n_tty: Fix read_buf race condition, increment read_head after pushing data

commit 8bfbe2de769afda051c56aba5450391670e769fc upstream.

Commit 19e2ad6a09f0c06dbca19c98e5f4584269d913dd ("n_tty: Remove overflow
tests from receive_buf() path") moved the increment of read_head into
the arguments list of read_buf_addr(). Function calls represent a
sequence point in C. Therefore read_head is incremented before the
character c is placed in the buffer. Since the circular read buffer is
a lock-less design since commit 6d76bd2618535c581f1673047b8341fd291abc67
("n_tty: Make N_TTY ldisc receive path lockless"), this creates a race
condition that leads to communication errors.

This patch modifies the code to increment read_head _after_ the data
is placed in the buffer and thus fixes the race for non-SMP machines.
To fix the problem for SMP machines, memory barriers must be added in
a separate patch.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 28ac3f3b7ec304cccf334a4c8216b371dbc9bbe7..d46b4ccec8cdfee72ab4227150bc6bcef3c5b914 100644 (file)
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -321,7 +321,8 @@ static void n_tty_check_unthrottle(struct tty_struct *tty)
 
 static inline void put_tty_queue(unsigned char c, struct n_tty_data *ldata)
 {
-       *read_buf_addr(ldata, ldata->read_head++) = c;
+       *read_buf_addr(ldata, ldata->read_head) = c;
+       ldata->read_head++;
 }
 
 /**