Re: [PATCH][next] Bluetooth: L2CAP: Avoid -Wflex-array-member-not-at-end warnings

[Gustavo A. R. Silva]

On 3/26/24 15:12, Luiz Augusto von Dentz wrote:
> Hi Gustavo,
>
> On Tue, Mar 26, 2024 at 4:02 PM Gustavo A. R. Silva
> <gustavoars@xxxxxxxxxx> wrote:
> > -Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
> > ready to enable it globally.
> >
> > There are currently a couple of objects (`req` and `rsp`), in a couple
> > of structures, that contain flexible structures (`struct l2cap_ecred_conn_req`
> > and `struct l2cap_ecred_conn_rsp`), for example:
> >
> > struct l2cap_ecred_rsp_data {
> >          struct {
> >                  struct l2cap_ecred_conn_rsp rsp;
> >                  __le16 scid[L2CAP_ECRED_MAX_CID];
> >          } __packed pdu;
> >          int count;
> > };
> >
> > in the struct above, `struct l2cap_ecred_conn_rsp` is a flexible
> > structure:
> >
> > struct l2cap_ecred_conn_rsp {
> >          __le16 mtu;
> >          __le16 mps;
> >          __le16 credits;
> >          __le16 result;
> >          __le16 dcid[];
> > };
> >
> > So, in order to avoid ending up with a flexible-array member in the
> > middle of another structure, we use the `struct_group_tagged()` (and
> > `__struct_group()` when the flexible structure is `__packed`) helper
> > to separate the flexible array from the rest of the members in the
> > flexible structure:
> >
> > struct l2cap_ecred_conn_rsp {
> >          struct_group_tagged(l2cap_ecred_conn_rsp_hdr, hdr,
> >
> >          ... the rest of members
> >
> >          );
> >          __le16 dcid[];
> > };
>
> Wouldn't it be better, more readable at least, to not define the
> l2cap_ecred_conn_rsp_hdr inside thought? Instead just do:
>
> struct_group_tagged(l2cap_ecred_conn_rsp_hdr, hdr,
> ...
>   };
>
>   struct l2cap_ecred_conn_rsp {
>          struct l2cap_ecred_conn_rsp_hdr hdr;
>           __le16 dcid[];
>   };
>
> Or was this done this way in order to maintain the same fields?

Exactly. But I can change it if people consider that's better.

> > With the change described above, we now declare objects of the type of
> > the tagged struct, in this example `struct l2cap_ecred_conn_rsp_hdr`,
> > without embedding flexible arrays in the middle of other structures:
> >
> > struct l2cap_ecred_rsp_data {
> >          struct {
> >                  struct l2cap_ecred_conn_rsp_hdr rsp;
> >                  __le16 scid[L2CAP_ECRED_MAX_CID];
> >          } __packed pdu;
> >          int count;
> > };
> >
> > Also, when the flexible-array member needs to be accessed, we use
> > `container_of()` to retrieve a pointer to the flexible structure.
> >
> > We also use the `DEFINE_RAW_FLEX()` helper for a couple of on-stack
> > definitions of a flexible structure where the size of the flexible-array
> > member is known at compile-time.
> >
> > So, with these changes, fix the following warnings:
> > net/bluetooth/l2cap_core.c:1260:45: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > net/bluetooth/l2cap_core.c:3740:45: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > net/bluetooth/l2cap_core.c:4999:45: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > net/bluetooth/l2cap_core.c:7116:47: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> >
> > Link: https://github.com/KSPP/linux/issues/202
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
> > ---
> >
> > Hi!
> >
> > I wonder if `struct l2cap_ecred_conn_rsp` should also be `__packed`.
> >
> > Thanks
> >   - Gustavo
> >
> >   include/net/bluetooth/l2cap.h | 20 +++++++++------
> >   net/bluetooth/l2cap_core.c    | 46 ++++++++++++++++-------------------
> >   2 files changed, 33 insertions(+), 33 deletions(-)
> >
> > diff --git a/include/net/bluetooth/l2cap.h b/include/net/bluetooth/l2cap.h
> > index a4278aa618ab..92a143517d83 100644
> > --- a/include/net/bluetooth/l2cap.h
> > +++ b/include/net/bluetooth/l2cap.h
> > @@ -463,18 +463,22 @@ struct l2cap_le_credits {
> >   #define L2CAP_ECRED_MAX_CID            5
> >
> >   struct l2cap_ecred_conn_req {
> > -       __le16 psm;
> > -       __le16 mtu;
> > -       __le16 mps;
> > -       __le16 credits;
> > +       __struct_group(l2cap_ecred_conn_req_hdr, hdr, __packed,
> > +               __le16 psm;
> > +               __le16 mtu;
> > +               __le16 mps;
> > +               __le16 credits;
> > +       );
> >          __le16 scid[];
> >   } __packed;
> >
> >   struct l2cap_ecred_conn_rsp {
> > -       __le16 mtu;
> > -       __le16 mps;
> > -       __le16 credits;
> > -       __le16 result;
> > +       struct_group_tagged(l2cap_ecred_conn_rsp_hdr, hdr,
> > +               __le16 mtu;
> > +               __le16 mps;
> > +               __le16 credits;
> > +               __le16 result;
> > +       );
> >          __le16 dcid[];
> >   };
> >
> > diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> > index 467b242d8be0..bf087eca489e 100644
> > --- a/net/bluetooth/l2cap_core.c
> > +++ b/net/bluetooth/l2cap_core.c
> > @@ -1257,7 +1257,7 @@ static void l2cap_le_connect(struct l2cap_chan *chan)
> >
> >   struct l2cap_ecred_conn_data {
> >          struct {
> > -               struct l2cap_ecred_conn_req req;
> > +               struct l2cap_ecred_conn_req_hdr req;
> >                  __le16 scid[5];
> >          } __packed pdu;
>
> Can't we just use DEFINE_RAW_FLEX for the pdu field above?

