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Date:   Thu, 16 May 2019 19:23:03 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     "Tobin C. Harding" <me@...in.cc>
Cc:     "Tobin C. Harding" <tobin@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] kobject: Clean up allocated memory on failure

On Thu, May 16, 2019 at 10:01:23PM +1000, Tobin C. Harding wrote:
> On Thu, May 16, 2019 at 08:40:29AM +0200, Greg Kroah-Hartman wrote:
> > On Thu, May 16, 2019 at 10:07:16AM +1000, Tobin C. Harding wrote:
> > > Currently kobject_add_varg() calls kobject_set_name_vargs() then returns
> > > the return value of kobject_add_internal().  kobject_set_name_vargs()
> > > allocates memory for the name string.  When kobject_add_varg() returns
> > > an error we do not know if memory was allocated or not.  If we check the
> > > return value of kobject_add_internal() instead of returning it directly
> > > we can free the allocated memory if kobject_add_internal() fails.  Doing
> > > this means that we now know that if kobject_add_varg() fails we do not
> > > have to do any clean up, this benefit goes back up the call chain
> > > meaning that we now do not need to do any cleanup if kobject_del()
> > > fails.  Moving further back (in a theoretical kobject user callchain)
> > > this means we now no longer need to call kobject_put() after calling
> > > kobject_init_and_add(), we can just call kfree() on the enclosing
> > > structure.  This makes the kobject API better follow the principle of
> > > least surprise.
> > > 
> > > Check return value of kobject_add_internal() and free previously
> > > allocated memory on failure.
> > > 
> > > Signed-off-by: Tobin C. Harding <tobin@...nel.org>
> > > ---
> > > 
> > > Hi Greg,
> > > 
> > > Pretty excited by this one, if this is correct it means that kobject
> > > initialisation code, in the error path, can now use either kobject_put()
> > > (to trigger the release method) OR kfree().  This means most of the
> > > call sites of kobject_init_and_add() will get fixed for free!
> > > 
> > > I've been wrong before so I'll state here that this is based on the
> > > assumption that kobject_init() does nothing that causes leaked memory.
> > > This is _not_ what the function docs in kobject.c say but it _is_ what
> > > the code seems to say since kobject_init() does nothing except
> > > initialise kobject data member values?  Or have I got the dog by the
> > > tail?
> > 
> > I think you are correct here.  In looking at the code paths, all should
> > be good and safe.
> > 
> > But, if you use your patch, then you have to call kfree, and you can not
> > call kobject_put(), otherwise kfree_const() will be called twice on the
> > same pointer, right?  So you will have to audit the kernel and change
> > everything again :)
> 
> Oh my bad, I got so excited by this I read the 'if (name) {' in kobject
> to be guarding the double call to kfree_const(), which clearly it doesn't.
> 
> > Or, maybe this patch would prevent that:
> > 
> > 
> > diff --git a/lib/kobject.c b/lib/kobject.c
> > index f2ccdbac8ed9..03cdec1d450a 100644
> > --- a/lib/kobject.c
> > +++ b/lib/kobject.c
> > @@ -387,7 +387,14 @@ static __printf(3, 0) int kobject_add_varg(struct kobject *kobj,
> >  		return retval;
> >  	}
> >  	kobj->parent = parent;
> > -	return kobject_add_internal(kobj);
> > +
> > +	retval = kobject_add_internal(kobj);
> > +	if (retval && !is_kernel_rodata((unsigned long)(kobj->name))) {
> > +		kfree_const(kobj->name);
> > +		kobj->name = NULL;
> > +	}
> > +
> > +	return retval;
> >  }
> >
> >  /**
> > 
> > 
> > But that feels like a huge hack to me.
> 
> I agree, does the job but too ugly.
> 
> > I think, to be safe, we should
> > keep the existing lifetime rules, as it mirrors what happens with
> > 'struct device', and that is what people _should_ be using, not "raw"
> > kobjects if at all possible.
> 
> Oh, I wasn't seeing this through the eyes of a driver developer, perhaps
> I should have started in drivers/ not in fs/ 

That's next on your list :)

Keep up the great work,

greg k-h

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