		LOG_ERR("Failed to export, ret %d", ret);
 		goto out;

subsys/llext/llext_priv.h

@@ -65,6 +65,7 @@ static inline const char *llext_string(struct llext_loader *ldr, struct llext *e
 
 int llext_link(struct llext_loader *ldr, struct llext *ext,
 	       const struct llext_load_param *ldr_parm);
+ssize_t llext_file_offset(struct llext_loader *ldr, uintptr_t offset);
 void llext_dependency_remove_all(struct llext *ext);
 
 #endif /* ZEPHYR_SUBSYS_LLEXT_PRIV_H_ */