Redesign the externalization APIs from the MAC Framework to

the MAC policy modules to improve robustness against C string
bugs and vulnerabilities.  Following these revisions, all
string construction of labels for export to userspace (or
elsewhere) is performed using the sbuf API, which prevents
the consumer from having to perform laborious and intricate
pointer and buffer checks.  This substantially simplifies
the externalization logic, both at the MAC Framework level,
and in individual policies; this becomes especially useful
when policies export more complex label data, such as with
compartments in Biba and MLS.

Bundled in here are some other minor fixes associated with
externalization: including avoiding malloc while holding the
process mutex in mac_lomac, and hence avoid a failure mode
when printing labels during a downgrade operation due to
the removal of the M_NOWAIT case.

This has been running in the MAC development tree for about
three weeks without problems.

Obtained from:	TrustedBSD Project
Sponsored by: DARPA, Network Associates Laboratories

sys/kern/kern_mac.c

@@ -56,6 +56,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/mac.h>
 #include <sys/module.h>
 #include <sys/proc.h>
+#include <sys/sbuf.h>
 #include <sys/systm.h>
 #include <sys/sysproto.h>
 #include <sys/sysent.h>
@@ -398,65 +399,44 @@ mac_policy_list_unbusy(void)
 
 #define	MAC_EXTERNALIZE(type, label, elementlist, outbuf, 		\
     outbuflen) do {							\
-	char *curptr, *curptr_start, *element_name, *element_temp;	\
-	size_t left, left_start, len;					\
-	int claimed, first, first_start, ignorenotfound;		\
+	int claimed, first, ignorenotfound, savedlen;			\
+	char *element_name, *element_temp;				\
+	struct sbuf sb;							\
 									\
 	error = 0;							\
-	element_temp = elementlist;					\
-	curptr = outbuf;						\
-	curptr[0] = '\0';						\
-	left = outbuflen;						\
 	first = 1;							\
+	sbuf_new(&sb, outbuf, outbuflen, SBUF_FIXEDLEN);		\
+	element_temp = elementlist;					\
 	while ((element_name = strsep(&element_temp, ",")) != NULL) {	\
-		curptr_start = curptr;					\
-		left_start = left;					\
-		first_start = first;					\
 		if (element_name[0] == '?') {				\
 			element_name++;					\
 			ignorenotfound = 1;				\
-		} else							\
+		 } else							\
 			ignorenotfound = 0;				\
-		claimed = 0;						\
+		savedlen = sbuf_len(&sb);				\
 		if (first) {						\
-			len = snprintf(curptr, left, "%s/",		\
-			    element_name);				\
+			error = sbuf_printf(&sb, "%s/", element_name);	\
 			first = 0;					\
 		} else							\
-			len = snprintf(curptr, left, ",%s/",		\
-			    element_name);				\
-		if (len >= left) {					\
-			error = EINVAL;		/* XXXMAC: E2BIG */	\
+			error = sbuf_printf(&sb, ",%s/", element_name);	\
+		if (error == -1) {					\
+			error = EINVAL;	/* XXX: E2BIG? */		\
 			break;						\
 		}							\
-		curptr += len;						\
-		left -= len;						\
-									\
+		claimed = 0;						\
 		MAC_CHECK(externalize_ ## type, label, element_name,	\
-		    curptr, left, &len, &claimed);			\
+		    &sb, &claimed);					\
 		if (error)						\
 			break;						\
-		if (claimed == 1) {					\
-			if (len >= outbuflen) {				\
-				error = EINVAL;	/* XXXMAC: E2BIG */	\
-				break;					\
-			}						\
-			curptr += len;					\
-			left -= len;					\
-		} else if (claimed == 0 && ignorenotfound) {		\
-			/*						\
-			 * Revert addition of the label element		\
-			 * name.					\
-			 */						\
-			curptr = curptr_start;				\
-			*curptr = '\0';					\
-			left = left_start;				\
-			first = first_start;				\
-		} else {						\
-			error = EINVAL;		/* XXXMAC: ENOLABEL */	\
+		if (claimed == 0 && ignorenotfound) {			\
+			/* Revert last label name. */			\
+			sbuf_setpos(&sb, savedlen);			\
+		} else if (claimed != 1) {				\
+			error = EINVAL;	/* XXX: ENOLABEL? */		\
 			break;						\
 		}							\
 	}								\
+	sbuf_finish(&sb);						\
 } while (0)
 
