RDMA/mlx5: Fix fortify source warning while accessing Eth segment

 ------------[ cut here ]------------
 memcpy: detected field-spanning write (size 56) of single field "eseg->inline_hdr.start" at /var/lib/dkms/mlnx-ofed-kernel/5.8/build/drivers/infiniband/hw/mlx5/wr.c:131 (size 2)
 WARNING: CPU: 0 PID: 293779 at /var/lib/dkms/mlnx-ofed-kernel/5.8/build/drivers/infiniband/hw/mlx5/wr.c:131 mlx5_ib_post_send+0x191b/0x1a60 [mlx5_ib]
 Modules linked in: 8021q garp mrp stp llc rdma_ucm(OE) rdma_cm(OE) iw_cm(OE) ib_ipoib(OE) ib_cm(OE) ib_umad(OE) mlx5_ib(OE) ib_uverbs(OE) ib_core(OE) mlx5_core(OE) pci_hyperv_intf mlxdevm(OE) mlx_compat(OE) tls mlxfw(OE) psample nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 ip_set nf_tables libcrc32c nfnetlink mst_pciconf(OE) knem(OE) vfio_pci vfio_pci_core vfio_iommu_type1 vfio iommufd irqbypass cuse nfsv3 nfs fscache netfs xfrm_user xfrm_algo ipmi_devintf ipmi_msghandler binfmt_misc crct10dif_pclmul crc32_pclmul polyval_clmulni polyval_generic ghash_clmulni_intel sha512_ssse3 snd_pcsp aesni_intel crypto_simd cryptd snd_pcm snd_timer joydev snd soundcore input_leds serio_raw evbug nfsd auth_rpcgss nfs_acl lockd grace sch_fq_codel sunrpc drm efi_pstore ip_tables x_tables autofs4 psmouse virtio_net net_failover failover floppy
  [last unloaded: mlx_compat(OE)]
 CPU: 0 PID: 293779 Comm: ssh Tainted: G           OE      6.2.0-32-generic Freescale#32~22.04.1-Ubuntu
 Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
 RIP: 0010:mlx5_ib_post_send+0x191b/0x1a60 [mlx5_ib]
 Code: 0c 01 00 a8 01 75 25 48 8b 75 a0 b9 02 00 00 00 48 c7 c2 10 5b fd c0 48 c7 c7 80 5b fd c0 c6 05 57 0c 03 00 01 e8 95 4d 93 da <0f> 0b 44 8b 4d b0 4c 8b 45 c8 48 8b 4d c0 e9 49 fb ff ff 41 0f b7
 RSP: 0018:ffffb5b48478b570 EFLAGS: 00010046
 RAX: 0000000000000000 RBX: 0000000000000001 RCX: 0000000000000000
 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
 RBP: ffffb5b48478b628 R08: 0000000000000000 R09: 0000000000000000
 R10: 0000000000000000 R11: 0000000000000000 R12: ffffb5b48478b5e8
 R13: ffff963a3c609b5e R14: ffff9639c3fbd800 R15: ffffb5b480475a80
 FS:  00007fc03b444c80(0000) GS:ffff963a3dc00000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000556f46bdf000 CR3: 0000000006ac6003 CR4: 00000000003706f0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Call Trace:
  <TASK>
  ? show_regs+0x72/0x90
  ? mlx5_ib_post_send+0x191b/0x1a60 [mlx5_ib]
  ? __warn+0x8d/0x160
  ? mlx5_ib_post_send+0x191b/0x1a60 [mlx5_ib]
  ? report_bug+0x1bb/0x1d0
  ? handle_bug+0x46/0x90
  ? exc_invalid_op+0x19/0x80
  ? asm_exc_invalid_op+0x1b/0x20
  ? mlx5_ib_post_send+0x191b/0x1a60 [mlx5_ib]
  mlx5_ib_post_send_nodrain+0xb/0x20 [mlx5_ib]
  ipoib_send+0x2ec/0x770 [ib_ipoib]
