n_tty: Fix 4096-byte canonical reads

Although the maximum allowable canonical line is specified to
be 255 bytes (MAX_CANON), the practical limit has actually been
the size of the line discipline read buffer (N_TTY_BUF_SIZE == 4096).

Commit 32f13521ca68bc624ff6effc77f308a52b038bf0,
n_tty: Line copy to user buffer in canonical mode, limited the
line copy to 4095 bytes. With a completely full line discipline
read buffer and a userspace buffer > 4095, _no_ data was copied,
and the read() syscall returned 0, indicating EOF.

Fix the interval arithmetic to compute the correct number of bytes
to copy to userspace in the range [1..4096].

Cc: <stable@vger.kernel.org> # 3.12.x
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index d8a779d3a611e5e1106fb36c4c462a17552fe144..c222a561c5aceb0d3faeb3b42b5a9d85307cb0fa 100644 (file)
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1998,7 +1998,10 @@ static int canon_copy_from_read_buf(struct tty_struct *tty,
                found = 1;
 
        size = N_TTY_BUF_SIZE - tail;
-       n = (found + eol + size) & (N_TTY_BUF_SIZE - 1);
+       n = eol - tail;
+       if (n > 4096)
+               n += 4096;
+       n += found;
        c = n;
 
        if (found && read_buf(ldata, eol) == __DISABLED_CHAR) {