 #define	MAC_INTERNALIZE(type, label, instring) do {			\

sys/security/mac/mac_framework.c

@@ -56,6 +56,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/mac.h>
 #include <sys/module.h>
 #include <sys/proc.h>
+#include <sys/sbuf.h>
 #include <sys/systm.h>
 #include <sys/sysproto.h>
 #include <sys/sysent.h>
@@ -398,65 +399,44 @@ mac_policy_list_unbusy(void)
 
 #define	MAC_EXTERNALIZE(type, label, elementlist, outbuf, 		\
     outbuflen) do {							\
-	char *curptr, *curptr_start, *element_name, *element_temp;	\
-	size_t left, left_start, len;					\
-	int claimed, first, first_start, ignorenotfound;		\
+	int claimed, first, ignorenotfound, savedlen;			\
+	char *element_name, *element_temp;				\
+	struct sbuf sb;							\
 									\
 	error = 0;							\
-	element_temp = elementlist;					\
-	curptr = outbuf;						\
-	curptr[0] = '\0';						\
-	left = outbuflen;						\
 	first = 1;							\
+	sbuf_new(&sb, outbuf, outbuflen, SBUF_FIXEDLEN);		\
+	element_temp = elementlist;					\
 	while ((element_name = strsep(&element_temp, ",")) != NULL) {	\
-		curptr_start = curptr;					\
-		left_start = left;					\
-		first_start = first;					\
 		if (element_name[0] == '?') {				\
 			element_name++;					\
 			ignorenotfound = 1;				\
-		} else							\
+		 } else							\
 			ignorenotfound = 0;				\
-		claimed = 0;						\
+		savedlen = sbuf_len(&sb);				\
 		if (first) {						\
-			len = snprintf(curptr, left, "%s/",		\
-			    element_name);				\
+			error = sbuf_printf(&sb, "%s/", element_name);	\
 			first = 0;					\
 		} else							\
-			len = snprintf(curptr, left, ",%s/",		\
-			    element_name);				\
-		if (len >= left) {					\
-			error = EINVAL;		/* XXXMAC: E2BIG */	\
+			error = sbuf_printf(&sb, ",%s/", element_name);	\
+		if (error == -1) {					\
+			error = EINVAL;	/* XXX: E2BIG? */		\
 			break;						\
 		}							\
-		curptr += len;						\
-		left -= len;						\
-									\
+		claimed = 0;						\
 		MAC_CHECK(externalize_ ## type, label, element_name,	\
-		    curptr, left, &len, &claimed);			\
+		    &sb, &claimed);					\
 		if (error)						\
 			break;						\
-		if (claimed == 1) {					\
-			if (len >= outbuflen) {				\
-				error = EINVAL;	/* XXXMAC: E2BIG */	\
-				break;					\
-			}						\
-			curptr += len;					\
-			left -= len;					\
-		} else if (claimed == 0 && ignorenotfound) {		\
-			/*						\
-			 * Revert addition of the label element		\
-			 * name.					\
-			 */						\
-			curptr = curptr_start;				\
-			*curptr = '\0';					\
-			left = left_start;				\
-			first = first_start;				\
-		} else {						\
-			error = EINVAL;		/* XXXMAC: ENOLABEL */	\
+		if (claimed == 0 && ignorenotfound) {			\
+			/* Revert last label name. */			\
+			sbuf_setpos(&sb, savedlen);			\
+		} else if (claimed != 1) {				\
+			error = EINVAL;	/* XXX: ENOLABEL? */		\
 			break;						\
 		}							\
 	}								\
+	sbuf_finish(&sb);						\
 } while (0)
 
 #define	MAC_INTERNALIZE(type, label, instring) do {			\

sys/security/mac/mac_internal.h

@@ -56,6 +56,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/mac.h>
 #include <sys/module.h>
 #include <sys/proc.h>
+#include <sys/sbuf.h>
 #include <sys/systm.h>
 #include <sys/sysproto.h>
 #include <sys/sysent.h>
@@ -398,65 +399,44 @@ mac_policy_list_unbusy(void)
 
