Re: [PATCH V4 6/6] i2c: tegra: remove BUG, BUG_ON

[Bitan Biswas <bbiswas@xxxxxxxxxx>]

On 6/10/19 2:00 PM, Dmitry Osipenko wrote:
> 10.06.2019 22:41, Bitan Biswas пишет:
> > On 6/10/19 11:12 AM, Dmitry Osipenko wrote:
> > > 10.06.2019 20:08, Bitan Biswas пишет:
> > > > Remove redundant BUG_ON calls or replace with WARN_ON_ONCE
> > > > as needed. Remove BUG() and make Rx and Tx case handling
> > > > similar.
> > > >
> > > > Signed-off-by: Bitan Biswas <bbiswas@xxxxxxxxxx>
> > > > ---
> > > >    drivers/i2c/busses/i2c-tegra.c | 11 ++++++-----
> > > >    1 file changed, 6 insertions(+), 5 deletions(-)
> > >
> > > Looks that this is still not correct. What if it transfer-complete flag
> > > is set and buffer is full on RX? In this case the transfer will succeed
> > > while it was a failure.
> > >
> > > > diff --git a/drivers/i2c/busses/i2c-tegra.c
> > > > b/drivers/i2c/busses/i2c-tegra.c
> > > > index 4dfb4c1..30619d6 100644
> > > > --- a/drivers/i2c/busses/i2c-tegra.c
> > > > +++ b/drivers/i2c/busses/i2c-tegra.c
> > > > @@ -515,7 +515,6 @@ static int tegra_i2c_empty_rx_fifo(struct
> > > > tegra_i2c_dev *i2c_dev)
> > > >         * prevent overwriting past the end of buf
> > > >         */
> > > >        if (rx_fifo_avail > 0 && buf_remaining > 0) {
> > > > -        BUG_ON(buf_remaining > 3);
> > >
> > > Actually error should be returned here since out-of-bounds memory
> > > accesses must be avoided, hence:
> > >
> > >      if (WARN_ON_ONCE(buf_remaining > 3))
> > >          return -EINVAL;
> > buf_remaining will be less than equal to 3 because of the expression
> > earlier
> > https://elixir.bootlin.com/linux/v5.2-rc4/source/drivers/i2c/busses/i2c-tegra.c#L520
>
> Ah yes, indeed!
I see that I am wrong and buf_remaining > 3 needs to be prevented at

https://elixir.bootlin.com/linux/v5.2-rc4/source/drivers/i2c/busses/i2c-tegra.c#L528

because of word_to_transfer is limited to rx_fifo_avail:

https://elixir.bootlin.com/linux/v5.2-rc4/source/drivers/i2c/busses/i2c-tegra.c#L515

I shall add the check for less than 3 in both RX and TX cases in a 
separate patch in this series.

> > > >            val = i2c_readl(i2c_dev, I2C_RX_FIFO);
> > > >            val = cpu_to_le32(val);
> > > >            memcpy(buf, &val, buf_remaining);
> > > > @@ -523,7 +522,6 @@ static int tegra_i2c_empty_rx_fifo(struct
> > > > tegra_i2c_dev *i2c_dev)
> > > >            rx_fifo_avail--;
> > > >        }
> > > >    -    BUG_ON(rx_fifo_avail > 0 && buf_remaining > 0);
> > >
> > > Better not to ignore this as well:
> > >
> > >      if (WARN_ON_ONCE(rx_fifo_avail > 0 &&
> > >               buf_remaining > 0))
> > >          return -EINVAL;
> > Please check below line.
> > https://elixir.bootlin.com/linux/v5.2-rc4/source/drivers/i2c/busses/i2c-tegra.c#L532
> >
> >
> > It ensures that buf_remaining will be 0 and we never hit the BUG_ON as
> > follows:
>
> [1] Okay, but it doesn't ensure about rx_fifo_avail. So it could be:
>
> 	if (WARN_ON_ONCE(rx_fifo_avail))
> 		return -EINVAL;
I shall add the WARN_ON_ONCE.


