>>> I do not observe the following error if
>>> * kernel is compiled with gcc 10, or
>>> * kernel version is 5.13.9 or 5.14rc5.
>>> The reported UBSAN error is only seen with 5.10 series compiled with
>>> CLang 12.

Sorry, what I originally reported is partially wrong.
I also observe UBSAN with 5.13.11 compiled with CLang 11 as below,
which has the same pattern as 5.10:

Aug 17 10:22:19 raspi4b-router kernel: IPv6: ADDRCONF(NETDEV_CHANGE): wlan0: link becomes ready
Aug 17 10:22:19 raspi4b-router systemd[1]: iwd_start_ap@Yamashita_guest.service: Succeeded.
Aug 17 10:22:19 raspi4b-router systemd[1]: Finished iwd starting Yamashita_guest access point.
Aug 17 10:22:19 raspi4b-router kernel: ================================================================================
Aug 17 10:22:19 raspi4b-router kernel: UBSAN: object-size-mismatch in ./include/linux/skbuff.h:2038:28
Aug 17 10:22:19 raspi4b-router kernel: member access within address 00000000eea757fb with insufficient space
Aug 17 10:22:19 raspi4b-router kernel: for an object of type 'struct sk_buff'
Aug 17 10:22:19 raspi4b-router kernel: CPU: 0 PID: 7 Comm: kworker/u8:0 Tainted: G         C        5.13.11-clang11 #1
Aug 17 10:22:19 raspi4b-router kernel: Hardware name: Raspberry Pi 4 Model B Rev 1.4 (DT)
Aug 17 10:22:19 raspi4b-router kernel: Workqueue: brcmf_wq/mmc0:0001:1 brcmf_sdio_dataworker [brcmfmac]
Aug 17 10:22:19 raspi4b-router kernel: Call trace:
Aug 17 10:22:19 raspi4b-router kernel:  dump_backtrace+0x0/0x2a4
Aug 17 10:22:19 raspi4b-router kernel:  dump_stack+0xc8/0x150
Aug 17 10:22:19 raspi4b-router kernel:  ubsan_type_mismatch_common+0x198/0x2a4
Aug 17 10:22:19 raspi4b-router kernel:  __ubsan_handle_type_mismatch_v1+0x40/0x50
Aug 17 10:22:19 raspi4b-router kernel:  brcmf_sdio_dpc+0x2078/0x28f0 [brcmfmac]
Aug 17 10:22:19 raspi4b-router kernel:  brcmf_sdio_dataworker+0x50/0x8c [brcmfmac]
Aug 17 10:22:19 raspi4b-router kernel:  process_one_work+0x318/0x4fc
Aug 17 10:22:19 raspi4b-router kernel:  worker_thread+0x2f0/0x660
Aug 17 10:22:19 raspi4b-router kernel:  kthread+0x304/0x340
Aug 17 10:22:19 raspi4b-router kernel:  ret_from_fork+0x10/0x18
Aug 17 10:22:19 raspi4b-router kernel: ================================================================================
Aug 17 10:22:19 raspi4b-router systemd[1]: Starting Network Service...

I attach full journalctl -b on kernel 5.13.11.

Best regards, Ryutaroh