Re: livepatch: reuse module loader code to write relocations

+++ Miroslav Benes [13/01/16 10:19 +0100]:

On Fri, 8 Jan 2016, Jessica Yu wrote:

quoted

 static int klp_write_object_relocations(struct module *pmod,
 					struct klp_object *obj)
 {
-	int ret = 0;
-	unsigned long val;
-	struct klp_reloc *reloc;
+	int i, len, ret = 0;
+	char *secname;
+	const char *objname;

 	if (WARN_ON(!klp_is_object_loaded(obj)))
 		return -EINVAL;

-	if (WARN_ON(!obj->relocs))
-		return -EINVAL;
+	objname = klp_is_module(obj) ? obj->name : "vmlinux";

 	module_disable_ro(pmod);
+	/* For each klp rela section for this object */
+	for (i = 1; i < pmod->info->hdr->e_shnum; i++) {
+		if (!(pmod->info->sechdrs[i].sh_flags & SHF_RELA_LIVEPATCH))
+			continue;

One more thing. If the module does not specify it is a live patch module
in modinfo (with MODULE_INFO(livepatch, "Y")), but it is a perfect live
patch module otherwise (it calls klp_register_patch in its init function),
the kernel crashes here. pmod->info is not initialized at all. This should
be fixed. Perhaps the easiest would be to call
klp_write_object_relocations() in klp_init_object_loaded() only if
is_livepatch_module() returns true. Similar to a check for obj->relocs
before.

Hm yes, that's a problem. To remedy this, I think it makes sense to
require all livepatch modules to identify themselves with the modinfo
attribute, since it is a very simple requirement. If some module calls
klp_register_patch() and it does not have the livepatch attribute,
klp_register_patch() can just return an error. We can call
is_livepatch_module() at the beginning of klp_register_patch(), and
proceed only if the check succeeds, since we'll then know that the
required structures have been properly initialized in the module
loader. What do you think?

Jessica