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

1 files changed, 5 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;