linux-kernel - Re: Bug with paravirt ops and livepatches

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Date:	Fri, 1 Apr 2016 17:01:16 +0200 (CEST)
From:	Jiri Kosina <jikos@...nel.org>
To:	Miroslav Benes <mbenes@...e.cz>
cc:	Chris J Arges <chris.j.arges@...onical.com>, jeyu@...hat.com,
	jpoimboe@...hat.com, eugene.shatokhin@...alab.ru,
	live-patching@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	pmladek@...e.cz
Subject: Re: Bug with paravirt ops and livepatches

On Tue, 29 Mar 2016, Jiri Kosina wrote:

> Agreed; I think we should be safe applying all the alternatives (with 
> paravirt being really just a special case of those) to the coming module 
> at the very last phase; they really are required only during runtime, 
> but nothing else should be depending on them. Right? If anyone is able 
> to come up with and counter-example, please speak up :)

So I have quickly gone through all the architectures that actually do 
overload __weak module_finalize() by their own implementation, and except 
for applying self-modifying code changes and registering unwind tables, 
there doesn't seem to be any relevant heavy-lifting, that'd need to be 
done before relocations have been written.

Is the (completely untested) sort-of-a-patch below a complete rubbish 
(on top of current livepatching.git's for-next)?




diff --git a/kernel/module.c b/kernel/module.c
index 5f71aa6..c003648 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3211,7 +3211,7 @@ int __weak module_finalize(const Elf_Ehdr *hdr,
 	return 0;
 }
 
-static int post_relocation(struct module *mod, const struct load_info *info)
+static void post_relocation(struct module *mod, const struct load_info *info)
 {
 	/* Sort exception table now relocations are done. */
 	sort_extable(mod->extable, mod->extable + mod->num_exentries);
@@ -3222,9 +3222,6 @@ static int post_relocation(struct module *mod, const struct load_info *info)
 
 	/* Setup kallsyms-specific fields. */
 	add_kallsyms(mod, info);
-
-	/* Arch-specific module finalizing. */
-	return module_finalize(info->hdr, info->sechdrs, mod);
 }
 
 /* Is this module of this name done loading?  No locks held. */
@@ -3562,9 +3559,7 @@ static int load_module(struct load_info *info, const char __user *uargs,
 	if (err < 0)
 		goto free_modinfo;
 
-	err = post_relocation(mod, info);
-	if (err < 0)
-		goto free_modinfo;
+	post_relocation(mod, info);
 
 	flush_module_icache(mod);
 
@@ -3589,6 +3584,11 @@ static int load_module(struct load_info *info, const char __user *uargs,
 	if (err)
 		goto bug_cleanup;
 
+	/* Arch-specific module finalizing. */
+	err = module_finalize(info->hdr, info->sechdrs, mod);
+	if (err)
+		goto bug_cleanup;
+
 	/* Module is ready to execute: parsing args may do that. */
 	after_dashes = parse_args(mod->name, mod->args, mod->kp, mod->num_kp,
 				  -32768, 32767, mod,

-- 
Jiri Kosina
SUSE Labs