linux-kernel - Re: [PATCH V4 6/6] i2c: tegra: remove BUG, BUG

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Date:   Mon, 10 Jun 2019 12:41:03 -0700
From:   Bitan Biswas <bbiswas@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        Thierry Reding <treding@...dia.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        <linux-i2c@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, Peter Rosin <peda@...ntia.se>,
        Wolfram Sang <wsa@...-dreams.de>
CC:     Shardar Mohammed <smohammed@...dia.com>,
        Sowjanya Komatineni <skomatineni@...dia.com>,
        Mantravadi Karthik <mkarthik@...dia.com>
Subject: Re: [PATCH V4 6/6] i2c: tegra: remove BUG, BUG_ON



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@...dia.com>
>> ---
>>   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

> 
>>   		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:

 >> -	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.

>>   		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)) {


-regards,
  Bitan