 #define	MAC_EXTERNALIZE(type, label, elementlist, outbuf, 		\
     outbuflen) do {							\
-	char *curptr, *curptr_start, *element_name, *element_temp;	\
-	size_t left, left_start, len;					\
-	int claimed, first, first_start, ignorenotfound;		\
+	int claimed, first, ignorenotfound, savedlen;			\
+	char *element_name, *element_temp;				\
+	struct sbuf sb;							\
 									\
 	error = 0;							\
-	element_temp = elementlist;					\
-	curptr = outbuf;						\
-	curptr[0] = '\0';						\
-	left = outbuflen;						\
 	first = 1;							\
+	sbuf_new(&sb, outbuf, outbuflen, SBUF_FIXEDLEN);		\
+	element_temp = elementlist;					\
 	while ((element_name = strsep(&element_temp, ",")) != NULL) {	\
-		curptr_start = curptr;					\
-		left_start = left;					\
-		first_start = first;					\
 		if (element_name[0] == '?') {				\
 			element_name++;					\
 			ignorenotfound = 1;				\
-		} else							\
+		 } else							\
 			ignorenotfound = 0;				\
-		claimed = 0;						\
+		savedlen = sbuf_len(&sb);				\
 		if (first) {						\
-			len = snprintf(curptr, left, "%s/",		\
-			    element_name);				\
+			error = sbuf_printf(&sb, "%s/", element_name);	\
 			first = 0;					\
 		} else							\
-			len = snprintf(curptr, left, ",%s/",		\
-			    element_name);				\
-		if (len >= left) {					\
-			error = EINVAL;		/* XXXMAC: E2BIG */	\
+			error = sbuf_printf(&sb, ",%s/", element_name);	\
+		if (error == -1) {					\
+			error = EINVAL;	/* XXX: E2BIG? */		\
 			break;						\
 		}							\
-		curptr += len;						\
-		left -= len;						\
-									\
+		claimed = 0;						\
 		MAC_CHECK(externalize_ ## type, label, element_name,	\
-		    curptr, left, &len, &claimed);			\
+		    &sb, &claimed);					\
 		if (error)						\
 			break;						\
-		if (claimed == 1) {					\
-			if (len >= outbuflen) {				\
-				error = EINVAL;	/* XXXMAC: E2BIG */	\
-				break;					\
-			}						\
-			curptr += len;					\
-			left -= len;					\
-		} else if (claimed == 0 && ignorenotfound) {		\
-			/*						\
-			 * Revert addition of the label element		\
-			 * name.					\
-			 */						\
-			curptr = curptr_start;				\
-			*curptr = '\0';					\
-			left = left_start;				\
-			first = first_start;				\
-		} else {						\
-			error = EINVAL;		/* XXXMAC: ENOLABEL */	\
+		if (claimed == 0 && ignorenotfound) {			\
+			/* Revert last label name. */			\
+			sbuf_setpos(&sb, savedlen);			\
+		} else if (claimed != 1) {				\
+			error = EINVAL;	/* XXX: ENOLABEL? */		\
 			break;						\
 		}							\
 	}								\
+	sbuf_finish(&sb);						\
 } while (0)
 
 #define	MAC_INTERNALIZE(type, label, instring) do {			\

sys/security/mac/mac_net.c

@@ -56,6 +56,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/mac.h>
 #include <sys/module.h>
 #include <sys/proc.h>
+#include <sys/sbuf.h>
 #include <sys/systm.h>
 #include <sys/sysproto.h>
 #include <sys/sysent.h>
@@ -398,65 +399,44 @@ mac_policy_list_unbusy(void)
 
