Re: [PATCH net-next 6/6] net: qualcomm: rmnet: don't use C bit-fields in rmnet checksum header

[Alex Elder]

On 3/4/21 11:26 PM, Bjorn Andersson wrote:

On Thu 04 Mar 16:34 CST 2021, Alex Elder wrote:

Replace the use of C bit-fields in the rmnet_map_ul_csum_header
structure with a single two-byte (big endian) structure member,
and use field masks to encode or get values within it.

Previously rmnet_map_ipv4_ul_csum_header() would update values in
the host byte-order fields, and then forcibly fix their byte order
using a combination of byte order operations and types.

Instead, just compute the value that needs to go into the new
structure member and save it with a simple byte-order conversion.

Make similar simplifications in rmnet_map_ipv6_ul_csum_header().

Finally, in rmnet_map_checksum_uplink_packet() a set of assignments
zeroes every field in the upload checksum header.  Replace that with
a single memset() operation.

Signed-off-by: Alex Elder <el...@linaro.org>
---
  .../ethernet/qualcomm/rmnet/rmnet_map_data.c  | 34 ++++++-------------
  include/linux/if_rmnet.h                      | 21 ++++++------
  2 files changed, 21 insertions(+), 34 deletions(-)

diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c 
b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
index 29d485b868a65..db76bbf000aa1 100644
--- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
+++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
@@ -198,23 +198,19 @@ rmnet_map_ipv4_ul_csum_header(void *iphdr,
                              struct rmnet_map_ul_csum_header *ul_header,
                              struct sk_buff *skb)
  {
-       __be16 *hdr = (__be16 *)ul_header;
        struct iphdr *ip4h = iphdr;
        u16 offset;
+       u16 val;

offset = skb_transport_header(skb) - (unsigned char *)iphdr;

        ul_header->csum_start_offset = htons(offset);

- ul_header->csum_insert_offset = skb->csum_offset;

-       ul_header->csum_enabled = 1;
+       val = be16_encode_bits(1, MAP_CSUM_UL_ENABLED_FMASK);

Why are you using be16_ here? Won't that cancel the htons() below?

Yes.  This was a mistake, and as you know, the kernel test
robot caught the problem with assigning a big endian value
to a host byte order variable.  This cannot have worked
before and I'm not sure what happened.

I've updated this series and am re-testing everything.  That
includes running the patches through sparse, but also includes
running with checksum offload enabled and verifying errors
are not reported when checksum offload active.

                                        -Alex

Regards,
Bjorn

        if (ip4h->protocol == IPPROTO_UDP)
-               ul_header->udp_ind = 1;
-       else
-               ul_header->udp_ind = 0;
+               val |= be16_encode_bits(1, MAP_CSUM_UL_UDP_FMASK);
+       val |= be16_encode_bits(skb->csum_offset, MAP_CSUM_UL_OFFSET_FMASK);

- /* Changing remaining fields to network order */

-       hdr++;
-       *hdr = htons((__force u16)*hdr);
+       ul_header->csum_info = htons(val);

skb->ip_summed = CHECKSUM_NONE; @@ -241,24 +237,19 @@ rmnet_map_ipv6_ul_csum_header(void *ip6hdr,

                              struct rmnet_map_ul_csum_header *ul_header,
                              struct sk_buff *skb)
  {
-       __be16 *hdr = (__be16 *)ul_header;
        struct ipv6hdr *ip6h = ip6hdr;
        u16 offset;
+       u16 val;

offset = skb_transport_header(skb) - (unsigned char *)ip6hdr;

        ul_header->csum_start_offset = htons(offset);

- ul_header->csum_insert_offset = skb->csum_offset;

-       ul_header->csum_enabled = 1;
-
+       val = be16_encode_bits(1, MAP_CSUM_UL_ENABLED_FMASK);
        if (ip6h->nexthdr == IPPROTO_UDP)
-               ul_header->udp_ind = 1;
-       else
-               ul_header->udp_ind = 0;
+               val |= be16_encode_bits(1, MAP_CSUM_UL_UDP_FMASK);
+       val |= be16_encode_bits(skb->csum_offset, MAP_CSUM_UL_OFFSET_FMASK);

- /* Changing remaining fields to network order */

-       hdr++;
-       *hdr = htons((__force u16)*hdr);
+       ul_header->csum_info = htons(val);

skb->ip_summed = CHECKSUM_NONE; @@ -425,10 +416,7 @@ void rmnet_map_checksum_uplink_packet(struct sk_buff *skb,

        }

sw_csum:

-       ul_header->csum_start_offset = 0;
-       ul_header->csum_insert_offset = 0;
-       ul_header->csum_enabled = 0;
-       ul_header->udp_ind = 0;
+       memset(ul_header, 0, sizeof(*ul_header));

priv->stats.csum_sw++;

  }
diff --git a/include/linux/if_rmnet.h b/include/linux/if_rmnet.h
index 1fbb7531238b6..149d696feb520 100644
--- a/include/linux/if_rmnet.h
+++ b/include/linux/if_rmnet.h
@@ -33,17 +33,16 @@ struct rmnet_map_dl_csum_trailer {

struct rmnet_map_ul_csum_header {

        __be16 csum_start_offset;
-#if defined(__LITTLE_ENDIAN_BITFIELD)
-       u16 csum_insert_offset:14;
-       u16 udp_ind:1;
-       u16 csum_enabled:1;
-#elif defined (__BIG_ENDIAN_BITFIELD)
-       u16 csum_enabled:1;
-       u16 udp_ind:1;
-       u16 csum_insert_offset:14;
-#else
-#error "Please fix <asm/byteorder.h>"
-#endif
+       __be16 csum_info;               /* MAP_CSUM_UL_*_FMASK */
  } __aligned(1);

+/* csum_info field:

+ *  ENABLED:   1 = checksum computation requested
+ *  UDP:       1 = UDP checksum (zero checkum means no checksum)
+ *  OFFSET:    where (offset in bytes) to insert computed checksum
+ */
+#define MAP_CSUM_UL_OFFSET_FMASK       GENMASK(13, 0)
+#define MAP_CSUM_UL_UDP_FMASK          GENMASK(14, 14)
+#define MAP_CSUM_UL_ENABLED_FMASK      GENMASK(15, 15)
+
  #endif /* !(_LINUX_IF_RMNET_H_) */
--
2.20.1