Re: Potential data race in psmouse_interrupt
From: Dmitry Vyukov
Date: Fri Aug 28 2015 - 14:08:52 EST
On Fri, Aug 28, 2015 at 7:51 PM, Dmitry Torokhov
<dmitry.torokhov@xxxxxxxxx> wrote:
> On Fri, Aug 28, 2015 at 10:34 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>> Hello,
>>
>> I am looking at this code in __ps2_command again:
>>
>> /*
>> * The reset command takes a long time to execute.
>> */
>> timeout = msecs_to_jiffies(command == PS2_CMD_RESET_BAT ? 4000 : 500);
>>
>> timeout = wait_event_timeout(ps2dev->wait,
>> !(READ_ONCE(ps2dev->flags) & PS2_FLAG_CMD1), timeout);
>>
>> if (smp_load_acquire(&ps2dev->cmdcnt) &&
>> !(smp_load_acquire(&ps2dev->flags) & PS2_FLAG_CMD1)) {
>> timeout = ps2_adjust_timeout(ps2dev, command, timeout);
>> wait_event_timeout(ps2dev->wait,
>> !(smp_load_acquire(&ps2dev->flags) &
>> PS2_FLAG_CMD), timeout);
>> }
>>
>> if (param)
>> for (i = 0; i < receive; i++)
>> param[i] = ps2dev->cmdbuf[(receive - 1) - i];
>>
>>
>> Here are two moments I don't understand:
>> 1. The last parameter of ps2_adjust_timeout is timeout in jiffies (it
>> is compared against 100ms). However, timeout is assigned to result of
>> wait_event_timeout, which returns 0 or 1. This does not make sense to
>> me. What am I missing?
>
> The fact that wait_event_timeout can return value greater than one:
>
> * Returns:
> * 0 if the @condition evaluated to %false after the @timeout elapsed,
> * 1 if the @condition evaluated to %true after the @timeout elapsed,
> * or the remaining jiffies (at least 1) if the @condition evaluated
> ^^^^^^^^^^^^^^^^^^^^^^^^^
OK, makes sense now!
>> 2. This code pays great attention to timeouts, but in the end I don't
>> see how it handles timeouts. That is, if a timeout is happened, we
>> still copyout (garbage) from cmdbuf. What am I missing here?
>
> Once upon a time wait_event() did not return positive value when
> timeout expired and then condition satisfied. So we just examine the
> final state (psmpouse->cmdcnt should be 0 if command actually
> succeeded) and even if we copy in garbage nobody should care since
> we'll return error in this case.
I see.
But the cmdcnt is re-read after copying out response. So it is
possible that we read garbage response, but then read cmdcnt==0 and
return OK to caller.
So far I have something along the following lines to fix data races in libps2.c
diff --git a/drivers/input/serio/libps2.c b/drivers/input/serio/libps2.c
index 7551699..51c747f 100644
--- a/drivers/input/serio/libps2.c
+++ b/drivers/input/serio/libps2.c
@@ -43,7 +43,7 @@ int ps2_sendbyte(struct ps2dev *ps2dev, unsigned
char byte, int timeout)
if (serio_write(ps2dev->serio, byte) == 0)
wait_event_timeout(ps2dev->wait,
- !(ps2dev->flags & PS2_FLAG_ACK),
+ !(READ_ONCE(ps2dev->flags) & PS2_FLAG_ACK),
msecs_to_jiffies(timeout));
serio_pause_rx(ps2dev->serio);
@@ -187,6 +187,7 @@ int __ps2_command(struct ps2dev *ps2dev, unsigned
char *param, int command)
int receive = (command >> 8) & 0xf;
int rc = -1;
int i;
+ unsigned char cmdcnt;
