[PATCH 3.12 115/212] n_tty: Fix 4096-byte canonical reads

[Greg Kroah-Hartman]

3.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>

commit c77569d2f3ef7844ee4ac7005a57da6898b302a8 upstream.

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].

Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/tty/n_tty.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2005,7 +2005,10 @@ static int canon_copy_from_read_buf(stru
 		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) {


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