linux-kernel - Re: [PATCH 2/2] cxl/cdat: Fix header sum value in CDAT checksum

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Message-ID: <962fa507-b9f5-4646-8090-cf63f737e84f@intel.com>
Date:   Mon, 20 Nov 2023 08:43:39 -0700
From:   Dave Jiang <dave.jiang@...el.com>
To:     Ira Weiny <ira.weiny@...el.com>,
        Jonathan Cameron <jonathan.cameron@...wei.com>,
        Fan Ni <nifan.cxl@...il.com>
CC:     <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] cxl/cdat: Fix header sum value in CDAT checksum



On 11/17/23 18:14, Ira Weiny wrote:
> The addition of the DCD support for CXL type-3 devices extended the CDAT
> table large enough that the checksum being returned was incorrect.[1]
> 
> This was because the checksum value was using the header length field
> rather than each of the 4 bytes of the length field.  This was
> previously not seen because the length of the CDAT data was less than
> 256 thus resulting in an equivalent checksum value.
> 
> Properly calculate the checksum for the CDAT header.
> 
> [1] https://lore.kernel.org/all/20231116-fix-cdat-devm-free-v1-1-b148b40707d7@intel.com/
> 
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> ---
>  hw/cxl/cxl-cdat.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/cxl/cxl-cdat.c b/hw/cxl/cxl-cdat.c
> index 24829cf2428d..d93e2e4e64f2 100644
> --- a/hw/cxl/cxl-cdat.c
> +++ b/hw/cxl/cxl-cdat.c
> @@ -95,8 +95,15 @@ static void ct3_build_cdat(CDATObject *cdat, Error **errp)
>      /* For now, no runtime updates */
>      cdat_header->sequence = 0;
>      cdat_header->length += sizeof(CDATTableHeader);
> -    sum += cdat_header->revision + cdat_header->sequence +
> -        cdat_header->length;
> +
> +    do {
> +        uint8_t *buf = (uint8_t *)cdat_header;
> +
> +        for (i = 0; i < sizeof(*cdat_header); i++) {
> +            sum += buf[i];
> +        }
> +    } while (0);

Why the empty do/while loop?

> +
>      /* Sum of all bytes including checksum must be 0 */
>      cdat_header->checksum = ~sum + 1;
>  
>