  ipoib_start_xmit+0x5a0/0x770 [ib_ipoib]
  dev_hard_start_xmit+0x8e/0x1e0
  ? validate_xmit_skb_list+0x4d/0x80
  sch_direct_xmit+0x116/0x3a0
  __dev_xmit_skb+0x1fd/0x580
  __dev_queue_xmit+0x284/0x6b0
  ? _raw_spin_unlock_irq+0xe/0x50
  ? __flush_work.isra.0+0x20d/0x370
  ? push_pseudo_header+0x17/0x40 [ib_ipoib]
  neigh_connected_output+0xcd/0x110
  ip_finish_output2+0x179/0x480
  ? __smp_call_single_queue+0x61/0xa0
  __ip_finish_output+0xc3/0x190
  ip_finish_output+0x2e/0xf0
  ip_output+0x78/0x110
  ? __pfx_ip_finish_output+0x10/0x10
  ip_local_out+0x64/0x70
  __ip_queue_xmit+0x18a/0x460
  ip_queue_xmit+0x15/0x30
  __tcp_transmit_skb+0x914/0x9c0
  tcp_write_xmit+0x334/0x8d0
  tcp_push_one+0x3c/0x60
  tcp_sendmsg_locked+0x2e1/0xac0
  tcp_sendmsg+0x2d/0x50
  inet_sendmsg+0x43/0x90
  sock_sendmsg+0x68/0x80
  sock_write_iter+0x93/0x100
  vfs_write+0x326/0x3c0
  ksys_write+0xbd/0xf0
  ? do_syscall_64+0x69/0x90
  __x64_sys_write+0x19/0x30
  do_syscall_64+0x59/0x90
  ? do_user_addr_fault+0x1d0/0x640
  ? exit_to_user_mode_prepare+0x3b/0xd0
  ? irqentry_exit_to_user_mode+0x9/0x20
  ? irqentry_exit+0x43/0x50
  ? exc_page_fault+0x92/0x1b0
  entry_SYSCALL_64_after_hwframe+0x72/0xdc
 RIP: 0033:0x7fc03ad14a37
 Code: 10 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 00 f3 0f 1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 51 c3 48 83 ec 28 48 89 54 24 18 48 89 74 24
 RSP: 002b:00007ffdf8697fe8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
 RAX: ffffffffffffffda RBX: 0000000000008024 RCX: 00007fc03ad14a37
 RDX: 0000000000008024 RSI: 0000556f46bd8270 RDI: 0000000000000003
 RBP: 0000556f46bb1800 R08: 0000000000007fe3 R09: 0000000000000000
 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000002
 R13: 0000556f46bc66b0 R14: 000000000000000a R15: 0000556f46bb2f50
  </TASK>
 ---[ end trace 0000000000000000 ]---

Link: https://lore.kernel.org/r/8228ad34bd1a25047586270f7b1fb4ddcd046282.1706433934.git.leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>

drivers/infiniband/hw/mlx5/wr.c

@@ -78,7 +78,7 @@ static void set_eth_seg(const struct ib_send_wr *wr, struct mlx5_ib_qp *qp,
 		 */
 		copysz = min_t(u64, *cur_edge - (void *)eseg->inline_hdr.start,
 			       left);
-		memcpy(eseg->inline_hdr.start, pdata, copysz);
+		memcpy(eseg->inline_hdr.data, pdata, copysz);
 		stride = ALIGN(sizeof(struct mlx5_wqe_eth_seg) -
 			       sizeof(eseg->inline_hdr.start) + copysz, 16);
 		*size += stride / 16;

include/linux/mlx5/qp.h

@@ -269,7 +269,10 @@ struct mlx5_wqe_eth_seg {
 	union {
 		struct {
 			__be16 sz;
-			u8     start[2];
+			union {
+				u8     start[2];
+				DECLARE_FLEX_ARRAY(u8, data);
+			};
 		} inline_hdr;
 		struct {
 			__be16 type;