Re: [PATCH v2 02/21] cxl: Add checksum verification to CDAT from CXL

[Alison Schofield <alison.schofield@intel.com>]

On Mon, Mar 27, 2023 at 02:44:13PM -0700, Dave Jiang wrote:
> A CDAT table is available from a CXL device. The table is read by the
> driver and cached in software. With the CXL subsystem needing to parse the
> CDAT table, the checksum should be verified. Add checksum verification
> after the CDAT table is read from device.
> 
> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> 
> ---
> v2:
> - Drop ACPI checksum export and just use local verification. (Dan)
> ---
>  drivers/cxl/core/pci.c |   16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/drivers/cxl/core/pci.c b/drivers/cxl/core/pci.c
> index 25b7e8125d5d..e0d5e6525c0d 100644
> --- a/drivers/cxl/core/pci.c
> +++ b/drivers/cxl/core/pci.c
> @@ -528,6 +528,16 @@ static int cxl_cdat_read_table(struct device *dev,
>  	return 0;
>  }
>  
> +static unsigned char cdat_checksum(void *buf, size_t size)
> +{
> +	unsigned char sum, *data = buf;
> +	size_t i;
> +
> +	for (sum = 0, i = 0; i < size; i++)
> +		sum += data[i];
> +	return 0 - sum;

This return value isn't obvious to me. What's happening here?
Thanks for explaining,

Alison

> +}
> +
>  /**
>   * read_cdat_data - Read the CDAT data on this port
>   * @port: Port to read data from
> @@ -573,6 +583,12 @@ void read_cdat_data(struct cxl_port *port)
>  	}
>  
>  	port->cdat.table = cdat_table + sizeof(__le32);
> +	if (cdat_checksum(port->cdat.table, cdat_length)) {
> +		/* Don't leave table data allocated on error */
> +		devm_kfree(dev, cdat_table);
> +		dev_err(dev, "CDAT data checksum error\n");
> +	}
> +
>  	port->cdat.length = cdat_length;
>  }
>  EXPORT_SYMBOL_NS_GPL(read_cdat_data, CXL);
> 
>