Re: [PATCH 1/2] i2c: designware: Only check the first byte for SMBus block read length

[Andy Shevchenko]

On Sun, Jun 14, 2020 at 02:02:54PM -0700, Sultan Alsawaf wrote:
> From: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>
> 
> SMBus block reads can be broken because the read function will just skip
> over bytes it doesn't like until reaching a byte that conforms to the
> length restrictions for block reads. This is problematic when it isn't
> known if the incoming payload is indeed a conforming block read.
> 
> According to the SMBus specification, block reads will only send the
> payload length in the first byte, so we can fix this by only considering
> the first byte in a sequence for block read length purposes.

I'm wondering if this overlaps with [1]. AFAIU that one is also makes sure that
the length is not a garbage.

[1]: https://lore.kernel.org/linux-i2c/20200613104109.2989-1-mans@xxxxxxxxx/T/#u

> Fixes: c3ae106050b9 ("i2c: designware: Implement support for SMBus block read and write")
> Signed-off-by: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>
> ---
>  drivers/i2c/busses/i2c-designware-master.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-designware-master.c b/drivers/i2c/busses/i2c-designware-master.c
> index d6425ad6e6a3..16d38b8fc19a 100644
> --- a/drivers/i2c/busses/i2c-designware-master.c
> +++ b/drivers/i2c/busses/i2c-designware-master.c
> @@ -398,7 +398,6 @@ i2c_dw_recv_len(struct dw_i2c_dev *dev, u8 len)
>  	len += (flags & I2C_CLIENT_PEC) ? 2 : 1;
>  	dev->tx_buf_len = len - min_t(u8, len, dev->rx_outstanding);
>  	msgs[dev->msg_read_idx].len = len;
> -	msgs[dev->msg_read_idx].flags &= ~I2C_M_RECV_LEN;
>  
>  	return len;
>  }
> @@ -430,10 +429,11 @@ i2c_dw_read(struct dw_i2c_dev *dev)
>  			u32 flags = msgs[dev->msg_read_idx].flags;
>  
>  			regmap_read(dev->map, DW_IC_DATA_CMD, &tmp);
> -			/* Ensure length byte is a valid value */
> -			if (flags & I2C_M_RECV_LEN &&
> -			    tmp <= I2C_SMBUS_BLOCK_MAX && tmp > 0) {
> -				len = i2c_dw_recv_len(dev, tmp);
> +			if (flags & I2C_M_RECV_LEN) {
> +				/* Ensure length byte is a valid value */
> +				if (tmp <= I2C_SMBUS_BLOCK_MAX && tmp > 0)
> +					len = i2c_dw_recv_len(dev, tmp);
> +				msgs[dev->msg_read_idx].flags &= ~I2C_M_RECV_LEN;
>  			}
>  			*buf++ = tmp;
>  			dev->rx_outstanding--;
> -- 
> 2.27.0
> 

-- 
With Best Regards,
Andy Shevchenko