linux-kernel - Re: [PATCH 2/3] module: Add text_mutex lockdep assertions for page attribute changes

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Message-ID: <20190614140457.urqjlosesvdtmiia@pathway.suse.cz>
Date:   Fri, 14 Jun 2019 16:04:57 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Jessica Yu <jeyu@...nel.org>, Steven Rostedt <rostedt@...dmis.org>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        Johannes Erdfelt <johannes@...felt.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 2/3] module: Add text_mutex lockdep assertions for page
 attribute changes

On Thu 2019-06-13 20:07:23, Josh Poimboeuf wrote:
> External callers of the module page attribute change functions now need
> to have the text_mutex.  Enforce that with lockdep assertions.
> 
> diff --git a/kernel/module.c b/kernel/module.c
> index 6e6712b3aaf5..e43a90ee2d23 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3519,7 +3534,7 @@ static noinline int do_init_module(struct module *mod)
>  	/* Switch to core kallsyms now init is done: kallsyms may be walking! */
>  	rcu_assign_pointer(mod->kallsyms, &mod->core_kallsyms);
>  #endif
> -	module_enable_ro(mod, true);
> +	__module_enable_ro(mod, true);

This one must be called under text_mutex. Otherwise it might get
called when ftrace is in the middle of modifying the functions.

It should be enough to take text_mutex right around this call.
It will prevent making the code ro when ftrace is doing
the modification. It safe also the other way.
set_all_modules_text_ro() does not call frob_ro_after_init().
Therefore ftrace could not make the after_init section RO prematurely.

>  	mod_tree_remove_init(mod);
>  	module_arch_freeing_init(mod);
>  	mod->init_layout.base = NULL;
> @@ -3626,8 +3641,8 @@ static int complete_formation(struct module *mod, struct load_info *info)
>  	/* This relies on module_mutex for list integrity. */
>  	module_bug_finalize(info->hdr, info->sechdrs, mod);
>  
> -	module_enable_ro(mod, false);
> -	module_enable_nx(mod);
> +	__module_enable_ro(mod, false);
> +	__module_enable_nx(mod);

This one is OK. It is called when the module is in
MODULE_STATE_UNFORMED. Therefore it is ignored by ftrace.
The module state is manipulated and checked under module_mutex.

Best Regards,
Petr