I get the following UBSAN warning during boot on my laptop:

================================================================================
UBSAN: Undefined behaviour in drivers/net/wireless/broadcom/brcm80211/brcmsmac/phy/phy_qmath.c:280:21
index 32 is out of range for type 's16 [32]'
CPU: 0 PID: 879 Comm: NetworkManager Not tainted 4.9.0-rc4 #28
Hardware name: LENOVO Lenovo IdeaPad N581/INVALID, BIOS 5ECN96WW(V9.01) 03/14/2013
ffff8800b74a6478 ffffffff828e59d2 0000000041b58ab3 ffffffff8398330c
ffffffff828e5920 ffff8800b74a64a0 ffff8800b74a6450 0000000000000020
1ffffffff845848c ffffed0016e94bf1 ffffffffc22c2460 000000006b9c0514
Call Trace:
[<ffffffff828e59d2>] dump_stack+0xb2/0x110
[<ffffffff828e5920>] ? _atomic_dec_and_lock+0x150/0x150
[<ffffffff82968c9d>] ubsan_epilogue+0xd/0x4e
[<ffffffff82969875>] __ubsan_handle_out_of_bounds+0xfa/0x13e
[<ffffffff8296977b>] ? __ubsan_handle_shift_out_of_bounds+0x241/0x241
[<ffffffffc0d48379>] ? bcma_host_pci_read16+0x59/0xa0 [bcma]
[<ffffffffc0d48388>] ? bcma_host_pci_read16+0x68/0xa0 [bcma]
[<ffffffffc212ad78>] ? read_phy_reg+0xe8/0x180 [brcmsmac]
[<ffffffffc2184714>] qm_log10+0x2e4/0x350 [brcmsmac]
[<ffffffffc2142eb8>] wlc_phy_init_lcnphy+0x538/0x1f20 [brcmsmac]
[<ffffffffc2142980>] ? wlc_lcnphy_periodic_cal+0x5c0/0x5c0 [brcmsmac]
[<ffffffffc1ba0c93>] ? ieee80211_open+0xb3/0x110 [mac80211]
[<ffffffff82f73a02>] ? sk_busy_loop+0x1e2/0x840
[<ffffffff82f7a6ce>] ? __dev_change_flags+0xae/0x220
...

The report is valid: doing the math in this function, with an input value
N=63 the variable s16tableIndex gets a value of 31. This value is used as
an index in the array log_table with 32 entries. But the next line is:

	s16errorApproximation = (s16) qm_mulu16(u16offset,
				(u16) (log_table[s16tableIndex + 1] -
				       log_table[s16tableIndex]));

With s16tableIndex + 1 we are trying an out-of-bounds access to the array.

The log_table array provides log2 values in q.15 format and the above
statement tries an error approximation with the next value. To fix this
issue add the next value to the array and update the comment accordingly.
Signed-off-by: Tobias Regnery <tobias.regnery@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>