On 04/06/2018 07:11 AM, Rusty Russell wrote:
> I was really surprised that sendmsg() returned EBADF on a valid fd;
> turns out I was using sendmsg with SCM_RIGHTS to send a closed fd, which
> gives EBADF (see test program below).
>
> But this is only obliquely referenced in unix(7):
>
> SCM_RIGHTS
> Send or receive a set of open file descriptors from another process. The data portion contains an integer
> array of the file descriptors. The passed file descriptors behave as though they have been created with
> dup(2).
>
> EBADF is not mentioned in the unix(7) ERRORS (it's mentioned in dup(2)).
Thanks, Rusty. I applied the patch below.
Cheers,
Michael
diff --git a/man7/unix.7 b/man7/unix.7
index 79f93899c..a8efc8940 100644
--- a/man7/unix.7
+++ b/man7/unix.7
@@ -467,6 +467,15 @@ see
The specified local address is already in use or the filesystem socket
object already exists.
.TP
+.B EBADF
+This error can occur for
+.BR sendmsg (2)
+when sending a file descriptor as ancillary data over
+a UNIX domain socket (see the description of
+.BR SCM_RIGHTS ,
+above), and indicates that the file descriptor number that
+is being sent is not valid (e.g., it is not an open file descripror).
+.TP
.B ECONNREFUSED
The remote address specified by
.BR connect (2)
> Thanks,
> Rusty.
>
> #include <assert.h>
> #include <errno.h>
> #include <stdbool.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <sys/socket.h>
> #include <unistd.h>
>
> int fdpass_send(int sockout, int fd)
> {
> /* From the cmsg(3) manpage: */
> struct msghdr msg = { 0 };
> struct cmsghdr *cmsg;
> struct iovec iov;
> char c = 0;
> union { /* Ancillary data buffer, wrapped in a union
> in order to ensure it is suitably aligned */
> char buf[CMSG_SPACE(sizeof(fd))];
> struct cmsghdr align;
> } u;
>
> msg.msg_control = u.buf;
> msg.msg_controllen = sizeof(u.buf);
> memset(&u, 0, sizeof(u));
> cmsg = CMSG_FIRSTHDR(&msg);
> cmsg->cmsg_level = SOL_SOCKET;
> cmsg->cmsg_type = SCM_RIGHTS;
> cmsg->cmsg_len = CMSG_LEN(sizeof(fd));
> memcpy(CMSG_DATA(cmsg), &fd, sizeof(fd));
>
> msg.msg_name = NULL;
> msg.msg_namelen = 0;
> msg.msg_iov = &iov;
> msg.msg_iovlen = 1;
> msg.msg_flags = 0;
>
> /* Keith Packard reports that 0-length sends don't work, so we
> * always send 1 byte. */
> iov.iov_base = &c;
> iov.iov_len = 1;
>
> return sendmsg(sockout, &msg, 0);
> }
>
> int fdpass_recv(int sockin)
> {
> /* From the cmsg(3) manpage: */
> struct msghdr msg = { 0 };
> struct cmsghdr *cmsg;
> struct iovec iov;
> int fd;
> char c;
> union { /* Ancillary data buffer, wrapped in a union
> in order to ensure it is suitably aligned */
> char buf[CMSG_SPACE(sizeof(fd))];
> struct cmsghdr align;
> } u;
>
> msg.msg_control = u.buf;
> msg.msg_controllen = sizeof(u.buf);
>
> msg.msg_name = NULL;
> msg.msg_namelen = 0;
> msg.msg_iov = &iov;
> msg.msg_iovlen = 1;
> msg.msg_flags = 0;
>
> iov.iov_base = &c;
> iov.iov_len = 1;
>
> if (recvmsg(sockin, &msg, 0) < 0)
> return -1;
>
> cmsg = CMSG_FIRSTHDR(&msg);
> if (!cmsg
> || cmsg->cmsg_len != CMSG_LEN(sizeof(fd))
> || cmsg->cmsg_level != SOL_SOCKET
> || cmsg->cmsg_type != SCM_RIGHTS) {
> errno = -EINVAL;
> return -1;
> }
>
> memcpy(&fd, CMSG_DATA(cmsg), sizeof(fd));
> return fd;
> }
>
> static void child(int sockfd)
> {
> int newfd = fdpass_recv(sockfd);
> assert(newfd < 0);
> exit(0);
> }
>
> int main(void)
> {
> int sv[2];
> int pid, ret;
>
> assert(socketpair(AF_UNIX, SOCK_STREAM, 0, sv) == 0);
>
> pid = fork();
> if (pid == 0) {
> close(sv[1]);
> child(sv[0]);
> }
>
> close(sv[0]);
> ret = fdpass_send(sv[1], sv[0]);
> printf("fdpass of bad fd return %i (%s)\n", ret, strerror(errno));
> return 0;
> }
> --
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--
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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