| From | Greg Kroah-Hartman <> | Subject | [PATCH 3.14 48/77] n_tty: Fix read_buf race condition, increment read_head after pushing data | Date | Tue, 13 Jan 2015 23:23:28 -0800 |
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3.14-stable review patch. If anyone has any objections, please let me know.
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From: Christian Riesch <christian.riesch@omicron.at>
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>
--- drivers/tty/n_tty.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
--- a/drivers/tty/n_tty.c +++ b/drivers/tty/n_tty.c @@ -321,7 +321,8 @@ static void n_tty_check_unthrottle(struc 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++; } /**
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