 #define	MAC_EXTERNALIZE(type, label, elementlist, outbuf, 		\
     outbuflen) do {							\
-	char *curptr, *curptr_start, *element_name, *element_temp;	\
-	size_t left, left_start, len;					\
-	int claimed, first, first_start, ignorenotfound;		\
+	int claimed, first, ignorenotfound, savedlen;			\
+	char *element_name, *element_temp;				\
+	struct sbuf sb;							\
 									\
 	error = 0;							\
-	element_temp = elementlist;					\
-	curptr = outbuf;						\
-	curptr[0] = '\0';						\
-	left = outbuflen;						\
 	first = 1;							\
+	sbuf_new(&sb, outbuf, outbuflen, SBUF_FIXEDLEN);		\
+	element_temp = elementlist;					\
 	while ((element_name = strsep(&element_temp, ",")) != NULL) {	\
-		curptr_start = curptr;					\
-		left_start = left;					\
-		first_start = first;					\
 		if (element_name[0] == '?') {				\
 			element_name++;					\
 			ignorenotfound = 1;				\
-		} else							\
+		 } else							\
 			ignorenotfound = 0;				\
-		claimed = 0;						\
+		savedlen = sbuf_len(&sb);				\
 		if (first) {						\
-			len = snprintf(curptr, left, "%s/",		\
-			    element_name);				\
+			error = sbuf_printf(&sb, "%s/", element_name);	\
 			first = 0;					\
 		} else							\
-			len = snprintf(curptr, left, ",%s/",		\
-			    element_name);				\
-		if (len >= left) {					\
-			error = EINVAL;		/* XXXMAC: E2BIG */	\
+			error = sbuf_printf(&sb, ",%s/", element_name);	\
+		if (error == -1) {					\
+			error = EINVAL;	/* XXX: E2BIG? */		\
 			break;						\
 		}							\
-		curptr += len;						\
-		left -= len;						\
-									\
+		claimed = 0;						\
 		MAC_CHECK(externalize_ ## type, label, element_name,	\
-		    curptr, left, &len, &claimed);			\
+		    &sb, &claimed);					\
 		if (error)						\
 			break;						\
-		if (claimed == 1) {					\
-			if (len >= outbuflen) {				\
-				error = EINVAL;	/* XXXMAC: E2BIG */	\
-				break;					\
-			}						\
-			curptr += len;					\
-			left -= len;					\
-		} else if (claimed == 0 && ignorenotfound) {		\
-			/*						\
-			 * Revert addition of the label element		\
-			 * name.					\
-			 */						\
-			curptr = curptr_start;				\
-			*curptr = '\0';					\
-			left = left_start;				\
-			first = first_start;				\
-		} else {						\
-			error = EINVAL;		/* XXXMAC: ENOLABEL */	\
+		if (claimed == 0 && ignorenotfound) {			\
+			/* Revert last label name. */			\
+			sbuf_setpos(&sb, savedlen);			\
+		} else if (claimed != 1) {				\
+			error = EINVAL;	/* XXX: ENOLABEL? */		\
 			break;						\
 		}							\
 	}								\
+	sbuf_finish(&sb);						\
 } while (0)
 
 #define	MAC_INTERNALIZE(type, label, instring) do {			\

sys/security/mac/mac_pipe.c

@@ -56,6 +56,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/mac.h>
 #include <sys/module.h>
 #include <sys/proc.h>
+#include <sys/sbuf.h>
 #include <sys/systm.h>
 #include <sys/sysproto.h>
 #include <sys/sysent.h>
@@ -398,65 +399,44 @@ mac_policy_list_unbusy(void)
 
