WARNING in format_decode

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 826f3285 Revert "BACKPORT, FROMGIT: crypto: speck - add su..
git tree: android-4.9
console output: https://syzkaller.appspot.com/x/log.txt?x=1528c175400000
kernel config: https://syzkaller.appspot.com/x/.config?x=3303f42e9d7e07c5
dashboard link: https://syzkaller.appspot.com/bug?extid=68bb74cd0b7caeeaf86f
compiler: gcc (GCC) 8.0.1 20180413 (experimental)

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+68bb74...@syzkaller.appspotmail.com

------------[ cut here ]------------
WARNING: CPU: 1 PID: 4713 at lib/vsprintf.c:1897 format_decode+0x7bb/0x9e0
lib/vsprintf.c:1897
Please remove unsupported % in format string
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 4713 Comm: syz-executor0 Not tainted 4.9.135+ #110
ffff8801ce997358 ffffffff81b36bf9 ffffffff82a38ba0 00000000ffffffff
0000000000000000 0000000000000001 0000000000000769 ffff8801ce997418
ffffffff813f6aa5 0000000041b58ab3 ffffffff82e29bcb ffffffff813f68e6
Call Trace:
[<ffffffff81b36bf9>] __dump_stack lib/dump_stack.c:15 [inline]
[<ffffffff81b36bf9>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
[<ffffffff813f6aa5>] panic+0x1bf/0x39f kernel/panic.c:179
[<ffffffff813f6d74>] __warn.cold.9+0xc1/0x17f kernel/panic.c:542
[<ffffffff810dbcc2>] warn_slowpath_fmt+0xc2/0x100 kernel/panic.c:565
[<ffffffff81b5751b>] format_decode+0x7bb/0x9e0 lib/vsprintf.c:1897
[<ffffffff81b60aa3>] vsnprintf+0x163/0x1840 lib/vsprintf.c:1993
[<ffffffff815822a3>] seq_vprintf+0xe3/0x1a0 fs/seq_file.c:403
[<ffffffff8158240b>] seq_printf+0xab/0xe0 fs/seq_file.c:418
[<ffffffff82723417>] get_tcp6_sock net/ipv6/tcp_ipv6.c:1787 [inline]
[<ffffffff82723417>] tcp6_seq_show+0xdf7/0x1ad0 net/ipv6/tcp_ipv6.c:1861
[<ffffffff81581135>] seq_read+0xa75/0x12d0 fs/seq_file.c:275
[<ffffffff8165bf9d>] proc_reg_read+0xfd/0x180 fs/proc/inode.c:203
[<ffffffff81509de5>] do_loop_readv_writev.part.1+0xd5/0x280
fs/read_write.c:718
[<ffffffff8150b48e>] do_loop_readv_writev fs/read_write.c:707 [inline]
[<ffffffff8150b48e>] do_readv_writev+0x56e/0x7b0 fs/read_write.c:873
[<ffffffff8150b754>] vfs_readv+0x84/0xc0 fs/read_write.c:897
[<ffffffff8150bb87>] do_preadv+0x197/0x240 fs/read_write.c:974
[<ffffffff8150f140>] SYSC_preadv fs/read_write.c:1024 [inline]
[<ffffffff8150f140>] SyS_preadv+0x30/0x40 fs/read_write.c:1019
[<ffffffff810056ef>] do_syscall_64+0x19f/0x550 arch/x86/entry/common.c:285
[<ffffffff82803413>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb
Kernel Offset: disabled
Rebooting in 86400 seconds..


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.