From mboxrd@z Thu Jan 1 00:00:00 1970
From: bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r@public.gmane.org
Subject: [Bug 15952] man page cmsg(3) inconsistency
Date: Tue, 05 May 2015 16:02:31 +0000
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https://bugzilla.kernel.org/show_bug.cgi?id=15952
Michael Kerrisk changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
CC| |mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
Resolution|--- |CODE_FIX
--- Comment #4 from Michael Kerrisk ---
It seems to me that at the time this report was made, there were at least *two*
problems with this code snippet:
msg.msg_control = buf;
[1] msg.msg_controllen = sizeof buf;
cmsg = CMSG_FIRSTHDR(&msg);
cmsg->cmsg_level = SOL_SOCKET;
cmsg->cmsg_type = SCM_RIGHTS;
cmsg->cmsg_len = CMSG_LEN(sizeof(int) * NUM_FD);
/* Initialize the payload: */
fdptr = (int *) CMSG_DATA(cmsg);
memcpy(fdptr, myfds, NUM_FD * sizeof(int));
/* Sum of the length of all control messages in the buffer: */
[2] msg.msg_controllen = cmsg->cmsg_len;
One of these is the problem referred to in this bug, at the line marked [2].
But the other is that there's a general confusion in the code where
msg.msg_controllen is being initialized twice.
Since the time of the report, the code has changed a little because on some
other reports, but the problem line [2] still exists. The solution is I believe
to remove line [2] and modify line [1] (which was already done as a result of
the other changes), as shown in this revised code snippet:
struct msghdr msg = {0};
struct cmsghdr *cmsg;
int myfds[NUM_FD]; /* Contains the file descriptors to pass. */
union {
/* ancillary data buffer, wrapped in a union in order to ensure
it is suitably aligned */
char buf[CMSG_SPACE(sizeof myfds)];
struct cmsghdr align;
} u;
int *fdptr;
msg.msg_control = u.buf;
msg.msg_controllen = sizeof u.buf;
cmsg = CMSG_FIRSTHDR(&msg);
cmsg->cmsg_level = SOL_SOCKET;
cmsg->cmsg_type = SCM_RIGHTS;
cmsg->cmsg_len = CMSG_LEN(sizeof(int) * NUM_FD);
/* Initialize the payload: */
fdptr = (int *) CMSG_DATA(cmsg);
memcpy(fdptr, myfds, NUM_FD * sizeof(int));
I've made this change, which I believe addresses the problem, so I'm closing
this bug. Please reopen, if you believe there is still a problem.
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