 #define	MAC_EXTERNALIZE(type, label, elementlist, outbuf, 		\
     outbuflen) do {							\
-	char *curptr, *curptr_start, *element_name, *element_temp;	\
-	size_t left, left_start, len;					\
-	int claimed, first, first_start, ignorenotfound;		\
+	int claimed, first, ignorenotfound, savedlen;			\
+	char *element_name, *element_temp;				\
+	struct sbuf sb;							\
 									\
 	error = 0;							\
-	element_temp = elementlist;					\
-	curptr = outbuf;						\
-	curptr[0] = '\0';						\
-	left = outbuflen;						\
 	first = 1;							\
+	sbuf_new(&sb, outbuf, outbuflen, SBUF_FIXEDLEN);		\
+	element_temp = elementlist;					\
 	while ((element_name = strsep(&element_temp, ",")) != NULL) {	\
-		curptr_start = curptr;					\
-		left_start = left;					\
-		first_start = first;					\
 		if (element_name[0] == '?') {				\
 			element_name++;					\
 			ignorenotfound = 1;				\
-		} else							\
+		 } else							\
 			ignorenotfound = 0;				\
-		claimed = 0;						\
+		savedlen = sbuf_len(&sb);				\
 		if (first) {						\
-			len = snprintf(curptr, left, "%s/",		\
-			    element_name);				\
+			error = sbuf_printf(&sb, "%s/", element_name);	\
 			first = 0;					\
 		} else							\
-			len = snprintf(curptr, left, ",%s/",		\
-			    element_name);				\
-		if (len >= left) {					\
-			error = EINVAL;		/* XXXMAC: E2BIG */	\
+			error = sbuf_printf(&sb, ",%s/", element_name);	\
+		if (error == -1) {					\
+			error = EINVAL;	/* XXX: E2BIG? */		\
 			break;						\
 		}							\
-		curptr += len;						\
-		left -= len;						\
-									\
+		claimed = 0;						\
 		MAC_CHECK(externalize_ ## type, label, element_name,	\
-		    curptr, left, &len, &claimed);			\
+		    &sb, &claimed);					\
 		if (error)						\
 			break;						\
-		if (claimed == 1) {					\
-			if (len >= outbuflen) {				\
-				error = EINVAL;	/* XXXMAC: E2BIG */	\
-				break;					\
-			}						\
-			curptr += len;					\
-			left -= len;					\
-		} else if (claimed == 0 && ignorenotfound) {		\
-			/*						\
-			 * Revert addition of the label element		\
-			 * name.					\
-			 */						\
-			curptr = curptr_start;				\
-			*curptr = '\0';					\
-			left = left_start;				\
-			first = first_start;				\
-		} else {						\
-			error = EINVAL;		/* XXXMAC: ENOLABEL */	\
+		if (claimed == 0 && ignorenotfound) {			\
+			/* Revert last label name. */			\
+			sbuf_setpos(&sb, savedlen);			\
+		} else if (claimed != 1) {				\
+			error = EINVAL;	/* XXX: ENOLABEL? */		\
 			break;						\
 		}							\
 	}								\
+	sbuf_finish(&sb);						\
 } while (0)
 
 #define	MAC_INTERNALIZE(type, label, instring) do {			\

sys/security/mac/mac_policy.h

@@ -60,6 +60,7 @@ struct mac_policy_conf;
 struct mbuf;
 struct mount;
 struct pipe;
+struct sbuf;
 struct socket;
 struct ucred;
 struct uio;
@@ -114,23 +115,17 @@ struct mac_policy_ops {
 	void	(*mpo_copy_vnode_label)(struct label *src,
 		    struct label *dest);
 	int	(*mpo_externalize_cred_label)(struct label *label,
-		    char *element_name, char *buffer, size_t buflen,
-		    size_t *len, int *claimed);
+		    char *element_name, struct sbuf *sb, int *claimed);
 	int	(*mpo_externalize_ifnet_label)(struct label *label,
-		    char *element_name, char *buffer, size_t buflen,
-		    size_t *len, int *claimed);
+		    char *element_name, struct sbuf *sb, int *claimed);
 	int	(*mpo_externalize_pipe_label)(struct label *label,
-		    char *element_name, char *buffer, size_t buflen,
-		    size_t *len, int *claimed);
+		    char *element_name, struct sbuf *sb, int *claimed);
 	int	(*mpo_externalize_socket_label)(struct label *label,
-		    char *element_name, char *buffer, size_t buflen,
-		    size_t *len, int *claimed);
+		    char *element_name, struct sbuf *sb, int *claimed);
 	int	(*mpo_externalize_socket_peer_label)(struct label *label,
-		    char *element_name, char *buffer, size_t buflen,
-		    size_t *len, int *claimed);
+		    char *element_name, struct sbuf *sb, int *claimed);
 	int	(*mpo_externalize_vnode_label)(struct label *label,
-		    char *element_name, char *buffer, size_t buflen,
-		    size_t *len, int *claimed);
+		    char *element_name, struct sbuf *sb, int *claimed);
 	int	(*mpo_internalize_cred_label)(struct label *label,
 		    char *element_name, char *element_data, int *claimed);
 	int	(*mpo_internalize_ifnet_label)(struct label *label,

sys/security/mac/mac_process.c

@@ -56,6 +56,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/mac.h>
 #include <sys/module.h>
 #include <sys/proc.h>
+#include <sys/sbuf.h>
 #include <sys/systm.h>
 #include <sys/sysproto.h>
 #include <sys/sysent.h>
@@ -398,65 +399,44 @@ mac_policy_list_unbusy(void)
 