No; DEFINE_FLEX() and DEFINE_RAW_FLEX() are for on-stack code only.

Thanks
--
Gustavo

> >          struct l2cap_chan *chan;
> > @@ -3737,7 +3737,7 @@ static void l2cap_ecred_list_defer(struct l2cap_chan *chan, void *data)
> >
> >   struct l2cap_ecred_rsp_data {
> >          struct {
> > -               struct l2cap_ecred_conn_rsp rsp;
> > +               struct l2cap_ecred_conn_rsp_hdr rsp;
> >                  __le16 scid[L2CAP_ECRED_MAX_CID];
> >          } __packed pdu;
>
> Ditto.
>
> >          int count;
> > @@ -3746,6 +3746,8 @@ struct l2cap_ecred_rsp_data {
> >   static void l2cap_ecred_rsp_defer(struct l2cap_chan *chan, void *data)
> >   {
> >          struct l2cap_ecred_rsp_data *rsp = data;
> > +       struct l2cap_ecred_conn_rsp *rsp_flex =
> > +               container_of(&rsp->pdu.rsp, struct l2cap_ecred_conn_rsp, hdr);
> >
> >          if (test_bit(FLAG_ECRED_CONN_REQ_SENT, &chan->flags))
> >                  return;
> > @@ -3755,7 +3757,7 @@ static void l2cap_ecred_rsp_defer(struct l2cap_chan *chan, void *data)
> >
> >          /* Include all channels pending with the same ident */
> >          if (!rsp->pdu.rsp.result)
> > -               rsp->pdu.rsp.dcid[rsp->count++] = cpu_to_le16(chan->scid);
> > +               rsp_flex->dcid[rsp->count++] = cpu_to_le16(chan->scid);
> >          else
> >                  l2cap_chan_del(chan, ECONNRESET);
> >   }
> > @@ -4995,10 +4997,7 @@ static inline int l2cap_ecred_conn_req(struct l2cap_conn *conn,
> >                                         u8 *data)
> >   {
> >          struct l2cap_ecred_conn_req *req = (void *) data;
> > -       struct {
> > -               struct l2cap_ecred_conn_rsp rsp;
> > -               __le16 dcid[L2CAP_ECRED_MAX_CID];
> > -       } __packed pdu;
> > +       DEFINE_RAW_FLEX(struct l2cap_ecred_conn_rsp, pdu, dcid, L2CAP_ECRED_MAX_CID);
> >          struct l2cap_chan *chan, *pchan;
> >          u16 mtu, mps;
> >          __le16 psm;
> > @@ -5017,7 +5016,7 @@ static inline int l2cap_ecred_conn_req(struct l2cap_conn *conn,
> >          cmd_len -= sizeof(*req);
> >          num_scid = cmd_len / sizeof(u16);
> >
> > -       if (num_scid > ARRAY_SIZE(pdu.dcid)) {
> > +       if (num_scid > L2CAP_ECRED_MAX_CID) {
> >                  result = L2CAP_CR_LE_INVALID_PARAMS;
> >                  goto response;
> >          }
> > @@ -5046,7 +5045,7 @@ static inline int l2cap_ecred_conn_req(struct l2cap_conn *conn,
> >
> >          BT_DBG("psm 0x%2.2x mtu %u mps %u", __le16_to_cpu(psm), mtu, mps);
> >
> > -       memset(&pdu, 0, sizeof(pdu));
> > +       memset(pdu, 0, sizeof(*pdu));
> >
> >          /* Check if we have socket listening on psm */
> >          pchan = l2cap_global_chan_by_psm(BT_LISTEN, psm, &conn->hcon->src,
