n_tty: Fix poll() after buffer-limited eof push read

n_tty: Fix poll() after buffer-limited eof push read

commit 40d5e0905a03 ("n_tty: Fix EOF push handling") fixed EOF push
for reads. However, that approach still allows a condition mismatch
between poll() and read(), where poll() returns POLLIN but read()
blocks. This state can happen when a previous read() returned because
the user buffer was full and the next character was an EOF not at the
beginning of the line. While the next read() will properly identify
the condition and advance the read buffer tail without improperly
indicating an EOF file condition (ie., read() will not mistakenly
return 0), poll() will mistakenly indicate POLLIN.

Although a possible solution would be to peek at the input buffer
in n_tty_poll(), the better solution in this patch is to eat the
EOF during the previous read() (ie., fix the problem by eliminating
the condition).

The current canon line buffer copy limits the scan for next end-of-line
to the smaller of either,
   a. the remaining user buffer size
   b. completed lines in the input buffer
When the remaining user buffer size is exactly one less than the
end-of-line marked by EOF push, the EOF is not scanned nor skipped
but left for subsequent reads. In the example below, the scan
index 'eol' has stopped at the EOF because it is past the scan
limit of 5 (not because it has found the next set bit in read_flags)

   user buffer [*nr = 5]    _ _ _ _ _

   read_flags               0 0 0 0 0   1
   input buffer             h e l l o [EOF]
                            ^           ^
                           /           /
                         tail        eol

   result: found = 0, tail += 5, *nr += 5

Instead, allow the scan to peek ahead 1 byte (while still limiting the
scan to completed lines in the input buffer). For the example above,

   result: found = 1, tail += 6, *nr += 5

Because the scan limit is now bumped +1 byte, when the scan is
completed, the tail advance and the user buffer copy limit is
re-clamped to *nr when EOF is _not_ found.

Fixes: 40d5e0905a03 ("n_tty: Fix EOF push handling")
Cc: <stable@vger.kernel.org> # 3.12+
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>