RDMA/core: Fix multiple -Warray-bounds warnings

GCC-13 (and Clang)[1] does not like to access a partially allocated object, since it cannot reason about it for bounds checking.

In this case 140 bytes are allocated for an object of type struct ib_umad_packet:

        packet = kzalloc(sizeof(*packet) + IB_MGMT_RMPP_HDR, GFP_KERNEL);

However, notice that sizeof(*packet) is only 104 bytes:

struct ib_umad_packet {
        struct ib_mad_send_buf *   msg;                  /*     0     8 */
        struct ib_mad_recv_wc *    recv_wc;              /*     8     8 */
        struct list_head           list;                 /*    16    16 */
        int                        length;               /*    32     4 */

        /* XXX 4 bytes hole, try to pack */

        struct ib_user_mad         mad __attribute__((__aligned__(8))); /*    40    64 */

        /* size: 104, cachelines: 2, members: 5 */
        /* sum members: 100, holes: 1, sum holes: 4 */
        /* forced alignments: 1, forced holes: 1, sum forced holes: 4 */
        /* last cacheline: 40 bytes */
} __attribute__((__aligned__(8)));

and 36 bytes extra bytes are allocated for a flexible-array member in struct ib_user_mad:

include/rdma/ib_mad.h:
120 enum {
...
123 IB_MGMT_RMPP_HDR = 36,
... }

struct ib_user_mad {
        struct ib_user_mad_hdr     hdr;                  /*     0    64 */
        /* --- cacheline 1 boundary (64 bytes) --- */
        __u64                      data[] __attribute__((__aligned__(8))); /*    64     0 */

        /* size: 64, cachelines: 1, members: 2 */
        /* forced alignments: 1 */
} __attribute__((__aligned__(8)));

So we have sizeof(*packet) + IB_MGMT_RMPP_HDR == 140 bytes

Then the address of the flex-array member (for which only 36 bytes were allocated) is casted and copied into a pointer to struct ib_rmpp_mad, which, in turn, is of size 256 bytes:

        rmpp_mad = (struct ib_rmpp_mad *) packet->mad.data;

struct ib_rmpp_mad {

        struct ib_mad_hdr          mad_hdr;              /*     0    24 */
        struct ib_rmpp_hdr         rmpp_hdr;             /*    24    12 */
        u8                         data[220];            /*    36   220 */

        /* size: 256, cachelines: 4, members: 3 */
};

The thing is that those 36 bytes allocated for flex-array member data in struct ib_user_mad onlly account for the size of both struct ib_mad_hdr and struct ib_rmpp_hdr, but nothing is left for array u8 data[220]. So, the compiler is legitimately complaining about accessing an object for which not enough memory was allocated.

Apparently, the only members of struct ib_rmpp_mad that are relevant (that are actually being used) in function ib_umad_write() are mad_hdr and rmpp_hdr. So, instead of casting packet->mad.data to (struct ib_rmpp_mad *) cast it just to the two members for which enough memory was allocated. That's actually the reason why

        IB_MGMT_RMPP_HDR == sizeof(struct ib_mad_hdr) + sizeof(struct ib_rmpp_hdr) == 36 bytes

Refactor the rest of the code, accordingly.

Fix the following warnings seen under GCC-13 and -Warray-bounds:
drivers/infiniband/core/user_mad.c:564:50: warning: array subscript ‘struct ib_rmpp_mad[0]’ is partly outside array bounds of ‘unsigned char[140]’ [-Warray-bounds=] drivers/infiniband/core/user_mad.c:566:42: warning: array subscript ‘struct ib_rmpp_mad[0]’ is partly outside array bounds of ‘unsigned char[140]’ [-Warray-bounds=] drivers/infiniband/core/user_mad.c:618:25: warning: array subscript ‘struct ib_rmpp_mad[0]’ is partly outside array bounds of ‘unsigned char[140]’ [-Warray-bounds=] drivers/infiniband/core/user_mad.c:622:44: warning: array subscript ‘struct ib_rmpp_mad[0]’ is partly outside array bounds of ‘unsigned char[140]’ [-Warray-bounds=]

