Re: [dpdk-stable] [dpdk-dev] [PATCH v1] mbuf: replace zero-length marker with unnamed union

[Kevin Traynor <ktraynor@redhat.com>]

On 03/03/2020 16:27, Gavin Hu wrote:
> gcc 10.0.1 reports: error: array subscript 0 is outside the bounds of an
> interior zero-length array 'RTE_MARKER64' {aka 'long unsigned int[0]'}
> [-Werror=zero-length-bounds] 310 |  *(uint64_t *)(&mbuf->rearm_data) =
> val;
> 
> Declaring zero-length arrays in other contexts, including as interior
> members of structure objects or as non-member objects, is discouraged.
> Accessing elements of zero-length arrays declared in such contexts is
> undefined and may be diagnosed.[1]
> 
> Fix by using unnamed union and struct.
> 
> https://bugs.dpdk.org/show_bug.cgi?id=396
> 
> Bugzilla ID: 396
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> 
> Fixes: 3e6181b07038 ("mbuf: use structure marker from EAL")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Gavin Hu <gavin.hu@arm.com>
> ---
>  lib/librte_mbuf/rte_mbuf_core.h | 54 +++++++++++++++++++--------------
>  1 file changed, 32 insertions(+), 22 deletions(-)
> 
> diff --git a/lib/librte_mbuf/rte_mbuf_core.h b/lib/librte_mbuf/rte_mbuf_core.h
> index b9a59c879..5390ddcfa 100644
> --- a/lib/librte_mbuf/rte_mbuf_core.h
> +++ b/lib/librte_mbuf/rte_mbuf_core.h
> @@ -480,31 +480,41 @@ struct rte_mbuf {
>  		rte_iova_t buf_physaddr; /**< deprecated */
>  	} __rte_aligned(sizeof(rte_iova_t));
>  
> -	/* next 8 bytes are initialised on RX descriptor rearm */
> -	RTE_MARKER64 rearm_data;
> -	uint16_t data_off;
> -
> -	/**
> -	 * Reference counter. Its size should at least equal to the size
> -	 * of port field (16 bits), to support zero-copy broadcast.
> -	 * It should only be accessed using the following functions:
> -	 * rte_mbuf_refcnt_update(), rte_mbuf_refcnt_read(), and
> -	 * rte_mbuf_refcnt_set(). The functionality of these functions (atomic,
> -	 * or non-atomic) is controlled by the CONFIG_RTE_MBUF_REFCNT_ATOMIC
> -	 * config option.
> -	 */
>  	RTE_STD_C11
>  	union {
> -		rte_atomic16_t refcnt_atomic; /**< Atomically accessed refcnt */
> -		/** Non-atomically accessed refcnt */
> -		uint16_t refcnt;
> -	};
> -	uint16_t nb_segs;         /**< Number of segments. */
> +		/* next 8 bytes are initialised on RX descriptor rearm */
> +		uint64_t rearm_data;
> +		RTE_STD_C11
> +		struct {
> +			uint16_t data_off;
> +
> +			/**
> +			 * Reference counter. Its size should at least equal to
> +			 * the size of port field (16 bits), to support
> +			 * zero-copy broadcast.  It should only be accessed
> +			 * using the following functions:
> +			 * rte_mbuf_refcnt_update(), rte_mbuf_refcnt_read(),
> +			 * and rte_mbuf_refcnt_set(). The functionality of
> +			 * these functions (atomic, or non-atomic) is
> +			 * controlled by the CONFIG_RTE_MBUF_REFCNT_ATOMIC
> +			 * config option.
> +			 */
> +			RTE_STD_C11
> +				union {
> +					/**< Atomically accessed refcnt */
> +					rte_atomic16_t refcnt_atomic;
> +					/** Non-atomically accessed refcnt */
> +					uint16_t refcnt;
> +				};
> +			uint16_t nb_segs;         /**< Number of segments. */
>  
> -	/** Input port (16 bits to support more than 256 virtual ports).
> -	 * The event eth Tx adapter uses this field to specify the output port.
> -	 */
> -	uint16_t port;
> +			/** Input port (16 bits to support more than 256
> +			 * virtual ports).  The event eth Tx adapter uses this
> +			 * field to specify the output port.
> +			 */
> +			uint16_t port;
> +		};
> +	};
>  
>  	uint64_t ol_flags;        /**< Offload features. */
>  
> 


Hi Gavin, this causes some errors on x86:

# gcc --version | head -1
gcc (GCC) 10.0.1 20200216 (Red Hat 10.0.1-0.8)


In file included from
../lib/librte_eal/common/include/arch/x86/rte_byteorder.h:13,
                 from ../drivers/net/sfc/sfc_ef10_rx.c:14:
../drivers/net/sfc/sfc_ef10_rx.c: In function ‘sfc_ef10_rx_process_event’:
../drivers/net/sfc/sfc_ef10_rx.c:309:39: error: subscripted value is
neither array nor pointer nor vector
  309 |  RTE_BUILD_BUG_ON(sizeof(m->rearm_data[0]) !=
sizeof(rxq->rearm_data));
      |                                       ^
../lib/librte_eal/common/include/rte_common.h:292:65: note: in
definition of macro ‘RTE_BUILD_BUG_ON’
  292 | #define RTE_BUILD_BUG_ON(condition) ((void)sizeof(char[1 -
2*!!(condition)]))
      |
^~~~~~~~~
../drivers/net/sfc/sfc_ef10_rx.c:310:15: error: subscripted value is
neither array nor pointer nor vector
  310 |  m->rearm_data[0] = rxq->rearm_data;
      |