Module Name: src
Committed By: riastradh
Date: Sat Feb 8 13:30:42 UTC 2025
Modified Files:
src/share/man/man3: CMSG_DATA.3
Log Message:
cmsg(3): Handle as many descriptors as the sender sends in example.
Due to a quirk of alignment, sizing the control buffer for
CMSG_SPACE(sizeof(int)) does not guarantee at most one file
descriptor fits in the buffer -- so if there is a chance the sender
may send more than one descriptor, the receiver has to loop over all
of them to avoid leaks.
Prompted by:
PR lib/59054: cmsg(3) uses malloc(3) unnecessarily
To generate a diff of this commit:
cvs rdiff -u -r1.4 -r1.5 src/share/man/man3/CMSG_DATA.3
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/share/man/man3/CMSG_DATA.3
diff -u src/share/man/man3/CMSG_DATA.3:1.4 src/share/man/man3/CMSG_DATA.3:1.5
--- src/share/man/man3/CMSG_DATA.3:1.4 Sat Feb 8 13:18:48 2025
+++ src/share/man/man3/CMSG_DATA.3 Sat Feb 8 13:30:42 2025
@@ -1,4 +1,4 @@
-.\" $NetBSD: CMSG_DATA.3,v 1.4 2025/02/08 13:18:48 riastradh Exp $
+.\" $NetBSD: CMSG_DATA.3,v 1.5 2025/02/08 13:30:42 riastradh Exp $
.\" $OpenBSD: CMSG_DATA.3,v 1.5 2008/03/24 16:11:07 deraadt Exp $
.\" Written by Jared Yanovich <jar...@openbsd.org>
.\" Public domain, July 3, 2005
@@ -116,7 +116,8 @@ if (sendmsg(s, &msg, 0) == -1)
err(1, "sendmsg");
.Ed
.Pp
-And an example that receives and decomposes the control message:
+And an example that receives the control message and handles all the
+file descriptors it receives:
.Bd -literal -offset indent
struct msghdr msg;
struct cmsghdr *cmsg;
@@ -131,18 +132,39 @@ msg.msg_controllen = sizeof(cmsgbuf.buf)
if (recvmsg(s, &msg, 0) == -1)
err(1, "recvmsg");
-if ((msg.msg_flags & (MSG_TRUNC|MSG_CTRUNC))
- errx(1, "control message truncated");
+if (msg.msg_flags & MSG_CTRUNC)
+ warnx("control message truncated");
for (cmsg = CMSG_FIRSTHDR(&msg); cmsg != NULL;
cmsg = CMSG_NXTHDR(&msg, cmsg)) {
- if (cmsg->cmsg_len == CMSG_LEN(sizeof(int)) &&
- cmsg->cmsg_level == SOL_SOCKET &&
+ if (cmsg->cmsg_level == SOL_SOCKET &&
cmsg->cmsg_type == SCM_RIGHTS) {
- fd = *(int *)CMSG_DATA(cmsg);
- /* Do something with the descriptor. */
+ int *fdp = (int *)CMSG_DATA(cmsg);
+ socklen_t nbytes = cmsg->cmsg_len - CMSG_LEN(0);
+ socklen_t nfds = nbytes/sizeof(fdp[0]);
+
+ assert(nbytes % sizeof(fdp[0]) == 0);
+
+ while (nfds --> 0) {
+ int fd = *fdp++;
+
+ /* Do something with the descriptor. */
+ }
}
}
.Ed
+.Pp
+Note that even if the receiver
+.Em intends
+to size its control buffer for
+.Em one
+file descriptor with
+.Li CMSG_SPACE(sizeof(int)) ,
+this size may be rounded up for alignment to enough space for more than
+one file descriptor.
+So if the sender may send more than one file descriptor at a time, the
+receiver cannot restrict itself to receiving at most one at a time, and
+must be prepared to handle all of them \(em otherwise they will simply
+leak on the receiver side.
.Sh SEE ALSO
.Xr recvmsg 2 ,
.Xr sendmsg 2 ,
