[PATCH 5/6] vcs: poll(): cope with a deallocated vt

Nicolas Pitre
2019-01-09T04:04:38Z

When VT_DISALLOCATE is used on a vt, user space waiting with poll() on
the corresponding /dev/vcs device is not awakened. This is now fixed by
returning POLLHUP|POLLERR to user space.

Also, in the normal screen update case, we don't set POLLERR anymore as
POLLPRI alone is a much more logical response in a non-error situation,
saving some confusion on the user space side. The only known user app
making use of poll() on /dev/vcs* is BRLTTY which is known to cope with
that change already, so the risk of breakage is pretty much nonexistent.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
---
drivers/tty/vt/vc_screen.c | 21 +++++++++++++++------
1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c
index 3dba60825c..1bbe2a30cd 100644
--- a/drivers/tty/vt/vc_screen.c
+++ b/drivers/tty/vt/vc_screen.c
@@ -80,7 +80,7 @@
struct vcs_poll_data {
struct notifier_block notifier;
unsigned int cons_num;
- bool seen_last_update;
+ int event;
wait_queue_head_t waitq;
struct fasync_struct *fasync;
};
@@ -94,7 +94,7 @@ vcs_notifier(struct notifier_block *nb, unsigned long code, void *_param)
container_of(nb, struct vcs_poll_data, notifier);
int currcons = poll->cons_num;

- if (code != VT_UPDATE)
+ if (code != VT_UPDATE && code != VT_DEALLOCATE)
return NOTIFY_DONE;

if (currcons == 0)
@@ -104,7 +104,7 @@ vcs_notifier(struct notifier_block *nb, unsigned long code, void *_param)
if (currcons != vc->vc_num)
return NOTIFY_DONE;

- poll->seen_last_update = false;
+ poll->event = code;
wake_up_interruptible(&poll->waitq);
kill_fasync(&poll->fasync, SIGIO, POLL_IN);
return NOTIFY_OK;
@@ -261,7 +261,7 @@ vcs_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)

poll = file->private_data;
if (count && poll)
- poll->seen_last_update = true;
+ poll->event = 0;
read = 0;
ret = 0;
while (count) {
@@ -616,12 +616,21 @@ static __poll_t
vcs_poll(struct file *file, poll_table *wait)
{
struct vcs_poll_data *poll = vcs_poll_data_get(file);
- __poll_t ret = DEFAULT_POLLMASK|EPOLLERR|EPOLLPRI;
+ __poll_t ret = DEFAULT_POLLMASK|EPOLLERR;

if (poll) {
poll_wait(file, &poll->waitq, wait);
- if (poll->seen_last_update)
+ switch (poll->event) {
+ case VT_UPDATE:
+ ret = DEFAULT_POLLMASK|EPOLLPRI;
+ break;
+ case VT_DEALLOCATE:
+ ret = DEFAULT_POLLMASK|EPOLLHUP|EPOLLERR;
+ break;
+ case 0:
ret = DEFAULT_POLLMASK;
+ break;
+ }
}
return ret;
}
--
2.20.1