Re: [PATCH v2 1/4] mm/mempolicy: Fix memory leaks in mempolicy_sysfs_init()

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Thu, 13 Mar 2025 11:52:18 -0400
Gregory Price <gourry@gourry.net> wrote:

> On Thu, Mar 13, 2025 at 03:31:38PM +0900, Rakie Kim wrote:
> > > Is this correct? If kobject_init_and_add fails, from other examples we
> > > need only free the mempolicy_kobj - because it failed to initialize and
> > > therefore should not have any references.  I think this causes an
> > > underflow.  
> > 
> > Regarding the reordering of mempolicy_kobj allocation:
> > 1) In kobject_init_and_add(), kobject_init() is always called, which  
> 
> Quite right, mea culpa.
> 
> > 
> > 2) The release function for mempolicy_kobj is responsible for freeing
> >    associated memory:
> > 
> >    static void mempolicy_kobj_release(struct kobject *kobj)
> >    {
> >        ...
> >        kfree(ngrp->nattrs);
> >        kfree(ngrp);
> >        kfree(kobj);
> >    }
> >   
> 
> I see what you're trying to do now after looking at the free-ordering
> at little closer.
> 
> Lets do the following:
> 
> 1) allocate node_attrs and mempolicy_kobj up front and keep your
>    reordering, this lets us clean up allocations on failure before
>    kobject_init is called
> 
> 2) after this remove all the other code and just let
>    mempolicy_kobj_release clean up node_attrs
> 
> 3) Add a (%d) to the error message to differentiate failures

Given how unlikely (and noisy) a memory allocation failure is,
maybe just drop the printing at all in those paths - allowing
early returns.

The lifetime rules around node_attrs in here are making readability
poor. It is implicitly owned by the mempolicy_kobj, but no direct association.
Maybe just encapsulating the kobject in a structure that contains
this as a [] array at the end.  Then we end up with single allocation of
stuff that is effectively one thing.


> 
> This is a little bit cleaner and is a bit less code. (Not built or
> tested, just a recommendation).
> 
> I'd recommend submitting this patch by itself to mm-stable, since the
> remainder of the patch line changes functionality and this fixes a bug
> in LTS kernels.
> 
> ~Gregory
> 
> ---
> 
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 530e71fe9147..05a410db08b4 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -3541,38 +3541,34 @@ static int __init mempolicy_sysfs_init(void)
>  	int err;
>  	static struct kobject *mempolicy_kobj;
> 
> -	mempolicy_kobj = kzalloc(sizeof(*mempolicy_kobj), GFP_KERNEL);
> -	if (!mempolicy_kobj) {
> +	node_attrs = kcalloc(nr_node_ids, sizeof(struct iw_node_attr *),
> +			     GFP_KERNEL);
> +	if (!node_attrs) {
>  		err = -ENOMEM;
>  		goto err_out;
>  	}
> 
> -	node_attrs = kcalloc(nr_node_ids, sizeof(struct iw_node_attr *),
> -			     GFP_KERNEL);
> -	if (!node_attrs) {
> +	mempolicy_kobj = kzalloc(sizeof(*mempolicy_kobj), GFP_KERNEL);
> +	if (!mempolicy_kobj) {
>  		err = -ENOMEM;
> -		goto mempol_out;
> +		kfree(node_attrs);
> +		goto err_out;
>  	}
> 
>  	err = kobject_init_and_add(mempolicy_kobj, &mempolicy_ktype, mm_kobj,
>  				   "mempolicy");
>  	if (err)
> -		goto node_out;
> +		goto mempol_out;
> 
>  	err = add_weighted_interleave_group(mempolicy_kobj);
> -	if (err) {
> -		pr_err("mempolicy sysfs structure failed to initialize\n");
> -		kobject_put(mempolicy_kobj);
> -		return err;
> -	}
> +	if (err)
> +		goto mempol_out;
> 
> -	return err;
> -node_out:
> -	kfree(node_attrs);
> +	return 0;
>  mempol_out:
> -	kfree(mempolicy_kobj);
> +	kobject_put(mempolicy_kobj);
>  err_out:
> -	pr_err("failed to add mempolicy kobject to the system\n");
> +	pr_err("mempolicy sysfs structure failed to initialize (%d)\n", err);
>  	return err;
>  }
> 
>