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From: Gavin Hu <Gavin.Hu@arm.com>
To: Kevin Traynor <ktraynor@redhat.com>, "dev@dpdk.org" <dev@dpdk.org>
CC: nd <nd@arm.com>, "thomas@monjalon.net" <thomas@monjalon.net>,
"david.marchand@redhat.com" <david.marchand@redhat.com>, "jerinj@marvell.com"
<jerinj@marvell.com>, Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>,
Ruifeng Wang <Ruifeng.Wang@arm.com>, Phil Yang <Phil.Yang@arm.com>,
"stable@dpdk.org" <stable@dpdk.org>, nd <nd@arm.com>
Thread-Topic: [dpdk-dev] [PATCH v1] mbuf: replace zero-length marker with
unnamed union
Thread-Index: AQHV8iEfnt/GvPVe10WEYhEzOB3ZNqg9OujA
Date: Sat, 7 Mar 2020 14:52:04 +0000
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Subject: Re: [dpdk-dev] [PATCH v1] mbuf: replace zero-length marker with
unnamed union
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Sender: "dev" <dev-bounces@dpdk.org>
Hi Kevin,
> -----Original Message-----
> From: Kevin Traynor <ktraynor@redhat.com>
> Sent: Wednesday, March 4, 2020 8:33 PM
> To: Gavin Hu <Gavin.Hu@arm.com>; dev@dpdk.org
> Cc: nd <nd@arm.com>; thomas@monjalon.net;
> david.marchand@redhat.com; jerinj@marvell.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; Ruifeng Wang
> <Ruifeng.Wang@arm.com>; Phil Yang <Phil.Yang@arm.com>;
> stable@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v1] mbuf: replace zero-length marker with
> unnamed union
>=20
> On 03/03/2020 16:27, Gavin Hu wrote:
> > gcc 10.0.1 reports: error: array subscript 0 is outside the bounds of a=
n
> > interior zero-length array 'RTE_MARKER64' {aka 'long unsigned int[0]'}
> > [-Werror=3Dzero-length-bounds] 310 | *(uint64_t *)(&mbuf->rearm_data) =
=3D
> > 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=3D396
> >
> > 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;
To address this historical issue, how about changing this line to uint64_t =
rearm_data[1]?=20
> > + 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 por=
t.
> > - */
> > - 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. */
> >
> >
>=20
>=20
> Hi Gavin, this causes some errors on x86:
>=20
> # gcc --version | head -1
> gcc (GCC) 10.0.1 20200216 (Red Hat 10.0.1-0.8)
>=20
>=20
> 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]) !=3D
> 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] =3D rxq->rearm_data;
> |