Re: [RFC,PATCH] svc: Fix the UDP address logic

[Tom Tucker <tom@opengridcomputing.com>]

Chuck Lever wrote:
> Tom Tucker wrote:
>> When the address information was moved to the svc_xprt structure, a bug
>> was introduced to UDP that caused an incorrect address to be used
>> when responding to RPC on the UDP transport. This was the result of
>> failing to completely implement the generic address logic for the UDP 
>> transport.
>>
>> Thanks to Greg for pointing this out...
>>
>> Since I confused myself, it's probably a good idea to describe how 
>> this is supposed to work. The transport is responsible for setting 
>> the xpt_local and xpt_remote addresses in the svc_xprt structure as
>> part of xpo_recvfrom processing. This cannot be done in a generic way 
>> and in fact varies between TCP, UDP and RDMA. A set of xpo_ functions
>> (e.g. getlocalname, getremotename) could have been added but this would
>> have resulted in additional caching and copying of the addresses around.
>> The generic svc_recv code copies the addresses from the svc_xprt 
>> structure into the rqstp structure as part of svc_recv processing. 
>> The xpt_local address should also be set on listening endpoints, for
>> tcp/rdma this is done as part of endpoint creation and for new 
>> connections in xpo_accept processing.
>> This patch was tested with Connectathon over V3 on UDP.
>>
>> Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
>> ---
>>
>>  net/sunrpc/svcsock.c |   16 ++++++++++------
>>  1 files changed, 10 insertions(+), 6 deletions(-)
>>
>> diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
>> index e1a27ee..0f8a3d5 100644
>> --- a/net/sunrpc/svcsock.c
>> +++ b/net/sunrpc/svcsock.c
>> @@ -473,17 +473,21 @@ svc_write_space(struct sock *sk)
>>  static inline void svc_udp_get_dest_address(struct svc_rqst *rqstp,
>>                          struct cmsghdr *cmh)
>>  {
>> -    struct svc_sock *svsk =
>> -        container_of(rqstp->rq_xprt, struct svc_sock, sk_xprt);
>> +    struct svc_xprt *xprt = rqstp->rq_xprt;
>> +    struct svc_sock *svsk = container_of(xprt, struct svc_sock, 
>> sk_xprt);
>>      switch (svsk->sk_sk->sk_family) {
>>      case AF_INET: {
>>          struct in_pktinfo *pki = CMSG_DATA(cmh);
>> -        rqstp->rq_daddr.addr.s_addr = pki->ipi_spec_dst.s_addr;
>> +        struct sockaddr_in *sin =
>> +            (struct sockaddr_in *)&xprt->xpt_local;
>> +        sin->sin_addr.s_addr = pki->ipi_spec_dst.s_addr;
>>          break;
>>          }
>>      case AF_INET6: {
>>          struct in6_pktinfo *pki = CMSG_DATA(cmh);
>> -        ipv6_addr_copy(&rqstp->rq_daddr.addr6, &pki->ipi6_addr);
>> +        struct sockaddr_in6 *sin6 =
>> +            (struct sockaddr_in6 *)&xprt->xpt_local;
>> +        ipv6_addr_copy(&sin6->sin6_addr, &pki->ipi6_addr);
>>          break;
>>          }
>>      }
>> @@ -506,7 +510,7 @@ svc_udp_recvfrom(struct svc_rqst *rqstp)
>>      struct cmsghdr *cmh = &buffer.hdr;
>>      int        err, len;
>>      struct msghdr msg = {
>> -        .msg_name = svc_addr(rqstp),
>> +        .msg_name = &svsk->sk_xprt.xpt_remote,
>>          .msg_control = cmh,
>>          .msg_controllen = sizeof(buffer),
>>          .msg_flags = MSG_DONTWAIT,
>> @@ -541,7 +545,7 @@ svc_udp_recvfrom(struct svc_rqst *rqstp)
>>          svc_xprt_received(&svsk->sk_xprt);
>>          return -EAGAIN;
>>      }
>> -    rqstp->rq_addrlen = sizeof(rqstp->rq_addr);
>> +    svsk->sk_xprt.xpt_remotelen = sizeof(svsk->sk_xprt.xpt_remote);
>
> This worries me a bit.  Why isn't rq_addrlen involved here?
rq_addrlen gets set in the generic code (see svc_recv). The source of 
the address is the transport, not the rqstp structure. The rqstp 
structure caches it for use when responding to the request in 
svc_process. Ironically, I think the HA case that Greg referred to is 
the only reason we can't get rid of rq_addr/rq_daddr altogether and 
access the address data directly from svc_xprt. Since this address may 
change for UDP, we have to store it in the rqstp structure and use it to 
reply because another request could arrive in the interim and change the 
address data in svc_xprt.
>   the xpt_remote field should be a sockaddr_storage, 
...it is.
> and the remote_len field should be the actual length of the address, 
> not the size of the storage space.
Perhaps it should be, but a) kernel_recvmsg doesn't give us a length, b) 
to determine it would require a switch, the degenerate case for which is 
sizeof(storage), and c) in it's current usage, big-enough is 
good-enough. A weaker point is "that's the way it's always been" ;-)

Tom
>
>>      if (skb->tstamp.tv64 == 0) {
>>          skb->tstamp = ktime_get_real();
>>          /* Don't enable netstamp, sunrpc doesn't


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