syzbot

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BUG: KCSAN: data-race in n_tty_receive_char_flow_ctrl / tty_set_termios

write to 0xffff88811aa70d08 of 44 bytes by task 6821 on cpu 0:
 tty_set_termios+0xc0/0x8c0 drivers/tty/tty_ioctl.c:335
 set_termios+0x35b/0x4d0 drivers/tty/tty_ioctl.c:516
 tty_mode_ioctl+0x379/0x5c0 drivers/tty/tty_ioctl.c:-1
 n_tty_ioctl_helper+0x91/0x210 drivers/tty/tty_ioctl.c:982
 n_tty_ioctl+0x101/0x200 drivers/tty/n_tty.c:2509
 tty_ioctl+0x83f/0xb80 drivers/tty/tty_io.c:2801
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xce/0x140 fs/ioctl.c:583
 __x64_sys_ioctl+0x43/0x50 fs/ioctl.c:583
 x64_sys_call+0x1816/0x3000 arch/x86/include/generated/asm/syscalls_64.h:17
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811aa70d21 of 1 bytes by task 12 on cpu 1:
 n_tty_is_char_flow_ctrl drivers/tty/n_tty.c:1204 [inline]
 n_tty_receive_char_flow_ctrl+0x23/0x1a0 drivers/tty/n_tty.c:1225
 n_tty_lookahead_flow_ctrl+0xed/0x130 drivers/tty/n_tty.c:1500
 tty_port_default_lookahead_buf+0x91/0xc0 drivers/tty/tty_port.c:59
 lookahead_bufs drivers/tty/tty_buffer.c:428 [inline]
 flush_to_ldisc+0x288/0x340 drivers/tty/tty_buffer.c:498
 process_one_work kernel/workqueue.c:3263 [inline]
 process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3346
 worker_thread+0x582/0x770 kernel/workqueue.c:3427
 kthread+0x489/0x510 kernel/kthread.c:463
 ret_from_fork+0x122/0x1b0 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

value changed: 0x11 -> 0x4b

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 12 Comm: kworker/u8:0 Tainted: G        W           syzkaller #0 PREEMPT(voluntary) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
Workqueue: events_unbound flush_to_ldisc
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