Unbreak writes of 0 bytes. Zero byte writes happen when only ancillary

control data but no payload data is passed.

Change m_uiotombuf() to return at least one empty mbuf if the requested
length was zero.  Add comment to sosend_dgram and sosend_generic().

Diagnoses by:		jhb
Regression test by:	rwatson
Pointy hat to.		andre

2 files changed, 15 insertions, 2 deletions

diff --git a/sys/kern/uipc_mbuf.c b/sys/kern/uipc_mbuf.c
index 4161ac8..e9ff7b0 100644
--- a/sys/kern/uipc_mbuf.c
+++ b/sys/kern/uipc_mbuf.c
@@ -1643,8 +1643,11 @@ m_uiotombuf(struct uio *uio, int how, int len, int align, int flags)
 	if (align >= MHLEN)
 		return (NULL);
 
-	/* Give us all or nothing. */
-	m = m_getm2(NULL, total + align, how, MT_DATA, flags);
+	/*
+	 * Give us the full allocation or nothing.
+	 * If len is zero return the smallest empty mbuf.
+	 */
+	m = m_getm2(NULL, max(total + align, 1), how, MT_DATA, flags);
 	if (m == NULL)
 		return (NULL);
 	m->m_data += align;
diff --git a/sys/kern/uipc_socket.c b/sys/kern/uipc_socket.c
index a87d04f..d1ce038 100644
--- a/sys/kern/uipc_socket.c
+++ b/sys/kern/uipc_socket.c
@@ -1055,6 +1055,11 @@ sosend_dgram(so, addr, uio, top, control, flags, td)
 		if (error)
 			goto out;
 #else
+		/*
+		 * Copy the data from userland into a mbuf chain.
+		 * If no data is to be copied in, a single empty mbuf
+		 * is returned.
+		 */
 		top = m_uiotombuf(uio, M_WAITOK, space, max_hdr,
 		    (M_PKTHDR | ((flags & MSG_EOR) ? M_EOR : 0)));
 		if (top == NULL) {
@@ -1230,6 +1235,11 @@ restart:
 					goto release;
 				}
 #else
+				/*
+				 * Copy the data from userland into a mbuf
+				 * chain.  If no data is to be copied in,
+				 * a single empty mbuf is returned.
+				 */
 				top = m_uiotombuf(uio, M_WAITOK, space,
 				    (atomic ? max_hdr : 0),
 				    (atomic ? M_PKTHDR : 0) |