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

[Nicolas Pitre]

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 <n...@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