CODEWITHSUNDEEP
cstructlinux-kernelkernelkernel-module
Sapiro
Sapiro

Reputation: 77

Determine if ipv4 or ipv6 data structure

In a kernel module, given a struct sockaddr with sa_family initialized as AF_UNSPEC, how can I reliably determine if it is a struct sockaddr_in or struct sockaddr_in6? On Linux 3.16.0-4-686-pae (x86).

    struct sockaddr {
                        unsigned short     sa_family; // AF_UNSPEC
                        char               sa_data[14]; // ?
                        };
    struct sockaddr_in {
                        unsigned short     sin_family;
                        unsigned short     sin_port;
                        struct in_addr     sin_addr;
                        char               sin_zero[8];
                        };
    struct sockaddr_in6 {
                        unsigned short     sin6_family;
                        unsigned short     sin6_port;
                        unsigned int       sin6_flowinfo;
                        struct in6_addr    sin6_addr;
                        unsigned int       sin6_scope_id;
                        };

Upvotes: 1

Views: 722

Answers (1)

haneefmubarak
haneefmubarak

Reputation: 2051

Usually, when something calls into the kernel and gives a struct sockaddr, it also has to give the length of the struct sockaddr. For example, see sendto():

ssize_t sendto (int sockfd, const void *buf, size_t buflen, int flags,
                    const struct sockaddr *addr, socklen_t addrlen);

Using the size of the buffer, you ought to be able to get a good guess as to what type of sockaddr you need to use:

if (addr.sa_family == AF_UNSPEC) {
    switch (addrlen) {
        case sizeof (struct sockaddr_in): {
            addr.sa_family = AF_INET;
            break;
        }

        case sizeof (struct sockaddr_in6): {
            addr.sa_family = AF_INET6;
            break;
        }

        default: {
            // handle error

            break;
        }
    }
}

In an ideal world, the sa_family would be set to seither AF_INET (IPv4) or AF_INET6 (IPv6) already, but unfortunately that doesn't appear to be the case here.

Upvotes: 2

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