> > @@ -5072,8 +5071,8 @@ static inline int l2cap_ecred_conn_req(struct l2cap_conn *conn,
> >
> >                  BT_DBG("scid[%d] 0x%4.4x", i, scid);
> >
> > -               pdu.dcid[i] = 0x0000;
> > -               len += sizeof(*pdu.dcid);
> > +               pdu->dcid[i] = 0x0000;
> > +               len += sizeof(*pdu->dcid);
> >
> >                  /* Check for valid dynamic CID range */
> >                  if (scid < L2CAP_CID_DYN_START || scid > L2CAP_CID_LE_DYN_END) {
> > @@ -5107,13 +5106,13 @@ static inline int l2cap_ecred_conn_req(struct l2cap_conn *conn,
> >                  l2cap_ecred_init(chan, __le16_to_cpu(req->credits));
> >
> >                  /* Init response */
> > -               if (!pdu.rsp.credits) {
> > -                       pdu.rsp.mtu = cpu_to_le16(chan->imtu);
> > -                       pdu.rsp.mps = cpu_to_le16(chan->mps);
> > -                       pdu.rsp.credits = cpu_to_le16(chan->rx_credits);
> > +               if (!pdu->credits) {
> > +                       pdu->mtu = cpu_to_le16(chan->imtu);
> > +                       pdu->mps = cpu_to_le16(chan->mps);
> > +                       pdu->credits = cpu_to_le16(chan->rx_credits);
> >                  }
> >
> > -               pdu.dcid[i] = cpu_to_le16(chan->scid);
> > +               pdu->dcid[i] = cpu_to_le16(chan->scid);
> >
> >                  __set_chan_timer(chan, chan->ops->get_sndtimeo(chan));
> >
> > @@ -5135,13 +5134,13 @@ static inline int l2cap_ecred_conn_req(struct l2cap_conn *conn,
> >          l2cap_chan_put(pchan);
> >
> >   response:
> > -       pdu.rsp.result = cpu_to_le16(result);
> > +       pdu->result = cpu_to_le16(result);
> >
> >          if (defer)
> >                  return 0;
> >
> >          l2cap_send_cmd(conn, cmd->ident, L2CAP_ECRED_CONN_RSP,
> > -                      sizeof(pdu.rsp) + len, &pdu);
> > +                      sizeof(*pdu) + len, pdu);
> >
> >          return 0;
> >   }
> > @@ -7112,14 +7111,11 @@ EXPORT_SYMBOL_GPL(l2cap_chan_connect);
> >   static void l2cap_ecred_reconfigure(struct l2cap_chan *chan)
> >   {
> >          struct l2cap_conn *conn = chan->conn;
> > -       struct {
> > -               struct l2cap_ecred_reconf_req req;
> > -               __le16 scid;
> > -       } pdu;
> > +       DEFINE_RAW_FLEX(struct l2cap_ecred_reconf_req, pdu, scid, 1);
> >
> > -       pdu.req.mtu = cpu_to_le16(chan->imtu);
> > -       pdu.req.mps = cpu_to_le16(chan->mps);
> > -       pdu.scid    = cpu_to_le16(chan->scid);
> > +       pdu->mtu = cpu_to_le16(chan->imtu);
> > +       pdu->mps = cpu_to_le16(chan->mps);
> > +       pdu->scid[0] = cpu_to_le16(chan->scid);
> >
> >          chan->ident = l2cap_get_ident(conn);
> >
> > --
> > 2.34.1