On Tue, Aug 25, 2015 at 9:03 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote: > On 08/25/2015 02:38 PM, Dmitry Vyukov wrote: >> On Tue, Aug 25, 2015 at 8:32 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: >>> On Tue, Aug 25, 2015 at 8:26 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote: >>>> On 08/25/2015 08:17 AM, Andrey Konovalov wrote: >>>>> Hi! >>>>> >>>>> We are working on a dynamic data race detector for the Linux kernel >>>>> called KernelThreadSanitizer (ktsan) >>>>> (https://github.com/google/ktsan/wiki). >>>>> >>>>> While booting the kernel (upstream revision 21bdb584af8c) we got a report: >>>>> >>>>> ================================================================== >>>>> ThreadSanitizer: data-race in uart_ioctl >>>>> >>>>> Read of size 8 by thread T424 (K971): >>>>> [<ffffffff81673c56>] uart_ioctl+0x36/0x11e0 >>>>> drivers/tty/serial/serial_core.c:1216 >>>>> [<ffffffff81643802>] tty_ioctl+0x4f2/0x11d0 drivers/tty/tty_io.c:2924 >>>>> [< inlined >] do_vfs_ioctl+0x44a/0x750 vfs_ioctl fs/ioctl.c:43 >>>>> [<ffffffff8127b0ca>] do_vfs_ioctl+0x44a/0x750 fs/ioctl.c:607 >>>>> [< inlined >] SyS_ioctl+0x79/0xa0 SYSC_ioctl fs/ioctl.c:622 >>>>> [<ffffffff8127b449>] SyS_ioctl+0x79/0xa0 fs/ioctl.c:613 >>>>> [<ffffffff81eae0ae>] entry_SYSCALL_64_fastpath+0x12/0x71 >>>>> arch/x86/entry/entry_64.S:186 >>>>> DBG: cpu = ffff88063fc1fe68 >>>>> DBG: cpu id = 0 >>>>> >>>>> Previous write of size 8 by thread T422 (K970): >>>>> [<ffffffff816737ef>] uart_open+0x12f/0x220 >>>>> drivers/tty/serial/serial_core.c:1629 >>>>> [<ffffffff81645be2>] tty_open+0x192/0x8f0 drivers/tty/tty_io.c:2105 >>>>> [<ffffffff812628fc>] chrdev_open+0x13c/0x290 fs/char_dev.c:388 >>>>> [<ffffffff812582fc>] do_dentry_open+0x3ac/0x550 fs/open.c:736 >>>>> [<ffffffff81259d68>] vfs_open+0xb8/0xe0 fs/open.c:853 >>>>> [< inlined >] path_openat+0x81c/0x2440 do_last fs/namei.c:3163 >>>>> [<ffffffff81272f1c>] path_openat+0x81c/0x2440 fs/namei.c:3295 >>>>> [<ffffffff8127656a>] do_filp_open+0xfa/0x170 fs/namei.c:3330 >>>>> [<ffffffff8125a243>] do_sys_open+0x183/0x2b0 fs/open.c:1025 >>>>> [< inlined >] SyS_open+0x35/0x50 SYSC_open fs/open.c:1043 >>>>> [<ffffffff8125a3a5>] SyS_open+0x35/0x50 fs/open.c:1038 >>>>> [<ffffffff81eae0ae>] entry_SYSCALL_64_fastpath+0x12/0x71 >>>>> arch/x86/entry/entry_64.S:186 >>>>> DBG: cpu = ffff88063fd1fe68 >>>>> >>>>> DBG: addr: ffff8801d2a0ce88 >>>>> DBG: first offset: 0, second offset: 0 >>>>> DBG: T424 clock: {T424: 211057, T422: 275728} >>>>> DBG: T422 clock: {T422: 275819} >>>>> ================================================================== >>>>> >>>>> It seems that one thread reads and uses tty->driver_data while it's >>>>> being initialized in another one. The second thread holds port->mutex, >>>>> but the first one does a few accesses to tty->driver_data before >>>>> locking it. >>>>> >>>>> Could you confirm if this is a real race? >>>> >>>> Although I don't understand what triggers ktsan to signal a race >>>> condition, this doesn't appear to be an actual race. >>>> >>>> For an ioctl() syscall to act on any given tty requires a successful >>>> open() syscall to have nearly completed (do_sys_open() => fd_install() >>>> initializes the file descriptor; ioctl() => fdget() derefs the descriptor). >>>> >>>> Perhaps what's tripping the race detection is that 2nd and subsequent >>>> opens also (redundantly) write the same values as from the first open? >>> >>> Since we use a fuzzer, yes, it is possible that open is called twice. >> >> Oh, no, sorry, this happens during booting. >> The race is on tty_struct, which is probably shared between several >> file descriptors. > > Yep, but there is 1:1 correspondence between tty_struct and uart_state; > so once the first open() initializes tty->driver_data, subsequent opens > are just re-writing the same value for tty->driver_data. > > Is ktsan just triggering on the fact there is a memory write, without > checking the value has changed? Yes, that is correct. Ktsan is based on C memory model which says that any write racing with other memory access leads to undefined behavior. As far as I see the setup is quite complex and not just writes to tty->driver_data. For example, it also calls uart_startup which does set_bit(TTY_IO_ERROR, &tty->flags) on the tty. Can't that lead to some issues? E.g. we set TTY_IO_ERROR, but since tty is already used by other threads, operations on the tty in other threads will spuriously fail. 195 static int uart_startup(struct tty_struct *tty, struct uart_state *state, 196 int init_hw) 197 { 198 struct tty_port *port = &state->port; 199 int retval; 200 201 if (port->flags & ASYNC_INITIALIZED) 202 return 0; 203 204 /* 205 * Set the TTY IO error marker - we will only clear this 206 * once we have successfully opened the port. 207 */ 208 set_bit(TTY_IO_ERROR, &tty->flags); 209 210 retval = uart_port_startup(tty, state, init_hw); 211 if (!retval) { 212 set_bit(ASYNCB_INITIALIZED, &port->flags); 213 clear_bit(TTY_IO_ERROR, &tty->flags); 214 } else if (retval > 0) 215 retval = 0; 216 217 return retval; 218 } -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