---
 drivers/infiniband/core/user_mad.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

index f83954180a33..ba847ccf1bf0 100644
@@ -497,8 +497,9 @@ static ssize_t ib_umad_write(struct file *filp, const char __user *buf,
 	struct ib_umad_packet *packet;
 	struct ib_mad_agent *agent;
 	struct rdma_ah_attr ah_attr;
+	struct ib_rmpp_hdr rmpp_hdr;
+	struct ib_mad_hdr mad_hdr;
 	struct ib_ah *ah;
-	struct ib_rmpp_mad *rmpp_mad;
 	__be64 *tid;
 	int ret, data_len, hdr_len, copy_offset, rmpp_active;
 	u8 base_version;

@@ -506,7 +507,7 @@ static ssize_t ib_umad_write(struct file *filp, const char __user *buf,
 	if (count < hdr_size(file) + IB_MGMT_RMPP_HDR)
 		return -EINVAL;
 
-	packet = kzalloc(sizeof *packet + IB_MGMT_RMPP_HDR, GFP_KERNEL);
+	packet = kzalloc(sizeof(*packet) + IB_MGMT_RMPP_HDR, GFP_KERNEL);
 	if (!packet)
 		return -ENOMEM;
 

@@ -560,13 +561,14 @@ static ssize_t ib_umad_write(struct file *filp, const char __user *buf,
 		goto err_up;
 	}
 
-	rmpp_mad = (struct ib_rmpp_mad *) packet->mad.data;
-	hdr_len = ib_get_mad_data_offset(rmpp_mad->mad_hdr.mgmt_class);
+	mad_hdr = *(struct ib_mad_hdr *)packet->mad.data;
+	rmpp_hdr = *(struct ib_rmpp_hdr *)((u8 *)packet->mad.data + sizeof(struct ib_mad_hdr));
+	hdr_len = ib_get_mad_data_offset(mad_hdr.mgmt_class);
 
-	if (ib_is_mad_class_rmpp(rmpp_mad->mad_hdr.mgmt_class)
+	if (ib_is_mad_class_rmpp(mad_hdr.mgmt_class)
 	    && ib_mad_kernel_rmpp_agent(agent)) {
 		copy_offset = IB_MGMT_RMPP_HDR;
-		rmpp_active = ib_get_rmpp_flags(&rmpp_mad->rmpp_hdr) &
+		rmpp_active = ib_get_rmpp_flags(&rmpp_hdr) &
 						IB_MGMT_RMPP_FLAG_ACTIVE;
 	} else {
 		copy_offset = IB_MGMT_MAD_HDR;

@@ -615,12 +617,12 @@ static ssize_t ib_umad_write(struct file *filp, const char __user *buf,
 		tid = &((struct ib_mad_hdr *) packet->msg->mad)->tid;
 		*tid = cpu_to_be64(((u64) agent->hi_tid) << 32 |
 				   (be64_to_cpup(tid) & 0xffffffff));
-		rmpp_mad->mad_hdr.tid = *tid;
+		mad_hdr.tid = *tid;
 	}
 
 	if (!ib_mad_kernel_rmpp_agent(agent)
-	   && ib_is_mad_class_rmpp(rmpp_mad->mad_hdr.mgmt_class)
-	   && (ib_get_rmpp_flags(&rmpp_mad->rmpp_hdr) & IB_MGMT_RMPP_FLAG_ACTIVE)) {
+	    && ib_is_mad_class_rmpp(mad_hdr.mgmt_class)
+	    && (ib_get_rmpp_flags(&rmpp_hdr) & IB_MGMT_RMPP_FLAG_ACTIVE)) {
 		spin_lock_irq(&file->send_lock);
 		list_add_tail(&packet->list, &file->send_list);
 		spin_unlock_irq(&file->send_lock);

2.34.1