Re: Incorrect behaviour or documentation problem of SO_RXQ_OVFL

[Adding Neil, who wrote the original text. Maybe he has also some
suggested improvement.]

Hello Petr and Tobias,

Thank you both for your reports about the incorrect documentation. See below.

On 15 November 2017 at 16:14, Petr Malat [off-list ref] wrote:

Hi!
Generic SO_RXQ_OVFL helpers sock_skb_set_dropcount() and sock_recv_drops()
implements returning of sk->sk_drops (the total number of dropped packets),
although the documentation says the number of dropped packets since the
last received one should be returned (quoting the current socket.7):
  SO_RXQ_OVFL (since Linux 2.6.33)
  Indicates that an unsigned 32-bit value ancillary message (cmsg)
  should be attached to received skbs indicating the number of packets
  dropped by the socket between the last received packet and this
  received packet.

I assume the documentation needs to be updated, as fixing this in the
code could break programs depending on the current behavior, although
the formerly planned functionality seems to be more usefull.

The problem can be revealed with the following program:

#include <netinet/in.h>
#include <netinet/ip.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>

int extract_drop(struct msghdr *msg)
{
        struct cmsghdr *cmsg;
        int rtn;

        for (cmsg = CMSG_FIRSTHDR(msg); cmsg; cmsg = CMSG_NXTHDR(msg,cmsg)) {
                if (cmsg->cmsg_level == SOL_SOCKET &&
                    cmsg->cmsg_type == SO_RXQ_OVFL) {
                        memcpy(&rtn, CMSG_DATA(cmsg), sizeof rtn);
                        return rtn;
                }
        }
        return -1;
}

int main(int argc, char *argv[])
{
        struct sockaddr_in addr = { .sin_family = AF_INET };
        char msg[48*1024], cmsgbuf[256];
        struct iovec iov = { .iov_base = msg, .iov_len = sizeof msg };
        int sk1, sk2, i, one = 1;

        sk1 = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
        sk2 = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);

        inet_pton(AF_INET, "127.0.0.1", &addr.sin_addr);
        addr.sin_port = htons(53333);

        bind(sk1, (struct sockaddr*)&addr, sizeof addr);
        connect(sk2, (struct sockaddr*)&addr, sizeof addr);

        // Kernel doubles this limit, but it accounts also the SKB overhead,
        // but it receives as long as there is at least 1 byte free.
        i = sizeof msg;
        setsockopt(sk1, SOL_SOCKET, SO_RCVBUF, &i, sizeof i);
        setsockopt(sk1, SOL_SOCKET, SO_RXQ_OVFL, &one, sizeof one);

        for (i = 0; i < 4; i++) {
                int rtn;

                send(sk2, msg, sizeof msg, 0);
                send(sk2, msg, sizeof msg, 0);
                send(sk2, msg, sizeof msg, 0);

                do {
                        struct msghdr msghdr = {
                                        .msg_iov = &iov, .msg_iovlen = 1,
                                        .msg_control = &cmsgbuf,
                                        .msg_controllen = sizeof cmsgbuf };
                        rtn = recvmsg(sk1, &msghdr, MSG_DONTWAIT);
                        if (rtn > 0) {
                                printf("rtn: %d drop %d\n", rtn,
                                                extract_drop(&msghdr));
                        } else {
                                printf("rtn: %d\n", rtn);
                        }
                } while (rtn > 0);
        }

        return 0;
}

which prints
  rtn: 49152 drop -1
  rtn: 49152 drop -1
  rtn: -1
  rtn: 49152 drop 1
  rtn: 49152 drop 1
  rtn: -1
  rtn: 49152 drop 2
  rtn: 49152 drop 2
  rtn: -1
  rtn: 49152 drop 3
  rtn: 49152 drop 3
  rtn: -1
although it should print (according to the documentation):
  rtn: 49152 drop 0
  rtn: 49152 drop 0
  rtn: -1
  rtn: 49152 drop 1
  rtn: 49152 drop 0
  rtn: -1
  rtn: 49152 drop 1
  rtn: 49152 drop 0
  rtn: -1
  rtn: 49152 drop 1
  rtn: 49152 drop 0
  rtn: -1

Please keep me on To:/CC: as I'm not on the list.

Thanks for the test program. Tobias reported the same issue, and I've
applied his suggested change to the page. (See below.)

Cheers,

Michael

diff --git a/man7/socket.7 b/man7/socket.7
index 79966a6fd..1a2cfe9cc 100644
--- a/man7/socket.7
+++ b/man7/socket.7

@@ -881,8 +881,7 @@ compete to receive datagrams on the same socket.
 .\" commit 3b885787ea4112eaa80945999ea0901bf742707f
 Indicates that an unsigned 32-bit value ancillary message (cmsg)
 should be attached to received skbs indicating
-the number of packets dropped by the socket between
-the last received packet and this received packet.
+the number of packets dropped by the socket since its creation.
 .TP
 .B SO_SNDBUF
 Sets or gets the maximum socket send buffer in bytes.

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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