if (receive > sizeof(ps2dev->cmdbuf)) {
WARN_ON(1);
@@ -225,23 +226,22 @@ int __ps2_command(struct ps2dev *ps2dev,
unsigned char *param, int command)
timeout = msecs_to_jiffies(command == PS2_CMD_RESET_BAT ? 4000 : 500);
timeout = wait_event_timeout(ps2dev->wait,
- !(ps2dev->flags &
PS2_FLAG_CMD1), timeout);
-
- if (ps2dev->cmdcnt && !(ps2dev->flags & PS2_FLAG_CMD1)) {
+ !(READ_ONCE(ps2dev->flags) & PS2_FLAG_CMD1), timeout);
+ if (READ_ONCE(&ps2dev->cmdcnt) &&
+ !(READ_ONCE(&ps2dev->flags) & PS2_FLAG_CMD1)) {
timeout = ps2_adjust_timeout(ps2dev, command, timeout);
wait_event_timeout(ps2dev->wait,
- !(ps2dev->flags & PS2_FLAG_CMD), timeout);
+ !(READ_ONCE(&ps2dev->flags) & PS2_FLAG_CMD), timeout);
}
- if (param)
- for (i = 0; i < receive; i++)
- param[i] = ps2dev->cmdbuf[(receive - 1) - i];
-
- if (ps2dev->cmdcnt && (command != PS2_CMD_RESET_BAT ||
ps2dev->cmdcnt != 1))
- goto out;
-
- rc = 0;
+ cmdcnt = smp_load_acquire(&ps2dev->cmdcnt);
+ if (!cmdcnt || command == PS2_CMD_RESET_BAT && ps2dev->cmdcnt == 1) {
+ if (param)
+ for (i = 0; i < receive; i++)
+ param[i] = ps2dev->cmdbuf[(receive - 1) - i];
+ rc = 0;
+ }
out:
serio_pause_rx(ps2dev->serio);
@@ -284,19 +284,21 @@ EXPORT_SYMBOL(ps2_init);
int ps2_handle_ack(struct ps2dev *ps2dev, unsigned char data)
{
+ unsigned long flags = ps2dev->flags;
+
switch (data) {
case PS2_RET_ACK:
ps2dev->nak = 0;
break;
case PS2_RET_NAK:
- ps2dev->flags |= PS2_FLAG_NAK;
+ flags |= PS2_FLAG_NAK;
ps2dev->nak = PS2_RET_NAK;
break;
case PS2_RET_ERR:
- if (ps2dev->flags & PS2_FLAG_NAK) {
- ps2dev->flags &= ~PS2_FLAG_NAK;
+ if (flags & PS2_FLAG_NAK) {
+ flags &= ~PS2_FLAG_NAK;
ps2dev->nak = PS2_RET_ERR;
break;
}
@@ -308,7 +310,7 @@ int ps2_handle_ack(struct ps2dev *ps2dev, unsigned
char data)
case 0x00:
case 0x03:
case 0x04:
- if (ps2dev->flags & PS2_FLAG_WAITID) {
+ if (flags & PS2_FLAG_WAITID) {
ps2dev->nak = 0;
break;
}
@@ -319,12 +321,12 @@ int ps2_handle_ack(struct ps2dev *ps2dev,
unsigned char data)
if (!ps2dev->nak) {
- ps2dev->flags &= ~PS2_FLAG_NAK;
+ flags &= ~PS2_FLAG_NAK;
if (ps2dev->cmdcnt)
- ps2dev->flags |= PS2_FLAG_CMD | PS2_FLAG_CMD1;
+ flags |= PS2_FLAG_CMD | PS2_FLAG_CMD1;
}
- ps2dev->flags &= ~PS2_FLAG_ACK;
+ WRITE_ONCE(ps2dev->flags, flags & ~PS2_FLAG_ACK);
wake_up(&ps2dev->wait);
if (data != PS2_RET_ACK)
@@ -342,17 +344,21 @@ EXPORT_SYMBOL(ps2_handle_ack);
int ps2_handle_response(struct ps2dev *ps2dev, unsigned char data)
{
- if (ps2dev->cmdcnt)
- ps2dev->cmdbuf[--ps2dev->cmdcnt] = data;
+ if (ps2dev->cmdcnt) {
+ unsigned char cmdcnt = ps2dev->cmdcnt - 1;
+
+ ps2dev->cmdbuf[cmdcnt] = data;
+ smp_store_release(&ps2dev->cmdcnt, cmdcnt);
+ }
if (ps2dev->flags & PS2_FLAG_CMD1) {
- ps2dev->flags &= ~PS2_FLAG_CMD1;
+ smp_store_release(&ps2dev->flags, ps2dev->flags &
~PS2_FLAG_CMD1);
if (ps2dev->cmdcnt)
wake_up(&ps2dev->wait);
}
if (!ps2dev->cmdcnt) {
- ps2dev->flags &= ~PS2_FLAG_CMD;
+ smp_store_release(&ps2dev->flags, ps2dev->flags &
~PS2_FLAG_CMD);
wake_up(&ps2dev->wait);
}
--
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