by Alex Elder

[permalink] [raw]

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

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 <[email protected]>
>> ---
>> .../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
>>

2021-03-05 21:07:45

by Alex Elder

[permalink] [raw]

Subject: Re: [PATCH net-next 2/6] net: qualcomm: rmnet: simplify some byte order logic

On 3/4/21 10:07 PM, Bjorn Andersson wrote:
> On Thu 04 Mar 16:34 CST 2021, Alex Elder wrote:
>
>> In rmnet_map_ipv4_ul_csum_header() and rmnet_map_ipv6_ul_csum_header()
>> the offset within a packet at which checksumming should commence is
>> calculated. This calculation involves byte swapping and a forced type
>> conversion that makes it hard to understand.
>>
>> Simplify this by computing the offset in host byte order, then
>> converting the result when assigning it into the header field.
>>
>> Signed-off-by: Alex Elder <[email protected]>
>> ---
>> .../ethernet/qualcomm/rmnet/rmnet_map_data.c | 22 ++++++++++---------
>> 1 file changed, 12 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
>> index 21d38167f9618..bd1aa11c9ce59 100644
>> --- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
>> +++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
>> @@ -197,12 +197,13 @@ rmnet_map_ipv4_ul_csum_header(void *iphdr,
>> struct rmnet_map_ul_csum_header *ul_header,
>> struct sk_buff *skb)
>> {
>> - struct iphdr *ip4h = (struct iphdr *)iphdr;
>> - __be16 *hdr = (__be16 *)ul_header, offset;
>> + __be16 *hdr = (__be16 *)ul_header;
>> + struct iphdr *ip4h = iphdr;
>> + u16 offset;
>> +
>> + offset = skb_transport_header(skb) - (unsigned char *)iphdr;
>> + ul_header->csum_start_offset = htons(offset);
>>
>> - offset = htons((__force u16)(skb_transport_header(skb) -
>
> Just curious, why does this require a __force, or even a cast?

The argument to htons() has type __u16. In this case it
is passed the difference between pointers, which will
have type ptrdiff_t, which is certainly bigger than
16 bits. I don't think the __force is needed, but the
cast to u16 might just be making the conversion to the
smaller type explicit. Here too though, I don't think
it's necessary.

-Alex

> Regardless, your proposed way of writing it is easier to read.
>
> Reviewed-by: Bjorn Andersson <[email protected]>
>
> Regards,
> Bjorn
>
>> - (unsigned char *)iphdr));
>> - ul_header->csum_start_offset = offset;
>> ul_header->csum_insert_offset = skb->csum_offset;
>> ul_header->csum_enabled = 1;
>> if (ip4h->protocol == IPPROTO_UDP)
>> @@ -239,12 +240,13 @@ rmnet_map_ipv6_ul_csum_header(void *ip6hdr,
>> struct rmnet_map_ul_csum_header *ul_header,
>> struct sk_buff *skb)
>> {
>> - struct ipv6hdr *ip6h = (struct ipv6hdr *)ip6hdr;
>> - __be16 *hdr = (__be16 *)ul_header, offset;
>> + __be16 *hdr = (__be16 *)ul_header;
>> + struct ipv6hdr *ip6h = ip6hdr;
>> + u16 offset;
>> +
>> + offset = skb_transport_header(skb) - (unsigned char *)ip6hdr;
>> + ul_header->csum_start_offset = htons(offset);
>>
>> - offset = htons((__force u16)(skb_transport_header(skb) -
>> - (unsigned char *)ip6hdr));
>> - ul_header->csum_start_offset = offset;
>> ul_header->csum_insert_offset = skb->csum_offset;
>> ul_header->csum_enabled = 1;
>>
>> --
>> 2.20.1
>>