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SubjectWARNING in add_uevent_var

syzbot hit the following crash on upstream commit
10b84daddbec72c6b440216a69de9a9605127f7a (Sat Mar 31 17:59:00 2018 +0000)
Merge branch 'perf-urgent-for-linus' of
syzbot dashboard link:

So far this crash happened 5 times on net-next, upstream.
C reproducer:
syzkaller reproducer:
Raw console output:
Kernel config:
compiler: gcc (GCC) 7.1.1 20170620

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------------[ cut here ]------------
add_uevent_var: buffer size too small
WARNING: CPU: 0 PID: 4431 at lib/kobject_uevent.c:594
add_uevent_var+0x2aa/0x2d0 lib/kobject_uevent.c:594
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 4431 Comm: syzkaller678536 Not tainted 4.16.0-rc7+ #374
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0x194/0x24d lib/dump_stack.c:53
panic+0x1e4/0x41c kernel/panic.c:183
__warn+0x1dc/0x200 kernel/panic.c:547
report_bug+0x1f4/0x2b0 lib/bug.c:186
fixup_bug.part.10+0x37/0x80 arch/x86/kernel/traps.c:178
fixup_bug arch/x86/kernel/traps.c:247 [inline]
do_error_trap+0x2d7/0x3e0 arch/x86/kernel/traps.c:296
do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:315
invalid_op+0x1b/0x40 arch/x86/entry/entry_64.S:986
RIP: 0010:add_uevent_var+0x2aa/0x2d0 lib/kobject_uevent.c:594
RSP: 0018:ffff8801adba6810 EFLAGS: 00010286
RAX: dffffc0000000008 RBX: ffff8801ad7a6d40 RCX: ffffffff815b193e
RDX: 0000000000000000 RSI: 1ffff10035b74cb2 RDI: 1ffff10035b74c87
RBP: ffff8801adba68d8 R08: 1ffff10035b74c49 R09: 0000000000000000
R10: 0000000000000002 R11: 0000000000000000 R12: 1ffff10035b74d03
R13: ffff8801ad7a6e58 R14: 00000000000003e6 R15: 0000000000000438
rfkill_dev_uevent+0x31/0x170 net/rfkill/core.c:813
dev_uevent+0x2b6/0x7e0 drivers/base/core.c:913
kobject_uevent_env+0x3fe/0xd30 lib/kobject_uevent.c:476
kobject_uevent+0x1f/0x30 lib/kobject_uevent.c:565
device_add+0xd04/0x1650 drivers/base/core.c:1834
rfkill_register+0x254/0xd60 net/rfkill/core.c:1019
wiphy_register+0x19d1/0x2050 net/wireless/core.c:872
ieee80211_register_hw+0x1162/0x3100 net/mac80211/main.c:1041
hwsim_new_radio_nl+0x67a/0x8c0 drivers/net/wireless/mac80211_hwsim.c:3214
genl_family_rcv_msg+0x7b7/0xfb0 net/netlink/genetlink.c:599
genl_rcv_msg+0xb2/0x140 net/netlink/genetlink.c:624
netlink_rcv_skb+0x14b/0x380 net/netlink/af_netlink.c:2447
genl_rcv+0x28/0x40 net/netlink/genetlink.c:635
netlink_unicast_kernel net/netlink/af_netlink.c:1311 [inline]
netlink_unicast+0x4c4/0x6b0 net/netlink/af_netlink.c:1337
netlink_sendmsg+0xa4a/0xe60 net/netlink/af_netlink.c:1900
sock_sendmsg_nosec net/socket.c:630 [inline]
sock_sendmsg+0xca/0x110 net/socket.c:640
___sys_sendmsg+0x767/0x8b0 net/socket.c:2046
__sys_sendmsg+0xe5/0x210 net/socket.c:2080
SYSC_sendmsg net/socket.c:2091 [inline]
SyS_sendmsg+0x2d/0x50 net/socket.c:2087
do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
RIP: 0033:0x43fda9
RSP: 002b:00007ffde548edb8 EFLAGS: 00000213 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 000000000043fda9
RDX: 0000000000000000 RSI: 0000000020b3dfc8 RDI: 0000000000000003
RBP: 00000000006cb018 R08: 00000000004002c8 R09: 00000000004002c8
R10: 00000000004002c8 R11: 0000000000000213 R12: 00000000004016d0
R13: 0000000000401760 R14: 0000000000000000 R15: 0000000000000000
Dumping ftrace buffer:
(ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..

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