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[91.79.162.197]) by smtp.googlemail.com with ESMTPSA id l1sm2795180wmg.13.2019.06.11.04.34.24 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 11 Jun 2019 04:34:25 -0700 (PDT) Subject: Re: [PATCH V4 6/6] i2c: tegra: remove BUG, BUG_ON To: Bitan Biswas , Laxman Dewangan , Thierry Reding , Jonathan Hunter , linux-i2c@vger.kernel.org, linux-tegra@vger.kernel.org, linux-kernel@vger.kernel.org, Peter Rosin , Wolfram Sang Cc: Shardar Mohammed , Sowjanya Komatineni , Mantravadi Karthik References: <1560186515-30797-1-git-send-email-bbiswas@nvidia.com> <1560186515-30797-6-git-send-email-bbiswas@nvidia.com> <06ab30b6-bf79-c628-0a04-d0307511a06f@gmail.com> <851d7837-5b98-228e-d8c9-3c41be1fb2e0@nvidia.com> From: Dmitry Osipenko Message-ID: Date: Tue, 11 Jun 2019 14:34:23 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 11.06.2019 10:38, Bitan Biswas пишет: > > > 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 >>>>> --- >>>>>    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. When word_to_transfer is more than rx_fifo_avail, then the rx_fifo_avail becomes zero and hence the nibbles won't be copied. Please take a closer look, the current code is correct, but the buf_remaining > 3 is unneeded because it can't ever happen. The code is structured the way that it's difficult to follow, apparently the person who added the BUG_ON check in the first place couldn't follow it either. Maybe it's worth to invest some more effort into refactoring at least that part of the code. At minimum a clarifying comments would be helpful. [snip] >>>> 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. There are two possible error conditions: 1) Underflow: the XFER_COMPLETE happens before message is fully sent. 2) Overflow: message is fully sent, but there is no XFER_COMPLETE and then hardware asks to transfer more. We are addressing the second case here, while you seems are confusing it with the first case.