[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 <oss@xxxxxxxxx> 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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