> > > > -    BUG_ON(rx_fifo_avail > 0 && buf_remaining > 0);
> >
> > > >        i2c_dev->msg_buf_remaining = buf_remaining;
> > > >        i2c_dev->msg_buf = buf;
> > > >    @@ -581,7 +579,6 @@ static int tegra_i2c_fill_tx_fifo(struct
> > > > tegra_i2c_dev *i2c_dev)
> > > >         * boundary and fault.
> > > >         */
> > > >        if (tx_fifo_avail > 0 && buf_remaining > 0) {
> > > > -        BUG_ON(buf_remaining > 3);
> > >
> > > And here, cause this will corrupt stack:
> > >
> > >          if (WARN_ON_ONCE(buf_remaining > 3))
> > >              return -EINVAL;
> > Please check the line
> > https://elixir.bootlin.com/linux/v5.2-rc4/source/drivers/i2c/busses/i2c-tegra.c#L576
> >
> >
> > It ensures buf_remaining will be less or equal to 3.
>
> Okay, agree here.
>
> > > >            memcpy(&val, buf, buf_remaining);
> > > >            val = le32_to_cpu(val);
> > > >    @@ -850,7 +847,8 @@ static irqreturn_t tegra_i2c_isr(int irq, void
> > > > *dev_id)
> > > >                if (i2c_dev->msg_buf_remaining)
> > > >                    tegra_i2c_empty_rx_fifo(i2c_dev);
> > > >                else
> > > > -                BUG();
> > > > +                tegra_i2c_mask_irq(i2c_dev,
> > > > +                           I2C_INT_RX_FIFO_DATA_REQ);
> > >
> > > Then here:
> > >
> > >      if (WARN_ON_ONCE(!i2c_dev->msg_buf_remaining) ||
> > >          tegra_i2c_empty_rx_fifo(i2c_dev)) {
> > >          i2c_dev->msg_err |= I2C_ERR_UNKNOWN_INTERRUPT;
> > >          goto err;
> > >      }
> > Can you please elaborate why the condition needs to be as follows
> > instead of " if (WARN_ON_ONCE(i2c_dev->msg_buf_remaining)) " ?
> >
> > >       if (WARN_ON_ONCE(!i2c_dev->msg_buf_remaining) ||
> > >           tegra_i2c_empty_rx_fifo(i2c_dev)) {
>
> Because this is a "receive" transfer and hence it is a error condition
> if the data-message was already fully received and then there is another
> request from hardware to receive more data. So
> "!i2c_dev->msg_buf_remaining" is the error condition here because there
> is no more space in the buffer.
>
> Looking at this again, seems checking for "if
> (WARN_ON_ONCE(rx_fifo_avail))" in the above hunk [1] will be already
> enough since a not fully drained RX FIFO means that there is no enough
> space in the buffer. Then it could be:
>
>          if (tegra_i2c_empty_rx_fifo(i2c_dev)) {
>                  i2c_dev->msg_err |= I2C_ERR_UNKNOWN_INTERRUPT;
>                  goto err;
> 	}
In the case "if (status & I2C_INT_PACKET_XFER_COMPLETE) {" , we do not 
have any tegra_i2c_empty_rx_fifo call today. In this current driver I do 
not see any code that checks for the buffer space and prevents RX FIFO 
from being drained. The transfer complete when seen must have already 
consumed all bytes of msg_buf_remaining in the call at the line

https://elixir.bootlin.com/linux/v5.2-rc4/source/drivers/i2c/busses/i2c-tegra.c#L860

So we can put the "WARN_ON_ONCE(i2c_dev->msg_buf_remaining) with msg_err 
assignment and goto err" to confirm if some corner case is not handled.

Planning to share updated patch.

-Thanks,
 Bitan