This is a note to let you know that I've just added the patch titled
n_tty: Fix read_buf race condition, increment read_head after pushing
to my tty git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
in the tty-testing branch.
The patch will show up in the next release of the linux-next tree
(usually sometime within the next 24 hours during the week.)
The patch will be merged to the tty-next branch sometime soon,
after it pasts testing, and the merge window is open.
If you have any questions about this process, please let me know.
>From 8bfbe2de769afda051c56aba5450391670e769fc Mon Sep 17 00:00:00 2001
From: Christian Riesch <christian.riesch@xxxxxxxxxx>
Date: Thu, 13 Nov 2014 05:53:26 +0100
Subject: n_tty: Fix read_buf race condition, increment read_head after pushing
data
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@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
drivers/tty/n_tty.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 8358daa865e5..9e3b21624c82 100644
--- 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++;
}
/**
--
2.1.3
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