linux-kernel - Re: [PATCH v2] virtio_blk: Fix a slient kernel panic

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Message-ID: <20160718160421.GA20047@huangminfeis-MacBook-Pro.local>
Date:	Tue, 19 Jul 2016 00:18:32 +0800
From:	Minfei Huang <mnfhuang@...il.com>
To:	Cornelia Huck <cornelia.huck@...ibm.com>
Cc:	Minfei Huang <mnfhuang@...il.com>, mst@...hat.com,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, fanc.fnst@...fujitsu.com,
	Minfei Huang <minfei.hmf@...baba-inc.com>
Subject: Re: [PATCH v2] virtio_blk: Fix a slient kernel panic

On 07/18/16 at 05:21P, Cornelia Huck wrote:
> On Mon, 18 Jul 2016 22:01:29 +0800
> Minfei Huang <mnfhuang@...il.com> wrote:
> > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > index 42758b5..d920512 100644
> > --- a/drivers/block/virtio_blk.c
> > +++ b/drivers/block/virtio_blk.c
> > @@ -381,9 +381,9 @@ static int init_vq(struct virtio_blk *vblk)
> >  {
> >  	int err = 0;
> >  	int i;
> > -	vq_callback_t **callbacks;
> > -	const char **names;
> > -	struct virtqueue **vqs;
> > +	vq_callback_t **callbacks = NULL;
> > +	const char **names = NULL;
> > +	struct virtqueue **vqs = NULL;
> 
> If you init the variables to NULL anyway...

Hi, Cornelia.

Thanks for reviewing this patch.

Seems there is no need to init these variables to NULL. I will remove
them laster.

> 
> >  	unsigned short num_vqs;
> >  	struct virtio_device *vdev = vblk->vdev;
> > 
> > @@ -394,22 +394,16 @@ static int init_vq(struct virtio_blk *vblk)
> >  		num_vqs = 1;
> > 
> 
> ...just do
> 
> err = -ENOMEM;
> 
> here and...
> 
> >  	vblk->vqs = kmalloc(sizeof(*vblk->vqs) * num_vqs, GFP_KERNEL);
> > -	if (!vblk->vqs) {
> > -		err = -ENOMEM;
> > -		goto out;
> > -	}
> > +	if (!vblk->vqs)
> > +		return -ENOMEM;
> > 
> >  	names = kmalloc(sizeof(*names) * num_vqs, GFP_KERNEL);
> > -	if (!names)
> > -		goto err_names;
> > -
> >  	callbacks = kmalloc(sizeof(*callbacks) * num_vqs, GFP_KERNEL);
> > -	if (!callbacks)
> > -		goto err_callbacks;
> > -
> >  	vqs = kmalloc(sizeof(*vqs) * num_vqs, GFP_KERNEL);
> > -	if (!vqs)
> > -		goto err_vqs;
> > +	if (!names || !callbacks || !vqs) {
> > +		err = -ENOMEM;
> > +		goto out;
> > +	}
> 
> ...you could use the
> 
> foo = kmalloc(...);
> if (!foo)
> 	goto out;
> 
> sequence in any case. This avoids trying again and again if e.g. the
> names allocation already failed.

For this implementation, I have referred others which calls
vdev->config->find_vqs as well. Yes, this continues trying to allocate
memory, although memory allocation failed before.

> 
> Alternatively, you should be fine if you don't init the variables to
> NULL: The code is now either taking an early exit or setting all of the
> variables anyway.
> 

It's a big change if we refactor the helper ->find_vqs, since other
devices also call it.

Thanks
Minfei