Index: share/man/man9/sbuf.9
===================================================================
--- share/man/man9/sbuf.9
+++ share/man/man9/sbuf.9
@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd April 5, 2017
+.Dd August 8, 2017
 .Dt SBUF 9
 .Os
 .Sh NAME
@@ -271,6 +271,14 @@
 as resources allow, to hold additional data.
 .It Dv SBUF_INCLUDENUL
 This causes the final nulterm byte to be counted in the length of the data.
+.It Dv SBUF_DRAINTOEOR
+Treat top-level sections started with
+.Fn sbuf_start_section
+as a record boundary marker that will be used during drain operations to avoid
+records being split.
+If a record grows sufficiently large such that it fills the
+.Fa sbuf
+and therefore cannot be drained without being split, an error of EDEADLK is set.
 .El
 .Pp
 Note that if
Index: sys/kern/subr_sbuf.c
===================================================================
--- sys/kern/subr_sbuf.c
+++ sys/kern/subr_sbuf.c
@@ -73,6 +73,8 @@
 #define	SBUF_CANEXTEND(s)	((s)->s_flags & SBUF_AUTOEXTEND)
 #define	SBUF_ISSECTION(s)	((s)->s_flags & SBUF_INSECTION)
 #define	SBUF_NULINCLUDED(s)	((s)->s_flags & SBUF_INCLUDENUL)
+#define	SBUF_ISDRAINTOEOR(s)	((s)->s_flags & SBUF_DRAINTOEOR)
+#define	SBUF_DODRAINTOEOR(s)	(SBUF_ISSECTION(s) && SBUF_ISDRAINTOEOR(s))
 
 /*
  * Set / clear flags
@@ -308,6 +310,7 @@
 	SBUF_CLEARFLAG(s, SBUF_FINISHED);
 	s->s_error = 0;
 	s->s_len = 0;
+	s->s_rec_off = 0;
 	s->s_sect_len = 0;
 }
 
@@ -362,7 +365,10 @@
 
 	KASSERT(s->s_len > 0, ("Shouldn't drain empty sbuf %p", s));
 	KASSERT(s->s_error == 0, ("Called %s with error on %p", __func__, s));
-	len = s->s_drain_func(s->s_drain_arg, s->s_buf, s->s_len);
+	if (SBUF_DODRAINTOEOR(s) && s->s_rec_off == 0)
+		return (s->s_error = EDEADLK);
+	len = s->s_drain_func(s->s_drain_arg, s->s_buf,
+	    SBUF_DODRAINTOEOR(s) ? s->s_rec_off : s->s_len);
 	if (len < 0) {
 		s->s_error = -len;
 		return (s->s_error);
@@ -370,6 +376,7 @@
 	KASSERT(len > 0 && len <= s->s_len,
 	    ("Bad drain amount %d for sbuf %p", len, s));
 	s->s_len -= len;
+	s->s_rec_off -= len;
 	/*
 	 * Fast path for the expected case where all the data was
 	 * drained.
@@ -835,6 +842,7 @@
 		    ("s_sect_len != 0 when starting a section"));
 		if (old_lenp != NULL)
 			*old_lenp = -1;
+		s->s_rec_off = s->s_len;
 		SBUF_SETFLAG(s, SBUF_INSECTION);
 	} else {
 		KASSERT(old_lenp != NULL,
@@ -865,7 +873,7 @@
 	}
 	len = s->s_sect_len;
 	if (old_len == -1) {
-		s->s_sect_len = 0;
+		s->s_rec_off = s->s_sect_len = 0;
 		SBUF_CLEARFLAG(s, SBUF_INSECTION);
 	} else {
 		s->s_sect_len += old_len;
Index: sys/sys/sbuf.h
===================================================================
--- sys/sys/sbuf.h
+++ sys/sys/sbuf.h
@@ -49,6 +49,7 @@
 #define	SBUF_FIXEDLEN	0x00000000	/* fixed length buffer (default) */
 #define	SBUF_AUTOEXTEND	0x00000001	/* automatically extend buffer */
 #define	SBUF_INCLUDENUL	0x00000002	/* nulterm byte is counted in len */
+#define	SBUF_DRAINTOEOR	0x00000004	/* use section 0 as drain EOR marker */
 #define	SBUF_USRFLAGMSK	0x0000ffff	/* mask of flags the user may specify */
 #define	SBUF_DYNAMIC	0x00010000	/* s_buf must be freed */
 #define	SBUF_FINISHED	0x00020000	/* set by sbuf_finish() */
@@ -56,6 +57,7 @@
 #define	SBUF_INSECTION	0x00100000	/* set by sbuf_start_section() */
 	int		 s_flags;	/* flags */
 	ssize_t		 s_sect_len;	/* current length of section */
+	ssize_t		 s_rec_off;	/* current record start offset */
 };
 
 #ifndef HD_COLUMN_MASK