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Subject[PATCH] TTY: serial_core, add ->install
Date
We need to compute the uart state only on the first open. This is
usually what is done in the ->install hook. serial_core used to do this
in ->open on every open. So move it to ->install.

As a side effect, it ensures the state is set properly in the window
after tty_init_dev is called, but before uart_open. This fixes a bunch
of races between tty_open and flush_to_ldisc we were dealing with
recently.

One of such bugs was attempted to fix in commit fedb5760648a (serial:
fix race between flush_to_ldisc and tty_open), but it only took care of
a couple of functions (uart_start and uart_unthrottle). I was able to
reproduce the crash on a SLE system, but in uart_write_room which is
also called from flush_to_ldisc via process_echoes. I was *unable* to
reproduce the bug locally. It is due to having this patch in my queue
since 2012!

general protection fault: 0000 [#1] SMP KASAN PTI
CPU: 1 PID: 5 Comm: kworker/u4:0 Tainted: G L 4.12.14-396-default #1 SLE15-SP1 (unreleased)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-0-ga698c89-prebuilt.qemu.org 04/01/2014
Workqueue: events_unbound flush_to_ldisc
task: ffff8800427d8040 task.stack: ffff8800427f0000
RIP: 0010:uart_write_room+0xc4/0x590
RSP: 0018:ffff8800427f7088 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 000000000000002f RSI: 00000000000000ee RDI: ffff88003888bd90
RBP: ffffffffb9545850 R08: 0000000000000001 R09: 0000000000000400
R10: ffff8800427d825c R11: 000000000000006e R12: 1ffff100084fee12
R13: ffffc900004c5000 R14: ffff88003888bb28 R15: 0000000000000178
FS: 0000000000000000(0000) GS:ffff880043300000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000561da0794148 CR3: 000000000ebf4000 CR4: 00000000000006e0
Call Trace:
tty_write_room+0x6d/0xc0
__process_echoes+0x55/0x870
n_tty_receive_buf_common+0x105e/0x26d0
tty_ldisc_receive_buf+0xb7/0x1c0
tty_port_default_receive_buf+0x107/0x180
flush_to_ldisc+0x35d/0x5c0
...

0 in rbx means tty->driver_data is NULL in uart_write_room. 0x178 is
tried to be dereferenced (0x178 >> 3 is 0x2f in rdx) at
uart_write_room+0xc4. 0x178 is exactly (struct uart_state *)NULL->refcount
used in uart_port_lock from uart_write_room.

So revert the upstream commit here as my local patch should fix the
whole family.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Li RongQing <lirongqing@baidu.com>
Cc: Wang Li <wangli39@baidu.com>
Cc: Zhang Yu <zhangyu31@baidu.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: stable <stable@vger.kernel.org>
---

============================= NOTE =============================

Could you test your use-case at Baidu, guys, please?


drivers/tty/serial/serial_core.c | 24 +++++++++++++-----------
1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 7c787e517fa5..33319544d9d2 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -130,9 +130,6 @@ static void uart_start(struct tty_struct *tty)
struct uart_port *port;
unsigned long flags;

- if (!state)
- return;
-
port = uart_port_lock(state, flags);
__uart_start(tty);
uart_port_unlock(port, flags);
@@ -730,9 +727,6 @@ static void uart_unthrottle(struct tty_struct *tty)
upstat_t mask = UPSTAT_SYNC_FIFO;
struct uart_port *port;

- if (!state)
- return;
-
port = uart_port_ref(state);
if (!port)
return;
@@ -1732,6 +1726,16 @@ static void uart_dtr_rts(struct tty_port *port, int raise)
uart_port_deref(uport);
}

+static int uart_install(struct tty_driver *driver, struct tty_struct *tty)
+{
+ struct uart_driver *drv = driver->driver_state;
+ struct uart_state *state = drv->state + tty->index;
+
+ tty->driver_data = state;
+
+ return tty_standard_install(driver, tty);
+}
+
/*
* Calls to uart_open are serialised by the tty_lock in
* drivers/tty/tty_io.c:tty_open()
@@ -1744,11 +1748,8 @@ static void uart_dtr_rts(struct tty_port *port, int raise)
*/
static int uart_open(struct tty_struct *tty, struct file *filp)
{
- struct uart_driver *drv = tty->driver->driver_state;
- int retval, line = tty->index;
- struct uart_state *state = drv->state + line;
-
- tty->driver_data = state;
+ struct uart_state *state = tty->driver_data;
+ int retval;

retval = tty_port_open(&state->port, tty, filp);
if (retval > 0)
@@ -2433,6 +2434,7 @@ static void uart_poll_put_char(struct tty_driver *driver, int line, char ch)
#endif

static const struct tty_operations uart_ops = {
+ .install = uart_install,
.open = uart_open,
.close = uart_close,
.write = uart_write,
--
2.21.0
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