When an underlying ioctl(2) handler returns an error, our ioctl(2)

interface considers that it hits a fatal error, and will not copyout
the request structure back for _IOW and _IOWR ioctls, keeping them
untouched.

The previous implementation of the SIOCGIFDESCR ioctl intends to
feed the buffer length back to userland.  However, if we return
an error, the feedback would be defeated and ifconfig(8) would
trap into an infinite loop.

This commit changes SIOCGIFDESCR to set buffer field to NULL to
indicate the previous ENAMETOOLONG case.

Reported by:	bschmidt
MFC after:	2 weeks

1 files changed, 3 insertions, 4 deletions

diff --git a/sys/net/if.c b/sys/net/if.c
index e4a200549636..98c8afa4a2b9 100644
--- a/sys/net/if.c
+++ b/sys/net/if.c
@@ -2049,14 +2049,13 @@ ifhwioctl(u_long cmd, struct ifnet *ifp, caddr_t data, struct thread *td)
 	case SIOCGIFDESCR:
 		error = 0;
 		sx_slock(&ifdescr_sx);
-		if (ifp->if_description == NULL) {
-			ifr->ifr_buffer.length = 0;
+		if (ifp->if_description == NULL)
 			error = ENOMSG;
-		} else {
+		else {
 			/* space for terminating nul */
 			descrlen = strlen(ifp->if_description) + 1;
 			if (ifr->ifr_buffer.length < descrlen)
-				error = ENAMETOOLONG;
+				ifr->ifr_buffer.buffer = NULL;
 			else
 				error = copyout(ifp->if_description,
 				    ifr->ifr_buffer.buffer, descrlen);