On 2023/8/14 16:52, Yi Yang wrote:
On 2023/8/14 14:32, Jiri Slaby wrote:
On 14. 08. 23, 6:01, Yi Yang wrote:
Soft lockup occurs when vt device used n_null ldisc, n_null_receivebuf()
is not implemented in null_ldisc. So tty_ldisc_receive_buf always return
0 in paste_selection(), this cause deadloop and cause soft lockup.
This can be reproduced as follows:
int ldisc = 0x1b; // 0x1b is n_null
struct{
char subcode;
struct tiocl_selection sel;
} data;
date.subcode = TIOCL_SETSEL;
data.sel.xs = 0;
data.sel.xe = 1;
data.sel.ys = 0;
data.sel.ye = 1;
data.sel.sel_mode = TIOCL_SELCHAR;
char bytes[2] = {TIOCL_PASTESEL, 0};
open("ttyxx", O_RDWR) // open a vt device
ioctl(fd, TIOCSETD, &ldisc) // set ldisc to n_null
ioctl(fd, TIOCLINUX, &data.subcode);
ioctl(fd, TIOCLINUX, bytes); // cause deadloop
Fix soft lockup by check if ldisc in paste_selection() is n_null.
Ugh, no. What if another ldisc returns with 0 too?
So instead, what about checking for progress instead of checking a
particular ldisc?
Sorry, my previous analysis have problem. The real reason why
tty_ldisc_receive_buf() returns 0 is that n_null ldisc does not
assign a value to receive_room. Therefore, min_t(int, count,
ld->tty->receive_room) is always 0 in the tty_ldisc_receive_buf().
Maybe i should check the receive_room size instead of checking the
specific ldisc, check if receive_room size is 0 can fix this issue
passed my test.
Check receive_room size no a good idear, receive_room may also be 0 when
ldisc is n_tty. Do you have any idear if do not check the particular ldisc?
Link:
https://lore.kernel.org/all/000000000000fe769905d315a1b7@xxxxxxxxxx/
Fixes: 8a8dabf2dd68 ("tty: handle the case where we cannot restore a
line discipline")
Signed-off-by: Yi Yang <yiyang13@xxxxxxxxxx>
---
drivers/tty/vt/selection.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c
index 6ef22f01cc51..9ba7f66fcf05 100644
--- a/drivers/tty/vt/selection.c
+++ b/drivers/tty/vt/selection.c
@@ -388,6 +388,12 @@ int paste_selection(struct tty_struct *tty)
ld = tty_ldisc_ref_wait(tty);
if (!ld)
return -EIO; /* ldisc was hung up */
+
+ /* tty_ldisc_receive_buf() will not do anything when ldisc is
n_null*/
+ if (ld->ops->num == N_NULL) {
+ tty_ldisc_deref(ld);
+ return -EIO;
+ }
tty_buffer_lock_exclusive(&vc->port);
add_wait_queue(&vc->paste_wait, &wait);
--
Yi Yang
