While reading through net-code i came across some code in tcp_sendpage which i
think
it is not working in the intended way all tie time. But as this code is at that
a
central place and pretty old, im suspicious if my analysis is really right.
The code in question is this
(from net/ipv4/tcp.c)
if (!(sk->sk_route_caps & NETIF_F_SG) ||
!(sk->sk_route_caps & NETIF_F_ALL_CSUM))
return sock_no_sendpage(sk->sk_socket, page, offset, size,
flags);
especially this part.
!(sk->sk_route_caps & NETIF_F_ALL_CSUM)
The problem occurs if a device only supports checksumming for ipv4 or ipv6. In
this cases only NETIF_F_IP_CSUM or NETIF_F_IPV6_CSUM is set.
Lets assume a device is only supporting checksumming in ipv6 via
NETIF_F_IPV6_CSUM,
and we are calling tcp_sendpage for a ipv4 connection. From my understanding
the intend
of the above code is that in this case sock_no_sendpage should be called. But
the bit-check
against NETIF_F_ALL_CSUM will be always > 0, as it is defined as
(from include/linux/netdev_features.h)
#define NETIF_F_V4_CSUM (NETIF_F_GEN_CSUM | NETIF_F_IP_CSUM)
#define NETIF_F_V6_CSUM (NETIF_F_GEN_CSUM | NETIF_F_IPV6_CSUM)
#define NETIF_F_ALL_CSUM (NETIF_F_V4_CSUM | NETIF_F_V6_CSUM)
so it will never get in that branch.
So imo the code should be something like this (in pseudocode)
(!(sk->sk_route_caps & NETIF_F_V4_CSUM) && proto == IPV4) ||
(!(sk->sk_route_caps & NETIF_F_V6_CSUM) && proto == IPV6)
So what am i missing?
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