livepatch: Fix having __klp_objects relics in non-livepatch modules

The linker script scripts/module.lds.S specifies that all input
__klp_objects sections should be consolidated into an output section of
the same name, and start/stop symbols should be created to enable
scripts/livepatch/init.c to locate this data.

This start/stop pattern is not ideal for modules because the symbols are
created even if no __klp_objects input sections are present.
Consequently, a dummy __klp_objects section also appears in the
resulting module. This unnecessarily pollutes non-livepatch modules.

Instead, since modules are relocatable files, the usual method for
locating consolidated data in a module is to read its section table.
This approach avoids the aforementioned problem.

The klp_modinfo already stores a copy of the entire section table with
the final addresses. Introduce a helper function that
scripts/livepatch/init.c can call to obtain the location of the
__klp_objects section from this data.

Fixes: dd590d4d57eb ("objtool/klp: Introduce klp diff subcommand for diffing object files")
Signed-off-by: Petr Pavlu <[e-mail]>
Acked-by: Joe Lawrence <[e-mail]>
Acked-by: Miroslav Benes <[e-mail]>
Reviewed-by: Aaron Tomlin <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Josh Poimboeuf <[e-mail]>

include/linux/livepatch.h

···
 int klp_module_coming(struct module *mod);
 void klp_module_going(struct module *mod);
 
+void *klp_find_section_by_name(const struct module *mod, const char *name,
+			       size_t *sec_size);
+
 void klp_copy_process(struct task_struct *child);
 void klp_update_patch_state(struct task_struct *task);
 

kernel/livepatch/core.c

···
 	mutex_unlock(&klp_mutex);
 }
 
+void *klp_find_section_by_name(const struct module *mod, const char *name,
+			       size_t *sec_size)
+{
+	struct klp_modinfo *info = mod->klp_info;
+
+	for (int i = 1; i < info->hdr.e_shnum; i++) {
+		Elf_Shdr *shdr = &info->sechdrs[i];
+
+		if (!strcmp(info->secstrings + shdr->sh_name, name)) {
+			*sec_size = shdr->sh_size;
+			return (void *)shdr->sh_addr;
+		}
+	}
+
+	*sec_size = 0;
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(klp_find_section_by_name);
+
 static int __init klp_init(void)
 {
 	klp_root_kobj = kobject_create_and_add("livepatch", kernel_kobj);

scripts/livepatch/init.c

···
 #include <linux/slab.h>
 #include <linux/livepatch.h>
 
-extern struct klp_object_ext __start_klp_objects[];
-extern struct klp_object_ext __stop_klp_objects[];
-
 static struct klp_patch *patch;
 
 static int __init livepatch_mod_init(void)
 {
+	struct klp_object_ext *obj_exts;
+	size_t obj_exts_sec_size;
 	struct klp_object *objs;
 	unsigned int nr_objs;
 	int ret;
 
-	nr_objs = __stop_klp_objects - __start_klp_objects;
-
+	obj_exts = klp_find_section_by_name(THIS_MODULE, "__klp_objects",
+					    &obj_exts_sec_size);
+	nr_objs = obj_exts_sec_size / sizeof(*obj_exts);
 	if (!nr_objs) {
 		pr_err("nothing to patch!\n");
 		ret = -EINVAL;
···
 	}
 
 	for (int i = 0; i < nr_objs; i++) {
-		struct klp_object_ext *obj_ext = __start_klp_objects + i;
+		struct klp_object_ext *obj_ext = obj_exts + i;
 		struct klp_func_ext *funcs_ext = obj_ext->funcs;
 		unsigned int nr_funcs = obj_ext->nr_funcs;
 		struct klp_func *funcs = objs[i].funcs;
···
 
 static void __exit livepatch_mod_exit(void)
 {
-	unsigned int nr_objs;
+	struct klp_object *obj;
 
-	nr_objs = __stop_klp_objects - __start_klp_objects;
-
-	for (int i = 0; i < nr_objs; i++)
-		kfree(patch->objs[i].funcs);
+	klp_for_each_object_static(patch, obj)
+		kfree(obj->funcs);
 
 	kfree(patch->objs);
 	kfree(patch);

scripts/module.lds.S

···
 	__patchable_function_entries : { *(__patchable_function_entries) }
 
 	__klp_funcs		0: ALIGN(8) { KEEP(*(__klp_funcs)) }
-
-	__klp_objects		0: ALIGN(8) {
-		__start_klp_objects = .;
-		KEEP(*(__klp_objects))
-		__stop_klp_objects = .;
-	}
+	__klp_objects		0: ALIGN(8) { KEEP(*(__klp_objects)) }
 
 #ifdef CONFIG_ARCH_USES_CFI_TRAPS
 	__kcfi_traps		: { KEEP(*(.kcfi_traps)) }