static inline bool addr4_match(__be32 a1, __be32 a2, u8 prefixlen)
{
	/* C99 6.5.7 (3): u32 << 32 is undefined behaviour */
	if (sizeof(long) == 4 && prefixlen == 0)
		return true;
	return !((a1 ^ a2) & htonl(~0UL << (32 - prefixlen)));
}


$ cat report0 
================================================================================
UBSAN: Undefined behaviour in ./include/net/xfrm.h:894:23
shift exponent -128 is negative
CPU: 0 PID: 6190 Comm: syz-executor1 Not tainted 4.18.0-rc1 #2
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.10.2-0-g5f4c7b1-prebuilt.qemu-project.org 04/01/2014
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x122/0x1c8 lib/dump_stack.c:113
 ubsan_epilogue+0x12/0x86 lib/ubsan.c:159
 __ubsan_handle_shift_out_of_bounds+0x29a/0x2ff lib/ubsan.c:425
 addr4_match include/net/xfrm.h:894 [inline]
 __xfrm4_selector_match net/xfrm/xfrm_policy.c:77 [inline]
 xfrm_selector_match+0xde9/0x11e0 net/xfrm/xfrm_policy.c:102
 xfrm_sk_policy_lookup+0x179/0x460 net/xfrm/xfrm_policy.c:1178
 xfrm_lookup+0x20e/0x1be0 net/xfrm/xfrm_policy.c:2149
 xfrm_lookup_route+0x42/0x1f0 net/xfrm/xfrm_policy.c:2282
 ip_route_output_flow+0x86/0xc0 net/ipv4/route.c:2588
 udp_sendmsg+0x15c1/0x2180 net/ipv4/udp.c:1086
 inet_sendmsg+0x103/0x490 net/ipv4/af_inet.c:798
 sock_sendmsg_nosec net/socket.c:645 [inline]
 sock_sendmsg+0xf9/0x180 net/socket.c:655
 __sys_sendto+0x239/0x3c0 net/socket.c:1833
 __do_sys_sendto net/socket.c:1845 [inline]
 __se_sys_sendto net/socket.c:1841 [inline]
 __x64_sys_sendto+0xef/0x1c0 net/socket.c:1841
 do_syscall_64+0xb8/0x3a0 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x455a09
Code: 1d ba fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 eb b9 fb ff c3 66 2e 0f 1f 84 00 00 00 00 
RSP: 002b:00007f0b710bdc68 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007f0b710be6d4 RCX: 0000000000455a09
RDX: 0000000000000000 RSI: 00000000200014c0 RDI: 0000000000000013
RBP: 000000000072bea0 R08: 0000000020001540 R09: 0000000000000010
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 00000000000005d7 R14: 00000000006fdcc8 R15: 0000000000000000
================================================================================
sr 1:0:0:0: [sr0] unaligned transfer
sr 1:0:0:0: [sr0] unaligned transfer
sr 1:0:0:0: [sr0] unaligned transfer
sr 1:0:0:0: [sr0] unaligned transfer
sr 1:0:0:0: [sr0] unaligned transfer
sr 1:0:0:0: [sr0] unaligned transfer
sr 1:0:0:0: [sr0] unaligned transfer
EXT4-fs (sda): re-mounted. Opts: jqfmt=vfsold,
sr 1:0:0:0: [sr0] unaligned transfer
sr 1:0:0:0: [sr0] unaligned transfer
audit: type=1326 audit(1529680282.002:2): auid=4294967295 uid=0 gid=0 ses=4294967295 subj=kernel pid=6558 comm="syz-executor0" exe="/syz-executor0" sig=31 arch=c000003e syscall=202 compat=0 ip=0x455a09 code=0x0
sr 1:0:0:0: [sr0] tag#0 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
sr 1:0:0:0: [sr0] tag#0 Sense Key : Not Ready [current] 
sr 1:0:0:0: [sr0] tag#0 Add. Sense: Medium not present
sr 1:0:0:0: [sr0] tag#0 CDB: Read(10) 28 00 00 00 00 00 00 00 40 00
print_req_error: 25 callbacks suppressed
print_req_error: I/O error, dev sr0, sector 0
sr 1:0:0:0: [sr0] tag#0 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
sr 1:0:0:0: [sr0] tag#0 Sense Key : Not Ready [current] 
sr 1:0:0:0: [sr0] tag#0 Add. Sense: Medium not present
sr 1:0:0:0: [sr0] tag#0 CDB: Read(10) 28 00 00 00 00 00 00 00 40 00
print_req_error: I/O error, dev sr0, sector 0
sr 1:0:0:0: [sr0] tag#0 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
sr 1:0:0:0: [sr0] tag#0 Sense Key : Not Ready [current] 
sr 1:0:0:0: [sr0] tag#0 Add. Sense: Medium not present
sr 1:0:0:0: [sr0] tag#0 CDB: Read(10) 28 00 00 00 00 00 00 00 40 00
print_req_error: I/O error, dev sr0, sector 0
sr 1:0:0:0: [sr0] tag#0 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
sr 1:0:0:0: [sr0] tag#0 Sense Key : Not Ready [current] 
sr 1:0:0:0: [sr0] tag#0 Add. Sense: Medium not present
sr 1:0:0:0: [sr0] tag#0 CDB: Read(10) 28 00 00 00 00 00 00 00 40 00
print_req_error: I/O error, dev sr0, sector 0
cgroup: cgroup2: unknown option ""
cgroup: cgroup2: unknown option ""