linux-kernel - Re: [PATCH v8 7/8] livepatch: Correctly handle atomic replace for not yet loaded modules

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Date:   Tue, 6 Mar 2018 15:10:38 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Joe Lawrence <joe.lawrence@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>, Jessica Yu <jeyu@...nel.org>,
        Evgenii Shatokhin <eshatokhin@...tuozzo.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 7/8] livepatch: Correctly handle atomic replace for
 not yet loaded modules

On Mon 2018-03-05 10:54:16, Miroslav Benes wrote:
> On Fri, 2 Mar 2018, Joe Lawrence wrote:
> 
> > On 03/01/2018 05:28 AM, Petr Mladek wrote:
> > > On Thu 2018-02-22 22:00:28, Miroslav Benes wrote:
> > >> On Wed, 21 Feb 2018, Petr Mladek wrote:
> > >>> This patch allows the late initialization.
> > >>>
> > >>> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> > >>> index ad508a86b2f9..da1438d47d83 100644
> > >>> --- a/kernel/livepatch/core.c
> > >>> +++ b/kernel/livepatch/core.c
> > >>> @@ -984,7 +988,12 @@ static void klp_free_patch(struct klp_patch *patch)
> > >>>  
> > >>>  static int klp_init_func(struct klp_object *obj, struct klp_func *func)
> > >>>  {
> > >>> -	if (!func->old_name || !func->new_func)
> > >>> +	if (!func->old_name)
> > >>> +		return -EINVAL;
> > >>> +
> > >>> +	/* NOPs do not know the address until the patched module is loaded. */
> > >>> +	if (!func->new_func &&
> > >>> +	    (!klp_is_func_type(func, KLP_FUNC_NOP) || klp_is_object_loaded(obj)))
> > >>>  		return -EINVAL;
> > >>
> > >> If we changed the order of klp_init_func() and klp_init_object_loaded() 
> > >> calls in klp_init_object(), the hunk would not be needed. Is that correct? 
> > > 
> > > Not really. klp_init_object_loaded() would set func->new_func only
> > > when the object was loaded. But we want to proceed here and create
> > > the kobject for NOPs even when it was not loaded.

Anyway, the above check have to be updated after we removed the
redundant func->new_func assignment in klp_alloc_func_nop().

func->new_func is always NULL for NOPs in klp_init_func().
It is set later in klp_init_object_loaded().

I am going to use the following check in v10:

	/*
	 * NOPs get the address later. The the patched module must be loaded,
	 * see klp_init_object_loaded().
	 */
	if (!func->new_func && !klp_is_func_type(func, KLP_FUNC_NOP))
		return -EINVAL;

> > > 
> > >>>  	INIT_LIST_HEAD(&func->stack_node);
> > >>> @@ -1039,6 +1048,9 @@ static int klp_init_object_loaded(struct klp_patch *patch,
> > >>>  			return -ENOENT;
> > >>>  		}
> > >>>  
> > >>> +		if (klp_is_func_type(func, KLP_FUNC_NOP))
> > >>> +			func->new_func = (void *)func->old_addr;
> > >>
> > >> Is there a reason why you left the same assignment in 
> > >> klp_alloc_func_nop()? This one is enough, no?
> > > 
> > > Good point! I am going to replace the obsolete assignment
> > > with a comment in v8.
> > 
> > Hi Petr, Miroslav,
> > 
> > I don't think the assignment in klp_alloc_func_nop() was necessarily
> > redundant.  It was removed in v9 and that breaks my atomic replace
> > sample module when I try to load it.  (Perhaps the sample patch has
> > issues, but here are my debug notes):

Sigh, I hate "trivial" last minute clean ups ;-)


> Ok, so it was not redundant. Or... I think it should be redundant, but we 
> need to define the if condition in klp_init_func() properly.

Exactly, see above.


> > I think this problem is contained to only replacement patches that need
> > the nop-revert feature... if the replacement patch provides a new
> > function definition, then it shouldn't be affected.
> > 
> > Man, we need a regression test suite for all these cases :)

It would be great...

> Any volunteer?

?

Best Regards,
Petr