KMSAN: uninit-value in strnlen

[syzbot <syzbot+cd06c321e7147d03a65e@syzkaller.appspotmail.com>]

Hello,

syzbot hit the following crash on  
https://github.com/google/kmsan.git/master commit
a7f95e9c8a95e9fbb388c3999b61a17667cd3bbe (Sat Apr 21 13:50:22 2018 +0000)
kmsan: disable assembly checksums
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=cd06c321e7147d03a65e

So far this crash happened 5 times on  
https://github.com/google/kmsan.git/master.
C reproducer: https://syzkaller.appspot.com/x/repro.c?id=5785171018121216
syzkaller reproducer:  
https://syzkaller.appspot.com/x/repro.syz?id=5117671628603392
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=6310764688179200
Kernel config: https://syzkaller.appspot.com/x/.config?id=328654897048964367
compiler: clang version 7.0.0 (trunk 329391)

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+cd06c321e7147d03a65e@syzkaller.appspotmail.com
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==================================================================
BUG: KMSAN: uninit-value in strnlen+0xc4/0x110 lib/string.c:499
CPU: 1 PID: 4507 Comm: syzkaller579712 Not tainted 4.16.0+ #85
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+0x185/0x1d0 lib/dump_stack.c:53
  kmsan_report+0x142/0x240 mm/kmsan/kmsan.c:1067
  __msan_warning_32+0x6c/0xb0 mm/kmsan/kmsan_instr.c:683
  strnlen+0xc4/0x110 lib/string.c:499
  dev_name_hash net/core/dev.c:209 [inline]
  dev_get_by_name_rcu net/core/dev.c:764 [inline]
  dev_get_by_name+0x6e/0x350 net/core/dev.c:791
  pppoe_connect+0xcb7/0x2360 drivers/net/ppp/pppoe.c:665
  SYSC_connect+0x41a/0x510 net/socket.c:1639
  SyS_connect+0x54/0x80 net/socket.c:1620
  do_syscall_64+0x309/0x430 arch/x86/entry/common.c:287
  entry_SYSCALL_64_after_hwframe+0x3d/0xa2
RIP: 0033:0x43fcf9
RSP: 002b:00007ffca4bd4978 EFLAGS: 00000213 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 000000000043fcf9
RDX: 0000000000000007 RSI: 0000000020000040 RDI: 0000000000000003
RBP: 00000000006ca018 R08: 00000000004002c8 R09: 00000000004002c8
R10: 00000000004002c8 R11: 0000000000000213 R12: 0000000000401620
R13: 00000000004016b0 R14: 0000000000000000 R15: 0000000000000000

Local variable description: ----address@SYSC_connect
Variable was created at:
  SYSC_connect+0x6f/0x510 net/socket.c:1622
  SyS_connect+0x54/0x80 net/socket.c:1620
==================================================================


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