linux-kernel - Re: [PATCH] et131x: fix allocation failures

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Message-ID: <87y515dy30.fsf@will.lan>
Date:	Fri, 21 Feb 2014 10:00:19 +0800
From:	"Zhao\, Gang" <gamerh2o@...il.com>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	devel@...verdev.osuosl.org, mark.einon@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] et131x: fix allocation failures

On Thu, 2014-02-20 at 17:03:39 +0800, Dan Carpenter wrote:
> On Thu, Feb 20, 2014 at 11:03:45AM +0800, Zhao, Gang wrote:
>> On Wed, 2014-02-19 at 19:43:15 +0800, One Thousand Gnomes wrote:
>> > On Wed, 19 Feb 2014 09:14:19 +0800
>> > "Zhao\, Gang" <gamerh2o@...il.com> wrote:
>> >
>> >> Alan, thanks for resending this patch. But it seems you overlooked
>> >> something we discussed earlier.
>> >> 
>> >> On Mon, 2014-02-17 at 22:13:08 +0800, Alan wrote:
>> >> > We should check the ring allocations don't fail.
>> >> > If we get a fail we need to clean up properly. The allocator assumes the
>> >> > deallocator will be used on failure, but it isn't. Make sure the
>> >> > right deallocator is always called and add a missing check against
>> >> > fbr allocation failure.
>> >> >
>> >> > [v2]: Correct check logic
>> >> >
>> >> > Signed-off-by: Alan Cox <alan@...ux.intel.com>
>> >> > ---
>> >> >  drivers/staging/et131x/et131x.c |    9 +++++++--
>> >> >  1 file changed, 7 insertions(+), 2 deletions(-)
>> >> >
>> >> > diff --git a/drivers/staging/et131x/et131x.c b/drivers/staging/et131x/et131x.c
>> >> > index 6413500..cc600df 100644
>> >> > --- a/drivers/staging/et131x/et131x.c
>> >> > +++ b/drivers/staging/et131x/et131x.c
>> >> > @@ -2124,7 +2124,11 @@ static int et131x_rx_dma_memory_alloc(struct et131x_adapter *adapter)
>> >> >  
>> >> >  	/* Alloc memory for the lookup table */
>> >> >  	rx_ring->fbr[0] = kmalloc(sizeof(struct fbr_lookup), GFP_KERNEL);
>> >> > +	if (rx_ring->fbr[0] == NULL)
>> >> > +		return -ENOMEM;
>> >> >  	rx_ring->fbr[1] = kmalloc(sizeof(struct fbr_lookup), GFP_KERNEL);
>> >> > +	if (rx_ring->fbr[1] == NULL)
>> >> > +		return -ENOMEM;
>> >> 
>> >> Shouldn't rx_ring->fbr[0] be freed when allocation of rx_ring->fbr[1]
>> >> fails ? Or we will leak memory here.
>> >
>> > No.. the tx_dma_memory_free and rx_dma_memory_free functions are
>> > designed to handle incomplete set up. They are now called on incomplete
>> > setup and will clean up all the resources.
>> >
>> 
>> Yes, you are right. By calling {tx, rx}_dma_memory_free the memory will
>> be freed.
>> 
>> But I think a comment is needed here, to make this more clear ? Without
>> proper comment the above code looks a little strange to let one think
>> it's right. :)
>
> No.  We don't need a comment.  If people start adding kfree() calls
> all over the place without thinking then we are already screwed and no
> comment is going to help us.

Hi, I thought this a little more.

AFAIK, most functions deal with this "fail in the middle" allocation
failure themselves. Honestly, relying on the caller to handle this type
of error seems a bad idea to me.

Code reviewer has to check *every* caller of this function to make sure
whether rx_ring->fbr[0] is leaked or not when allocation of
rx_ring->fbr[1] fails.(By examing if the caller called the correct
freeing function when this function returns error) This is just a waste
of time. By freeing rx_ring->fbr[0] in this function the above type of
memory leak can't be happen at the beginning.

So now my suggestion is freeing rx_ring->fbr[0] *and* set the pointer
rx_ring->fbr[0] to NULL when allocation of rx_ring->fbr[1] fails *in*
this function. The freeing function which can handle "fail in the
middle" allocation failure surely can handle this situation correctly,
isn't it ?

>
> regards,
> dan carpenter
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