Commit d21b0be246bf ("vt: introduce unicode mode for /dev/vcs") guarded
against using devices containing attributes as this is not yet
implemented. It however failed to guard against writes to any devices
as this is also unimplemented.

Reported-by: Or Cohen <orcohen@paloaltonetworks.com>
Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Cc: <stable@vger.kernel.org> # v4.19+
Cc: Jiri Slaby <jslaby@suse.com>
Fixes: d21b0be246bf ("vt: introduce unicode mode for /dev/vcs")
Link: https://lore.kernel.org/r/nycvar.YSQ.7.76.1911051030580.30289@knanqh.ubzr
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Both /dev/vcs and /dev/vcs0 were in use in the past, but these days
/dev/vcs0 is mostly historical curiosity.

* "/dev/vcs" is the name that has always been in the Linux allocated
devices list.

* "vcs" is the device name in sysfs since Linux v2.6.12.

* MAKEDEV(1) in Debian used to create /dev/vcs0 only, but /dev/vcs was
  added in 1999: https://bugs.debian.org/45698

* MAKEDEV(1) in RedHat switched from /dev/vcs0 to /dev/vcs in 2000:

    * Fri Oct 20 2000 Nalin Dahyabhai <nalin@redhat.com>
      - change vcs0 to vcs (ditto for vcsa0)

Signed-off-by: Jakub Wilk <jwilk@jwilk.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Restore and document the forced initial POLLPRI event reporting when
poll() is used for the first time. This used to be the implemented
behavior before recent changes. Because of the way poll() is implemented,
this prevents losing an event happening between the last read() and the
first poll() invocation.

Since poll() for /dev/vcs* was not always supported, user space probes
for its availability as follows:

	int fd = open("/dev/vcsa", O_RDONLY);
	struct pollfd p = { .fd = fd, .events = POLLPRI };
	available = (poll(&p, 1, 0) == 1);

Semantically, it makes sense to signal the first event as such even if
it might be spurious. The screen could be modified, and modified back
to its initial state before we get to read it, so users must be prepared
for that anyway.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

We use POLLPRI not POLLIN to wait for data with poll() as there is
never any incoming data stream per se. Let's use the same semantic
with fasync() for consistency, including the fact that a vt may go away.

No known user space ever relied on the SIGIO reason so far, let alone
FASYNC, so the risk of breakage is pretty much nonexistent.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

The /dev/vcsa* devices have a fixed char-sized header that stores the
screen geometry and cursor location. Let's make sure it doesn't contain
random garbage when those values exceed 255. If ever it becomes necessary
to convey larger screen info to user space then a larger header in the
not-yet-implemented /dev/vcsua* devices should be considered.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

There is currently no provision for scrollback content in the core code,
leaving that to backend video drivers where this can be highly optimized.
There is currently no common method for those drivers to tell the core
what part of the scrollback is actually displayed and what size the
scrollback buffer is either. Because of that, the unicode screen buffer
has no provision for any scrollback.

At least we can provide backtranslated glyph values when the scrollback
is active which should be plenty good enough for now.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Dave Mielke <Dave@mielke.cc>
Acked-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Now that the core vt code knows how to preserve unicode values for each
displayed character, it is then possible to let user space access it via
/dev/vcs*.

Unicode characters are presented as 32 bit values in native endianity
via the /dev/vcsu* devices, mimicking the simple /dev/vcs* devices.
Unicode with attributes (similarly to /dev/vcsa*) is not supported at
the moment.

Data is available only as long as the console is in UTF-8 mode. ENODATA
is returned otherwise.

This was tested with the latest development version (to become
version 5.7) of BRLTTY. Amongst other things, this allows ⠋⠕⠗ ⠞⠓⠊⠎
⠃⠗⠁⠊⠇⠇⠑⠀⠞⠑⠭⠞⠀to appear directly on braille displays regardless of the
console font being used.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Dave Mielke <Dave@mielke.cc>
Acked-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

This is the mindless scripted replacement of kernel use of POLL*
variables as described by Al, done by this script:

    for V in IN OUT PRI ERR RDNORM RDBAND WRNORM WRBAND HUP RDHUP NVAL MSG; do
        L=`git grep -l -w POLL$V | grep -v '^t' | grep -v /um/ | grep -v '^sa' | grep -v '/poll.h$'|grep -v '^D'`
        for f in $L; do sed -i "-es/^\([^\"]*\)\(\<POLL$V\>\)/\\1E\\2/" $f; done
    done

with de-mangling cleanups yet to come.

NOTE! On almost all architectures, the EPOLL* constants have the same
values as the POLL* constants do.  But they keyword here is "almost".
For various bad reasons they aren't the same, and epoll() doesn't
actually work quite correctly in some cases due to this on Sparc et al.

The next patch from Al will sort out the final differences, and we
should be all done.

Scripted-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>