Re: [PATCH v4 net-next 5/9] net/sched: pass netlink extack to mqprio and taprio offload

[Jamal Hadi Salim]

On Mon, Apr 3, 2023 at 6:35 AM Vladimir Oltean <vladimir.oltean@xxxxxxx> wrote:
>
> With the multiplexed ndo_setup_tc() model which lacks a first-class
> struct netlink_ext_ack * argument, the only way to pass the netlink
> extended ACK message down to the device driver is to embed it within the
> offload structure.
>
> Do this for struct tc_mqprio_qopt_offload and struct tc_taprio_qopt_offload.
>
> Since struct tc_taprio_qopt_offload also contains a tc_mqprio_qopt_offload
> structure, and since device drivers might effectively reuse their mqprio
> implementation for the mqprio portion of taprio, we make taprio set the
> extack in both offload structures to point at the same netlink extack
> message.
>
> In fact, the taprio handling is a bit more tricky, for 2 reasons.
>
> First is because the offload structure has a longer lifetime than the
> extack structure. The driver is supposed to populate the extack
> synchronously from ndo_setup_tc() and leave it alone afterwards.
> To not have any use-after-free surprises, we zero out the extack pointer
> when we leave taprio_enable_offload().
>
> The second reason is because taprio does overwrite the extack message on
> ndo_setup_tc() error. We need to switch to the weak form of setting an
> extack message, which preserves a potential message set by the driver.
>
> Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>
> Reviewed-by: Simon Horman <simon.horman@xxxxxxxxxxxx>
Acked-by: Jamal Hadi Salim <jhs@xxxxxxxxxxxx>

cheers,
jamal

> ---
> v3->v4: none
> v2->v3: patch is new
>
>  include/net/pkt_sched.h |  2 ++
>  net/sched/sch_mqprio.c  |  5 ++++-
>  net/sched/sch_taprio.c  | 12 ++++++++++--
>  3 files changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/include/net/pkt_sched.h b/include/net/pkt_sched.h
> index bb0bd69fb655..b43ed4733455 100644
> --- a/include/net/pkt_sched.h
> +++ b/include/net/pkt_sched.h
> @@ -166,6 +166,7 @@ struct tc_mqprio_caps {
>  struct tc_mqprio_qopt_offload {
>         /* struct tc_mqprio_qopt must always be the first element */
>         struct tc_mqprio_qopt qopt;
> +       struct netlink_ext_ack *extack;
>         u16 mode;
>         u16 shaper;
>         u32 flags;
> @@ -193,6 +194,7 @@ struct tc_taprio_sched_entry {
>
>  struct tc_taprio_qopt_offload {
>         struct tc_mqprio_qopt_offload mqprio;
> +       struct netlink_ext_ack *extack;
>         u8 enable;
>         ktime_t base_time;
>         u64 cycle_time;
> diff --git a/net/sched/sch_mqprio.c b/net/sched/sch_mqprio.c
> index 9ee5a9a9b9e9..5287ff60b3f9 100644
> --- a/net/sched/sch_mqprio.c
> +++ b/net/sched/sch_mqprio.c
> @@ -33,9 +33,12 @@ static int mqprio_enable_offload(struct Qdisc *sch,
>                                  const struct tc_mqprio_qopt *qopt,
>                                  struct netlink_ext_ack *extack)
>  {
> -       struct tc_mqprio_qopt_offload mqprio = {.qopt = *qopt};
>         struct mqprio_sched *priv = qdisc_priv(sch);
>         struct net_device *dev = qdisc_dev(sch);
> +       struct tc_mqprio_qopt_offload mqprio = {
> +               .qopt = *qopt,
> +               .extack = extack,
> +       };
>         int err, i;
>
>         switch (priv->mode) {
> diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
> index 1f469861eae3..cbad43019172 100644
> --- a/net/sched/sch_taprio.c
> +++ b/net/sched/sch_taprio.c
> @@ -1520,7 +1520,9 @@ static int taprio_enable_offload(struct net_device *dev,
>                 return -ENOMEM;
>         }
>         offload->enable = 1;
> +       offload->extack = extack;
>         mqprio_qopt_reconstruct(dev, &offload->mqprio.qopt);
> +       offload->mqprio.extack = extack;
>         taprio_sched_to_offload(dev, sched, offload, &caps);
>
>         for (tc = 0; tc < TC_MAX_QUEUE; tc++)
> @@ -1528,14 +1530,20 @@ static int taprio_enable_offload(struct net_device *dev,
>
>         err = ops->ndo_setup_tc(dev, TC_SETUP_QDISC_TAPRIO, offload);
>         if (err < 0) {
> -               NL_SET_ERR_MSG(extack,
> -                              "Device failed to setup taprio offload");
> +               NL_SET_ERR_MSG_WEAK(extack,
> +                                   "Device failed to setup taprio offload");
>                 goto done;
>         }
>
>         q->offloaded = true;
>
>  done:
> +       /* The offload structure may linger around via a reference taken by the
> +        * device driver, so clear up the netlink extack pointer so that the
> +        * driver isn't tempted to dereference data which stopped being valid
> +        */
> +       offload->extack = NULL;
> +       offload->mqprio.extack = NULL;
>         taprio_offload_free(offload);
>
>         return err;
> --
> 2.34.1
>