Date: Tue, 3 Nov 2015 17:09:48 +0100 (CET)
From: Miroslav Benes <mbenes@suse.cz>
To: Josh Poimboeuf <jpoimboe@redhat.com>
cc: Chris J Arges <chris.j.arges@canonical.com>, live-patching@vger.kernel.org,
        jeyu@redhat.com, Seth Jennings <sjenning@redhat.com>,
        Jiri Kosina <jikos@kernel.org>, Vojtech Pavlik <vojtech@suse.com>,
        linux-api@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] livepatch: old_name.number scheme in livepatch sysfs
 directory
In-Reply-To: <20151103145843.GH27488@treble.redhat.com>
Message-ID: <alpine.LNX.2.00.1511031705340.6257@pobox.suse.cz>
References: <20151102203241.GF27488@treble.redhat.com> <1446505187-28970-1-git-send-email-chris.j.arges@canonical.com> <alpine.LNX.2.00.1511031146070.6257@pobox.suse.cz> <20151103145843.GH27488@treble.redhat.com>
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On Tue, 3 Nov 2015, Josh Poimboeuf wrote:

> On Tue, Nov 03, 2015 at 11:52:08AM +0100, Miroslav Benes wrote:
> > On Mon, 2 Nov 2015, Chris J Arges wrote:
> > 
> > [...]
> > 
> > > +static int klp_get_func_pos_callback(void *data, const char *name,
> > > +				      struct module *mod, unsigned long addr)
> > > +{
> > > +	struct klp_find_arg *args = data;
> > > +
> > > +	if ((mod && !args->objname) || (!mod && args->objname))
> > > +		return 0;
> > > +
> > > +	if (strcmp(args->name, name))
> > > +		return 0;
> > > +
> > > +	if (args->objname && strcmp(args->objname, mod->name))
> > > +		return 0;
> > > +
> > > +	/* on address match, return 1 to break kallsyms_on_each_symbol loop */
> > > +	if (args->addr == addr)
> > > +		return 1;
> > > +
> > > +	/* if we don't match addr, count instance of named symbol */
> > > +	args->count++;
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +static int klp_get_func_pos(struct klp_object *obj, struct klp_func *func)
> > > +{
> > > +	struct klp_find_arg args = {
> > > +		.objname = obj->name,
> > > +		.name = func->old_name,
> > > +		.addr = func->old_addr,
> > > +		.count = 0,
> > > +	};
> > > +
> > > +	mutex_lock(&module_mutex);
> > > +	kallsyms_on_each_symbol(klp_get_func_pos_callback, &args);
> > > +	mutex_unlock(&module_mutex);
> > > +
> > > +	return args.count;
> > > +}
> > > +
> > >  static int klp_init_func(struct klp_object *obj, struct klp_func *func)
> > >  {
> > >  	INIT_LIST_HEAD(&func->stack_node);
> > >  	func->state = KLP_DISABLED;
> > >  
> > >  	return kobject_init_and_add(&func->kobj, &klp_ktype_func,
> > > -				    &obj->kobj, "%s", func->old_name);
> > > +				    &obj->kobj, "%s,%d", func->old_name,
> > > +				    klp_get_func_pos(obj, func));
> > >  }
> > 
> > There is a problem which I missed before. klp_init_func() is called before 
> > klp_find_verify_func_addr() in klp_init_object(). This means that 
> > func->old_addr is either not verified yet or worse it is still 0. This 
> > means that klp_get_func_pos_callback() never returns 1 and is thus called 
> > on each symbol. So if you for example patched cmdline_proc_show the 
> > resulting directory in sysfs would be called cmdline_proc_show,1 because 
> > addr is never matched. Had old_addr been specified the name would have 
> > been probably correct, but not for sure.
> > 
> > This should be fixed as well.
> 
> Even worse, klp_init_func() can be called even if the object hasn't been
> loaded.  In that case there's no way to know what the value of n is, and
> therefore no way to reliably create the sysfs entry.

Ah, right.

> Should we create "func,n" in klp_init_object_loaded() instead of
> klp_init_func()?

So that the function entries in sysfs would be created only when the 
object is loaded? Well, why not, but in that case it could easily confuse 
the user. Object entry would be empty for not loaded object. I would not 
dare to propose to remove such object entries. It would make things worse. 
So maybe we could introduce an attribute in sysfs object entry which would 
say if the object is loaded or not. Or something like that. Hm, we can 
easily mess this up :)

Miroslav
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