RE: [PATCH rdma-next 2/2] RDMA/core: Check that process is still alive before sending it to the users

[Parav Pandit <parav@mellanox.com>]

> -----Original Message-----
> From: linux-rdma-owner@vger.kernel.org <linux-rdma-
> owner@vger.kernel.org> On Behalf Of Leon Romanovsky
> Sent: Wednesday, October 2, 2019 7:33 AM
> To: Doug Ledford <dledford@redhat.com>; Jason Gunthorpe
> <jgg@mellanox.com>
> Cc: Leon Romanovsky <leonro@mellanox.com>; RDMA mailing list <linux-
> rdma@vger.kernel.org>
> Subject: [PATCH rdma-next 2/2] RDMA/core: Check that process is still alive
> before sending it to the users
> 
> From: Leon Romanovsky <leonro@mellanox.com>
> 
> The PID information can disappear asynchronically because task can be killed
> and moved to zombie state. In such case, PID will be zero in similar way to the
> kernel tasks. Recognize such situation where we are asking to return orphaned
> object and simply skip filling PID attribute.
> 
> As part of this change, document the same scenario in counter.c code.
> 
> Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
> ---
>  drivers/infiniband/core/counters.c | 14 ++++++++++++--
>  drivers/infiniband/core/nldev.c    | 31 ++++++++++++++++++++++--------
>  2 files changed, 35 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/infiniband/core/counters.c
> b/drivers/infiniband/core/counters.c
> index 12ba2685abcf..47c551a0bcb0 100644
> --- a/drivers/infiniband/core/counters.c
> +++ b/drivers/infiniband/core/counters.c
> @@ -149,8 +149,18 @@ static bool auto_mode_match(struct ib_qp *qp, struct
> rdma_counter *counter,
>  	struct auto_mode_param *param = &counter->mode.param;
>  	bool match = true;
> 
> -	/* Ensure that counter belongs to the right PID */
> -	if (task_pid_nr(counter->res.task) != task_pid_nr(qp->res.task))
> +	/*
> +	 * Ensure that counter belongs to the right PID.
> +	 * This operation can race with user space which kills
> +	 * the process and leaves QP and counters orphans.
> +	 *
> +	 * It is not a big deal because exitted task will leave both
> +	 * QP and counter in the same bucket of zombie process. Just ensure
> +	 * that process is still alive before procedding.
> +	 *
> +	 */
> +	if (task_pid_nr(counter->res.task) != task_pid_nr(qp->res.task) ||
> +	    !task_pid_nr(qp->res.task))
>  		return false;
> 
>  	if (auto_mask & RDMA_COUNTER_MASK_QP_TYPE) diff --git
> a/drivers/infiniband/core/nldev.c b/drivers/infiniband/core/nldev.c index
> 71bc08510064..c6fe0c52f6dc 100644
> --- a/drivers/infiniband/core/nldev.c
> +++ b/drivers/infiniband/core/nldev.c
> @@ -399,20 +399,35 @@ static int fill_res_info(struct sk_buff *msg, struct
> ib_device *device)  static int fill_res_name_pid(struct sk_buff *msg,
>  			     struct rdma_restrack_entry *res)  {
> +	int err = 0;
> +	pid_t pid;
> +
>  	/*
>  	 * For user resources, user is should read /proc/PID/comm to get the
>  	 * name of the task file.
>  	 */
>  	if (rdma_is_kernel_res(res)) {
> -		if (nla_put_string(msg,
> RDMA_NLDEV_ATTR_RES_KERN_NAME,
> -		    res->kern_name))
> -			return -EMSGSIZE;
> -	} else {
> -		if (nla_put_u32(msg, RDMA_NLDEV_ATTR_RES_PID,
> -		    task_pid_vnr(res->task)))
> -			return -EMSGSIZE;
> +		err = nla_put_string(msg,
> RDMA_NLDEV_ATTR_RES_KERN_NAME,
> +				     res->kern_name);
> +		goto out;
>  	}
> -	return 0;
> +
> +	pid = task_pid_vnr(res->task);
> +	/*
> +	 * PID == 0 returns in two scenarios:
> +	 * 1. It is kernel task, but because we checked above, it won't be
> possible.
Please drop above comment point 1. See more below.

> +	 * 2. Task is dead and in zombie state. There is no need to print PID
> anymore.
> +	 */
> +	if (pid)
> +		/*
> +		 * This part is racy, task can be killed and PID will be zero right
> +		 * here but it is ok, next query won't return PID. We don't
> promise
> +		 * real-time reflection of SW objects.
> +		 */
> +		err = nla_put_u32(msg, RDMA_NLDEV_ATTR_RES_PID, pid);
> +
> +out:
> +	return err ? -EMSGSIZE : 0;
>  }

Below code reads better along with rest of the comments in the patch.

if (kern_resource) {
	err = nla_put_string(msg, RDMA_NLDEV_ATTR_RES_KERN_NAME,
			     res->kern_name);
} else {
	pid_t pid;

	pid = task_pid_vnr(res->task);
	if (pid)
		err = nla_put_u32(msg, RDMA_NLDEV_ATTR_RES_PID, pid);
}

> 
>  static bool fill_res_entry(struct ib_device *dev, struct sk_buff *msg,
> --
> 2.20.1