linux-kernel - Re: [PATCH 3/5] net/mlx4: fix some error handling in mlx4_multi_func

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Date:	Thu, 11 Feb 2016 11:29:43 +0200
From:	Jack Morgenstein <jackm@....mellanox.co.il>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Yishai Hadas <yishaih@....mellanox.co.il>,
	Yishai Hadas <yishaih@...lanox.com>, netdev@...r.kernel.org,
	linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
	"jackm\@mellanox.com" <jackm@...lanox.com>,
	Majd Dibbiny <majd@...lanox.com>
Subject: Re: [PATCH 3/5] net/mlx4: fix some error handling in
 mlx4_multi_func_init()

On Wed, 10 Feb 2016 19:15:20 +0100
Rasmus Villemoes <linux@...musvillemoes.dk> wrote:

> On Wed, Feb 10 2016, Yishai Hadas <yishaih@....mellanox.co.il> wrote:
> 
> >> @@ -2429,7 +2429,7 @@ err_thread:
> >>   	flush_workqueue(priv->mfunc.master.comm_wq);
> >>   	destroy_workqueue(priv->mfunc.master.comm_wq);
> >>   err_slaves:
> >> -	while (--i) {
> >> +	while (i--) {
> >
> > This fix is wrong as it hits the case that i arrived the last value
> > then below code will access to a non valid entry in the array.
> >
> > The expected fix should be:
> > while (--i >= 0)
> >
> 
> Huh? They're completely equivalent (given that i is necessarily
> non-negative before we evaluate the loop condition)

No, they are not equivalent.
if i == the max value (dev->num_slaves) when entering your proposed
while loop, the kfree call index (i) will be out of range!  This can
happen, for example, if the failure occurs downstream from the "i"
for-loop (e.g., if the call to mlx4_init_resource_tracker() fails).

Therefore, we DO require the pre-decrement format.  Therefore, the
one-line fix proposed by Yishai is the correct fix.
>. I don't really
> care either way, but git grep says that 'while (i--)' is 5 times more
> common than 'while (--i >= 0)'.
Not relevant, while (i--) is simply not correct, because of the case
where the for-loop involving i completes successfully and an error
occurs later.

FYI, you also had another bug in your solution -- a double-free when
kzalloc for port 2 fails.  For your code, you should also have reset
s_state->vlan_filter[port] to NULL as shown below:
			for (port = 1; port <= MLX4_MAX_PORTS; port++) {
				struct mlx4_vport_state *admin_vport;
				struct mlx4_vport_state *oper_vport;

				s_state->vlan_filter[port] =
					kzalloc(sizeof(struct
				mlx4_vlan_fltr), GFP_KERNEL);
				if (!s_state->vlan_filter[port]) {
					if (--port) {
 						kfree(s_state->vlan_filter[port]);
	==> You should have added this		s_state->vlan_filter[port] = NULL;
					}
					goto err_slaves;
				}

However, again, the correct solution is to do what Yishai suggests:
	while (--i >= 0)
so that if i is already zero the while-loop will not be entered.

-Jack
> 
> Rasmus
> --
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