KMSAN: uninit-value in snd_midi_event_encode_byte

[syzbot <syzbot+194dffdb8b22fc5d207a@syzkaller.appspotmail.com>]

Hello,

syzbot found the following crash on:

HEAD commit:    2dca2cbde67a kmsan: fix build warnings with CONFIG_KMSAN=n
git tree:       https://github.com/google/kmsan.git/master
console output: https://syzkaller.appspot.com/x/log.txt?x=1660ef6a400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=820d6393634b55e3
dashboard link: https://syzkaller.appspot.com/bug?extid=194dffdb8b22fc5d207a
compiler:       clang version 8.0.0 (trunk 339414)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+194dffdb8b22fc5d207a@syzkaller.appspotmail.com

==================================================================
BUG: KMSAN: uninit-value in snd_midi_event_encode_byte+0x569/0xff0  
sound/core/seq/seq_midi_event.c:195
CPU: 0 PID: 1623 Comm: kworker/0:1H Not tainted 4.19.0-rc1+ #36
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: events_highpri snd_vmidi_output_work
Call Trace:
  __dump_stack lib/dump_stack.c:77 [inline]
  dump_stack+0x14b/0x190 lib/dump_stack.c:113
  kmsan_report+0x183/0x2b0 mm/kmsan/kmsan.c:956
  __msan_warning+0x70/0xc0 mm/kmsan/kmsan_instr.c:645
  snd_midi_event_encode_byte+0x569/0xff0 sound/core/seq/seq_midi_event.c:195
  snd_vmidi_output_work+0x34e/0x5b0 sound/core/seq/seq_virmidi.c:161
  process_one_work+0x1605/0x1f40 kernel/workqueue.c:2153
  worker_thread+0x11a2/0x2590 kernel/workqueue.c:2296
  kthread+0x465/0x4a0 kernel/kthread.c:247
  ret_from_fork+0x35/0x40 arch/x86/entry/entry_64.S:416

Uninit was stored to memory at:
  kmsan_save_stack_with_flags mm/kmsan/kmsan.c:256 [inline]
  kmsan_save_stack mm/kmsan/kmsan.c:271 [inline]
  kmsan_internal_chain_origin+0x128/0x210 mm/kmsan/kmsan.c:573
  __msan_chain_origin+0x69/0xc0 mm/kmsan/kmsan_instr.c:482
  __snd_rawmidi_transmit_peek sound/core/rawmidi.c:1103 [inline]
  snd_rawmidi_transmit+0xa75/0xbf0 sound/core/rawmidi.c:1228
  snd_vmidi_output_work+0x2ac/0x5b0 sound/core/seq/seq_virmidi.c:159
  process_one_work+0x1605/0x1f40 kernel/workqueue.c:2153
  worker_thread+0x11a2/0x2590 kernel/workqueue.c:2296
  kthread+0x465/0x4a0 kernel/kthread.c:247
  ret_from_fork+0x35/0x40 arch/x86/entry/entry_64.S:416

Uninit was created at:
  kmsan_save_stack_with_flags mm/kmsan/kmsan.c:256 [inline]
  kmsan_internal_poison_shadow+0xb8/0x1b0 mm/kmsan/kmsan.c:181
  kmsan_kmalloc+0x98/0x100 mm/kmsan/kmsan_hooks.c:91
  __kmalloc_node+0x7bf/0x11c0 mm/slub.c:3828
  kmalloc_node include/linux/slab.h:555 [inline]
  kvmalloc_node+0x19d/0x3e0 mm/util.c:423
  kvmalloc include/linux/mm.h:577 [inline]
  snd_rawmidi_runtime_create sound/core/rawmidi.c:132 [inline]
  open_substream+0x3c8/0xaa0 sound/core/rawmidi.c:276
  rawmidi_open_priv+0x347/0x1000 sound/core/rawmidi.c:327
  snd_rawmidi_open+0x7d4/0x1120 sound/core/rawmidi.c:424
  soundcore_open+0x9be/0xa60 sound/sound_core.c:597
  chrdev_open+0xc26/0xdb0 fs/char_dev.c:417
  do_dentry_open+0xc9d/0x1660 fs/open.c:771
  vfs_open+0xaf/0xe0 fs/open.c:880
  do_last fs/namei.c:3418 [inline]
  path_openat+0x17a6/0x67c0 fs/namei.c:3534
  do_filp_open+0x259/0x610 fs/namei.c:3564
  do_sys_open+0x630/0x940 fs/open.c:1063
  __do_sys_open fs/open.c:1081 [inline]
  __se_sys_open fs/open.c:1076 [inline]
  __x64_sys_open+0xf2/0x130 fs/open.c:1076
  do_syscall_64+0x15b/0x220 arch/x86/entry/common.c:290
  entry_SYSCALL_64_after_hwframe+0x63/0xe7
==================================================================


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