Hi Tom,
On Fri, Jul 10, 2020 at 06:45:36AM -0700, trix@xxxxxxxxxx wrote:
> From: Tom Rix <trix@xxxxxxxxxx>
>
> clang static analysis flags this error
>
> j1939/main.c:292:2: warning: Attempt to free released memory [unix.Malloc]
> kfree(priv);
> ^~~~~~~~~~~
>
> The problem block of code is
>
> ret = j1939_can_rx_register(priv);
> if (ret < 0)
> goto out_priv_put;
>
> return priv;
>
> out_priv_put:
> j1939_priv_set(ndev, NULL);
> dev_put(ndev);
> kfree(priv);
>
> When j1939_can_rx_register fails, it frees priv via the
> j1939_priv_put release function __j1939_priv_release.
In j1939_can_rx_register()...
| static int j1939_can_rx_register(struct j1939_priv *priv)
| {
| struct net_device *ndev = priv->ndev;
| int ret;
... the function in entered with ref counter == 1.
(Due to kref_init(&priv->kref); in j1939_priv_create())
|
| j1939_priv_get(priv);
... then the ref counter is increased by 1, resulting in 2.
| ret = can_rx_register(dev_net(ndev), ndev, J1939_CAN_ID, J1939_CAN_MASK,
| j1939_can_recv, priv, "j1939", NULL);
| if (ret < 0) {
| j1939_priv_put(priv);
And in case of an error, the ref counter is decreased by one again.
| return ret;
| }
|
| return 0;
| }
So we cannot see why clang thinks the memory is double free()d.
> Since j1939_priv_put is used widely, remove the second
> free from j1939_netdev_start.
We might replace the manual kfree() and dev_put() by the dropping the
last ref count and rely on the automatic cleanup.
> Fixes: 9d71dd0c7009 ("can: add support of SAE J1939 protocol")
>
> Signed-off-by: Tom Rix <trix@xxxxxxxxxx>
> ---
> net/can/j1939/main.c | 1 -
> 1 file changed, 1 deletion(-)
>
> diff --git a/net/can/j1939/main.c b/net/can/j1939/main.c
> index 137054bff9ec..991a74bc491b 100644
> --- a/net/can/j1939/main.c
> +++ b/net/can/j1939/main.c
> @@ -289,7 +289,6 @@ struct j1939_priv *j1939_netdev_start(struct net_device *ndev)
> out_priv_put:
> j1939_priv_set(ndev, NULL);
> dev_put(ndev);
> - kfree(priv);
>
> return ERR_PTR(ret);
> }
regards,
Oleksij & Marc
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