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Subject: Re: KCSAN: data-race in echo_char / n_tty_receive_buf_common
To:     syzbot <syzbot+e518b0df8f4e19495d3e@syzkaller.appspotmail.com>,
        elver@google.com, gregkh@linuxfoundation.org,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com
References: <0000000000006881cf059683d5fb@google.com>
From:   Jiri Slaby <jslaby@suse.com>
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Date:   Mon, 4 Nov 2019 13:08:26 +0100
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On 04. 11. 19, 12:44, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    05f22368 x86, kcsan: Enable KCSAN for x86
> git tree:       https://github.com/google/ktsan.git kcsan
> console output: https://syzkaller.appspot.com/x/log.txt?x=106bdb60e00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=87d111955f40591f
> dashboard link:
> https://syzkaller.appspot.com/bug?extid=e518b0df8f4e19495d3e
> compiler:       gcc (GCC) 9.0.0 20181231 (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+e518b0df8f4e19495d3e@syzkaller.appspotmail.com
> 
> ==================================================================
> BUG: KCSAN: data-race in echo_char / n_tty_receive_buf_common
> 
> write to 0xffffc9000c433018 of 8 bytes by task 18008 on cpu 0:
>  add_echo_byte drivers/tty/n_tty.c:845 [inline]

IMO it's expected add_echo_byte is called without output_lock. That's
exactly what the barrier there is for. See:

commit ebec3f8f5271139df618ebdf8427e24ba102ba94
Author: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date:   Sat May 26 09:53:14 2018 +0900

    n_tty: Access echo_* variables carefully.

>  echo_char+0x14e/0x1c0 drivers/tty/n_tty.c:948
>  n_tty_receive_char_special+0xb5f/0x1c10 drivers/tty/n_tty.c:1339
>  n_tty_receive_buf_fast drivers/tty/n_tty.c:1610 [inline]
>  __receive_buf drivers/tty/n_tty.c:1644 [inline]
>  n_tty_receive_buf_common+0x1844/0x1b00 drivers/tty/n_tty.c:1742
>  n_tty_receive_buf+0x3a/0x50 drivers/tty/n_tty.c:1771
>  tiocsti drivers/tty/tty_io.c:2197 [inline]
>  tty_ioctl+0xb75/0xe10 drivers/tty/tty_io.c:2573
>  vfs_ioctl fs/ioctl.c:46 [inline]
>  file_ioctl fs/ioctl.c:509 [inline]
>  do_vfs_ioctl+0x991/0xc60 fs/ioctl.c:696
>  ksys_ioctl+0xbd/0xe0 fs/ioctl.c:713
>  __do_sys_ioctl fs/ioctl.c:720 [inline]
>  __se_sys_ioctl fs/ioctl.c:718 [inline]
>  __x64_sys_ioctl+0x4c/0x60 fs/ioctl.c:718
>  do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> read to 0xffffc9000c433018 of 8 bytes by task 7 on cpu 1:
>  flush_echoes drivers/tty/n_tty.c:828 [inline]
>  __receive_buf drivers/tty/n_tty.c:1648 [inline]
>  n_tty_receive_buf_common+0xe3f/0x1b00 drivers/tty/n_tty.c:1742
>  n_tty_receive_buf2+0x3d/0x60 drivers/tty/n_tty.c:1777
>  tty_ldisc_receive_buf+0x71/0xf0 drivers/tty/tty_buffer.c:461
>  tty_port_default_receive_buf+0x87/0xd0 drivers/tty/tty_port.c:38
>  receive_buf drivers/tty/tty_buffer.c:481 [inline]
>  flush_to_ldisc+0x1d5/0x260 drivers/tty/tty_buffer.c:533
>  process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
>  worker_thread+0xa0/0x800 kernel/workqueue.c:2415
>  kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
>  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352
> 
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 PID: 7 Comm: kworker/u4:0 Not tainted 5.4.0-rc3+ #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> Workqueue: events_unbound flush_to_ldisc
> ==================================================================

thanks,
-- 
js
suse labs