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SubjectKMSAN: uninit-value in simple_attr_read

syzbot found the following crash on:

HEAD commit: 8bbbc5cf kmsan: don't compile memmove
git tree: master
console output:
kernel config:
dashboard link:
compiler: clang version 10.0.0 ( c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
syz repro:
C reproducer:

IMPORTANT: if you fix the bug, please add the following tag to the commit:

BUG: KMSAN: uninit-value in strlen+0x5e/0xa0 lib/string.c:552
CPU: 1 PID: 11402 Comm: syz-executor230 Not tainted 5.6.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x1c9/0x220 lib/dump_stack.c:118
kmsan_report+0xf7/0x1e0 mm/kmsan/kmsan_report.c:118
__msan_warning+0x58/0xa0 mm/kmsan/kmsan_instr.c:215
strlen+0x5e/0xa0 lib/string.c:552
simple_attr_read+0x1ec/0x740 fs/libfs.c:935
debugfs_attr_read+0x13e/0x290 fs/debugfs/file.c:360
__vfs_read+0x1a9/0xc80 fs/read_write.c:425
vfs_read+0x346/0x6a0 fs/read_write.c:461
ksys_read+0x267/0x450 fs/read_write.c:587
__do_sys_read fs/read_write.c:597 [inline]
__se_sys_read+0x92/0xb0 fs/read_write.c:595
__x64_sys_read+0x4a/0x70 fs/read_write.c:595
do_syscall_64+0xb8/0x160 arch/x86/entry/common.c:296
RIP: 0033:0x440269
Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 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 fb 13 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fff09d3bc68 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 0000000000440269
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000004
RBP: 00000000006ca018 R08: 000000000000000a R09: 000000000000000a
R10: 0000000000010001 R11: 0000000000000246 R12: 0000000000401af0
R13: 0000000000401b80 R14: 0000000000000000 R15: 0000000000000000

Uninit was created at:
kmsan_save_stack_with_flags mm/kmsan/kmsan.c:144 [inline]
kmsan_internal_poison_shadow+0x66/0xd0 mm/kmsan/kmsan.c:127
kmsan_slab_alloc+0x8a/0xe0 mm/kmsan/kmsan_hooks.c:82
slab_alloc_node mm/slub.c:2793 [inline]
slab_alloc mm/slub.c:2802 [inline]
kmem_cache_alloc_trace+0x6f3/0xd70 mm/slub.c:2819
kmalloc include/linux/slab.h:555 [inline]
simple_attr_open+0xd4/0x400 fs/libfs.c:894
lowpan_enable_fops_open+0x94/0xb0 net/bluetooth/6lowpan.c:1105
open_proxy_open+0x657/0x800 fs/debugfs/file.c:189
do_dentry_open+0xf89/0x1820 fs/open.c:797
vfs_open+0xaf/0xe0 fs/open.c:914
do_last fs/namei.c:3490 [inline]
path_openat+0x4d57/0x6bd0 fs/namei.c:3607
do_filp_open+0x2b8/0x710 fs/namei.c:3637
do_sys_openat2+0x92e/0xd40 fs/open.c:1149
do_sys_open fs/open.c:1165 [inline]
__do_sys_openat fs/open.c:1179 [inline]
__se_sys_openat+0x24a/0x2b0 fs/open.c:1174
__x64_sys_openat+0x56/0x70 fs/open.c:1174
do_syscall_64+0xb8/0x160 arch/x86/entry/common.c:296

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