UBSAN reports issue in ip_idents_reserve

Hi.

I'm consistently getting following UBSAN warning on every bootup:

[   47.545820] ================================================================================
[   47.554340] UBSAN: Undefined behaviour in ./arch/x86/include/asm/atomic.h:156:11
[   47.561808] signed integer overflow:
[   47.565420] -2117905507 + -695755206 cannot be represented in type 'int'
[   47.572226] CPU: 0 PID: 389 Comm: ntpd Not tainted 4.8.0-rc6jiri+ #1
[   47.578636] Hardware name: Mellanox Technologies Ltd. Mellanox switch/Mellanox switch, BIOS 4.6.5 05/21/2015
[   47.588586]  ffffffff847bf8c0 00000000987b8f47 ffff8803829af5a8 ffffffff818354e3
[   47.596165]  0000000041b58ab3 ffffffff8277e711 ffffffff81835431 ffff8803829af5d0
[   47.603722]  ffff8803829af580 ffffffffd6879e3a 1ffffffff08f8214 ffffed0070535e6c
[   47.611298] Call Trace:
[   47.613795]  [<ffffffff818354e3>] dump_stack+0xb2/0x10f
[   47.619077]  [<ffffffff81835431>] ? _atomic_dec_and_lock+0xa1/0xa1
[   47.625327]  [<ffffffff818a884f>] ubsan_epilogue+0xd/0x4e
[   47.630811]  [<ffffffff818a9821>] handle_overflow+0x190/0x1de
[   47.636627]  [<ffffffff818a9691>] ? __ubsan_handle_negate_overflow+0x140/0x140
[   47.643914]  [<ffffffff81863130>] ? iov_iter_copy_from_user_atomic+0x6e0/0x6e0
[   47.651219]  [<ffffffff811e6f79>] ? __lock_acquire.isra.17+0xb79/0xe50
[   47.657832]  [<ffffffff81e581f2>] ? ip_generic_getfrag+0xd2/0x190
[   47.664011]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
[   47.669827]  [<ffffffff818a987d>] __ubsan_handle_add_overflow+0xe/0x10
[   47.676444]  [<ffffffff81e41d52>] ip_idents_reserve+0xb2/0xe0
[   47.682254]  [<ffffffff81e443e9>] __ip_select_ident+0x159/0x1b0
[   47.688248]  [<ffffffff81e44290>] ? update_or_create_fnhe+0x850/0x850
[   47.694782]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
[   47.700624]  [<ffffffff81e5ef40>] __ip_make_skb+0x8a0/0xab0
[   47.706259]  [<ffffffff81e5f3fd>] ip_make_skb+0x17d/0x1d0
[   47.711717]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
[   47.717526]  [<ffffffff81e5f280>] ? ip_flush_pending_frames+0x20/0x20
[   47.724032]  [<ffffffff81e46ef0>] ? ip_rt_update_pmtu+0x4f0/0x4f0
[   47.730231]  [<ffffffff81f35291>] ? xfrm_lookup_route+0x21/0xe0
[   47.736216]  [<ffffffff81ec0cdb>] udp_sendmsg+0x9db/0xf60
[   47.741668]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
[   47.747472]  [<ffffffff81ec0300>] ? udp_abort+0x70/0x70
[   47.752763]  [<ffffffff81ede3d8>] inet_sendmsg+0x198/0x220
[   47.758324]  [<ffffffff81ede292>] ? inet_sendmsg+0x52/0x220
[   47.763982]  [<ffffffff81ede240>] ? inet_recvmsg+0x300/0x300
[   47.769728]  [<ffffffff81d6fd25>] sock_sendmsg+0xa5/0xd0
[   47.775100]  [<ffffffff81d72f70>] SYSC_sendto+0x1d0/0x280
[   47.780551]  [<ffffffff81d72da0>] ? SYSC_connect+0x200/0x200
[   47.786283]  [<ffffffff814f66df>] ? poll_select_copy_remaining+0x2af/0x310
[   47.793265]  [<ffffffff814f6430>] ? set_fd_set+0x60/0x60
[   47.798665]  [<ffffffff811ee360>] ? do_raw_spin_trylock+0x90/0x90
[   47.804853]  [<ffffffff814f80e3>] ? SyS_select+0x1a3/0x200
[   47.810399]  [<ffffffff814f7f40>] ? core_sys_select+0x570/0x570
[   47.816415]  [<ffffffff8100467c>] ? exit_to_usermode_loop+0xec/0x110
[   47.822842]  [<ffffffff811e8abd>] ? lockdep_sys_exit+0x2d/0xb0
[   47.828769]  [<ffffffff81004016>] ? lockdep_sys_exit_thunk+0x16/0x30
[   47.835199]  [<ffffffff81d7433e>] SyS_sendto+0xe/0x10
[   47.840321]  [<ffffffff820381f2>] entry_SYSCALL_64_fastpath+0x1a/0xa9
[   47.846826] ================================================================================

Looks like this might be result of following commit:

commit 04ca6973f7c1a0d8537f2d9906a0cf8e69886d75
Author: Eric Dumazet [off-list ref]
Date:   Sat Jul 26 08:58:10 2014 +0200

    ip: make IP identifiers less predictable

Eric, could you please take look at that?

Thanks.

Jiri