RE: [PATCH v3] ethdev: fix strict aliasing lead to link cannot be up

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Chengwen Feng [mailto:fengchengwen@huawei.com]
> Sent: Thursday, 11 April 2024 14.04
> 
> Fix a problem introduced by a compiler upgrade (from gcc10 to gcc12.3),
> which will lead the hns3 NIC can't link up. The root cause is strict
> aliasing violation in rte_eth_linkstatus_set() with hns3 driver, see
> [1] for more details.
> 
> This commit use union to avoid such aliasing violation.
> 
> [1] Strict aliasing problem with rte_eth_linkstatus_set()
>     https://marc.info/?l=dpdk-dev&m=171274148514777&w=3
> 
> Cc: stable@dpdk.org
> 
> Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
> Signed-off-by: Dengdui Huang <huangdengdui@huawei.com>
> 
> ---
> v3: fix checkpatch warning "missing --in-reply-to".
> v2: add RTE_ATOMIC(uint64_t) wrap which address Morten's comment.
> 
> ---
>  lib/ethdev/ethdev_driver.h | 23 +++++++----------------
>  lib/ethdev/rte_ethdev.h    | 16 ++++++++++------
>  2 files changed, 17 insertions(+), 22 deletions(-)
> 
> diff --git a/lib/ethdev/ethdev_driver.h b/lib/ethdev/ethdev_driver.h
> index 0dbf2dd6a2..9d831d5c84 100644
> --- a/lib/ethdev/ethdev_driver.h
> +++ b/lib/ethdev/ethdev_driver.h
> @@ -1674,18 +1674,13 @@ static inline int
>  rte_eth_linkstatus_set(struct rte_eth_dev *dev,
>  		       const struct rte_eth_link *new_link)
>  {
> -	RTE_ATOMIC(uint64_t) *dev_link = (uint64_t __rte_atomic *)&(dev-
> >data->dev_link);
> -	union {
> -		uint64_t val64;
> -		struct rte_eth_link link;
> -	} orig;
> -
> -	RTE_BUILD_BUG_ON(sizeof(*new_link) != sizeof(uint64_t));
> +	struct rte_eth_link old_link;
> 
> -	orig.val64 = rte_atomic_exchange_explicit(dev_link, *(const
> uint64_t *)new_link,
> -					rte_memory_order_seq_cst);
> +	old_link.val64 = rte_atomic_exchange_explicit(&dev->data-
> >dev_link.val64,

You are right; old_link has local scope and is on the stack, so atomic store is not required.

And since rte_eth_linkstatus_set() is an internal function called from the driver only, it is probably safe to assume that *new_link is on the caller's stack and doesn't change while being accessed by this function.
I guess that new_link is passed by reference for performance and future-proofing reasons; it could have been passed by value instead. If it was passed by value, atomic access would certainly not be required.
In other words: You are right here too; new_link does not require atomic load.

> +						      new_link->val64,
> +						      rte_memory_order_seq_cst);
> 
> -	return (orig.link.link_status == new_link->link_status) ? -1 : 0;
> +	return (old_link.link_status == new_link->link_status) ? -1 : 0;
>  }
> 
>  /**
> @@ -1701,12 +1696,8 @@ static inline void
>  rte_eth_linkstatus_get(const struct rte_eth_dev *dev,
>  		       struct rte_eth_link *link)
>  {
> -	RTE_ATOMIC(uint64_t) *src = (uint64_t __rte_atomic *)&(dev->data-
> >dev_link);
> -	uint64_t *dst = (uint64_t *)link;
> -
> -	RTE_BUILD_BUG_ON(sizeof(*link) != sizeof(uint64_t));
> -
> -	*dst = rte_atomic_load_explicit(src, rte_memory_order_seq_cst);
> +	link->val64 = rte_atomic_load_explicit(&dev->data->dev_link.val64,
> +					       rte_memory_order_seq_cst);

It is not safe to assume that the link pointer points to local memory on the caller's stack.
The link pointer might point to a shared memory area, used by multiple threads/processes, so it needs to be stored atomically using rte_atomic_store_explicit(&link->val64, ..., rte_memory_order_seq_cst).

>  }
> 
>  /**
> diff --git a/lib/ethdev/rte_ethdev.h b/lib/ethdev/rte_ethdev.h
> index 147257d6a2..ccf43e468a 100644
> --- a/lib/ethdev/rte_ethdev.h
> +++ b/lib/ethdev/rte_ethdev.h
> @@ -332,12 +332,16 @@ struct rte_eth_stats {
>  /**
>   * A structure used to retrieve link-level information of an Ethernet
> port.
>   */
> -__extension__
> -struct __rte_aligned(8) rte_eth_link { /**< aligned for atomic64
> read/write */
> -	uint32_t link_speed;        /**< RTE_ETH_SPEED_NUM_ */
> -	uint16_t link_duplex  : 1;  /**< RTE_ETH_LINK_[HALF/FULL]_DUPLEX
> */
> -	uint16_t link_autoneg : 1;  /**< RTE_ETH_LINK_[AUTONEG/FIXED] */
> -	uint16_t link_status  : 1;  /**< RTE_ETH_LINK_[DOWN/UP] */
> +struct rte_eth_link {
> +	union {
> +		RTE_ATOMIC(uint64_t) val64; /**< used for atomic64
> read/write */
> +		struct {
> +			uint32_t link_speed;	    /**< RTE_ETH_SPEED_NUM_
> */
> +			uint16_t link_duplex  : 1;  /**<
> RTE_ETH_LINK_[HALF/FULL]_DUPLEX */
> +			uint16_t link_autoneg : 1;  /**<
> RTE_ETH_LINK_[AUTONEG/FIXED] */
> +			uint16_t link_status  : 1;  /**<
> RTE_ETH_LINK_[DOWN/UP] */
> +		};
> +	};
>  };
> 
>  /**@{@name Link negotiation
> --
> 2.17.1