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SubjectRe: KMSAN: uninit-value in snd_midi_event_encode_byte
From
syzbot has found a reproducer for the following crash on:

HEAD commit: 28f0ca98eadf kmsan: don't instrument do_syscall_64() and _..
git tree: https://github.com/google/kmsan.git/master
console output: https://syzkaller.appspot.com/x/log.txt?x=10556c92400000
kernel config: https://syzkaller.appspot.com/x/.config?x=3431f03869413153
dashboard link: https://syzkaller.appspot.com/bug?extid=194dffdb8b22fc5d207a
compiler: clang version 8.0.0 (trunk 339414)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=123a520a400000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16068dc1400000

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: 1 PID: 1659 Comm: kworker/1:1H Not tainted 4.19.0-rc1+ #40
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+0xce6/0x1740 fs/open.c:771
vfs_open+0xaf/0xe0 fs/open.c:880
do_last fs/namei.c:3418 [inline]
path_openat+0x1799/0x6870 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+0xad/0xc0 fs/open.c:1076
__x64_sys_open+0x4a/0x70 fs/open.c:1076
do_syscall_64+0xb8/0x100 arch/x86/entry/common.c:291
entry_SYSCALL_64_after_hwframe+0x63/0xe7
==================================================================

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