Most recent kernel where this bug did not occur: 
this bug occurs in the current and all preceding kernels in at 
least 2.6.x and 2.4.x

Distribution: not applicable (I'm currently test 2.6.15-rc5)

Hardware Environment: x86

Software Environment:

Problem Description:
On most Unix implementations, doing a write() of the form:

write(fd, buf, 0);

on a connected datagram socket delivers a datagram of length zero to
the peer.  (This provide no data of course, but does provide 
metadata ("a datgram arrived").)

This works (see test program below) on Solaris 8, HP-UX 11, Tru64 5.1B,
and FreeBSD 5.5.  It does not work on Linux: it would be good
to make Linux compatible with other implementations in this respect.

The problem is the following lines (which are probably designed to 
catch other unrelated paths) in net/socket.c:sock_aio_read():

       if (size==0)            /* Match SYS5 behaviour */
                return 0;

Steps to reproduce:

If we run the program below on FreeBSD (fr example), we see 
the following:

$ ./a.out
FreeBSD td149.testdrive.hp.com 6.0-RELEASE FreeBSD 6.0-RELEASE #0: Thu Nov  3
01:10:43 UTC 2005     root@ds10.freebie.xs4all.nl:/usr/obj/usr/src/sys/GENERIC 
alpha
Mon Dec 12 14:10:18 EST 2005
Did a write of 0 bytes
Obtained datagram with length 0

But on Linux, we see the following:

$ ./a.out
Linux tekapo 2.6.15-rc5 #6 SMP PREEMPT Wed Dec 7 11:08:45 CET 2005 i686 i686
i386 GNU/Linux
Mon Dec 12 15:07:46 CET 2005
Did a write of 0 bytes
No datagram could be read

/* write_zero_len_dgram.c */
#include <netinet/in.h>
#include <sys/socket.h>
#include <signal.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>

#define errExit(msg)            do { perror(msg); exit(EXIT_FAILURE); \
                                } while (0)

#define usageErr(msg, progName) \
                                do { fprintf(stderr, "Usage: "); \
                                     fprintf(stderr, msg, progName); \
                                     exit(EXIT_FAILURE); } while (0)
static void
handler(int sig)
{
} /* handler */

int
main(int argc, char *argv[])
{
    int rfd, sfd;
    struct sockaddr_in svaddr;
    char *buf;
    int port;
    ssize_t nw, nr;
    size_t len;
    struct sigaction sa;

    if (argc > 1 && strcmp(argv[1], "--help") == 0)
        usageErr("%s [len [port]]\n", argv[0]);

    system("uname -a; date");

    rfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (rfd == -1) errExit("socket");

    sfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sfd == -1) errExit("socket");

    len = (argc > 1) ? atoi(argv[1]) : 0;
    port = (argc > 2) ? atoi(argv[2]) : 55555;

    memset(&svaddr, 0, sizeof(struct sockaddr_in));
    svaddr.sin_family = AF_INET;
    svaddr.sin_addr.s_addr = htonl(INADDR_ANY); /* Wildcard address */
    svaddr.sin_port = htons(port);

    if (bind(rfd, (struct sockaddr *) &svaddr,
                sizeof(struct sockaddr_in)) == -1) errExit("bind");

    if (connect(sfd, (struct sockaddr *) &svaddr,
                sizeof(struct sockaddr_in)) == -1) errExit("connect");
    buf = malloc((len > 0) ? len : 10);
    if (buf == NULL) errExit("malloc");

    nw = write(sfd, buf, len);
    if (nw != len) errExit("write");
    printf("Did a write of %ld bytes\n", (long) len);

    sa.sa_flags = 0;
    sa.sa_handler = handler;
    sigemptyset(&sa.sa_mask);
    if (sigaction(SIGALRM, &sa, NULL) == -1) errExit("sigaction");

    alarm(2);

    nr = read(rfd, buf, (len > 0) ? len : 10);
    if (nr == -1) {
        if (errno != EINTR)
            errExit("read");
        else
            printf("No datagram could be read\n");
    } else
        printf("Obtained datagram with length %ld\n", (long) nr);

    exit(EXIT_SUCCESS);
} /* main */