 #define	MAC_EXTERNALIZE(type, label, elementlist, outbuf, 		\
     outbuflen) do {							\
-	char *curptr, *curptr_start, *element_name, *element_temp;	\
-	size_t left, left_start, len;					\
-	int claimed, first, first_start, ignorenotfound;		\
+	int claimed, first, ignorenotfound, savedlen;			\
+	char *element_name, *element_temp;				\
+	struct sbuf sb;							\
 									\
 	error = 0;							\
-	element_temp = elementlist;					\
-	curptr = outbuf;						\
-	curptr[0] = '\0';						\
-	left = outbuflen;						\
 	first = 1;							\
+	sbuf_new(&sb, outbuf, outbuflen, SBUF_FIXEDLEN);		\
+	element_temp = elementlist;					\
 	while ((element_name = strsep(&element_temp, ",")) != NULL) {	\
-		curptr_start = curptr;					\
-		left_start = left;					\
-		first_start = first;					\
 		if (element_name[0] == '?') {				\
 			element_name++;					\
 			ignorenotfound = 1;				\
-		} else							\
+		 } else							\
 			ignorenotfound = 0;				\
-		claimed = 0;						\
+		savedlen = sbuf_len(&sb);				\
 		if (first) {						\
-			len = snprintf(curptr, left, "%s/",		\
-			    element_name);				\
+			error = sbuf_printf(&sb, "%s/", element_name);	\
 			first = 0;					\
 		} else							\
-			len = snprintf(curptr, left, ",%s/",		\
-			    element_name);				\
-		if (len >= left) {					\
-			error = EINVAL;		/* XXXMAC: E2BIG */	\
+			error = sbuf_printf(&sb, ",%s/", element_name);	\
+		if (error == -1) {					\
+			error = EINVAL;	/* XXX: E2BIG? */		\
 			break;						\
 		}							\
-		curptr += len;						\
-		left -= len;						\
-									\
+		claimed = 0;						\
 		MAC_CHECK(externalize_ ## type, label, element_name,	\
-		    curptr, left, &len, &claimed);			\
+		    &sb, &claimed);					\
 		if (error)						\
 			break;						\
-		if (claimed == 1) {					\
-			if (len >= outbuflen) {				\
-				error = EINVAL;	/* XXXMAC: E2BIG */	\
-				break;					\
-			}						\
-			curptr += len;					\
-			left -= len;					\
-		} else if (claimed == 0 && ignorenotfound) {		\
-			/*						\
-			 * Revert addition of the label element		\
-			 * name.					\
-			 */						\
-			curptr = curptr_start;				\
-			*curptr = '\0';					\
-			left = left_start;				\
-			first = first_start;				\
-		} else {						\
-			error = EINVAL;		/* XXXMAC: ENOLABEL */	\
+		if (claimed == 0 && ignorenotfound) {			\
+			/* Revert last label name. */			\
+			sbuf_setpos(&sb, savedlen);			\
+		} else if (claimed != 1) {				\
+			error = EINVAL;	/* XXX: ENOLABEL? */		\
 			break;						\
 		}							\
 	}								\
+	sbuf_finish(&sb);						\
 } while (0)
 
 #define	MAC_INTERNALIZE(type, label, instring) do {			\

sys/security/mac/mac_syscalls.c

@@ -56,6 +56,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/mac.h>
 #include <sys/module.h>
 #include <sys/proc.h>
+#include <sys/sbuf.h>
 #include <sys/systm.h>
 #include <sys/sysproto.h>
 #include <sys/sysent.h>
@@ -398,65 +399,44 @@ mac_policy_list_unbusy(void)
 
 #define	MAC_EXTERNALIZE(type, label, elementlist, outbuf, 		\
     outbuflen) do {							\
-	char *curptr, *curptr_start, *element_name, *element_temp;	\
-	size_t left, left_start, len;					\
-	int claimed, first, first_start, ignorenotfound;		\
+	int claimed, first, ignorenotfound, savedlen;			\
+	char *element_name, *element_temp;				\
+	struct sbuf sb;							\
 									\
 	error = 0;							\
-	element_temp = elementlist;					\
-	curptr = outbuf;						\
-	curptr[0] = '\0';						\
-	left = outbuflen;						\
 	first = 1;							\
+	sbuf_new(&sb, outbuf, outbuflen, SBUF_FIXEDLEN);		\
+	element_temp = elementlist;					\
 	while ((element_name = strsep(&element_temp, ",")) != NULL) {	\
-		curptr_start = curptr;					\
-		left_start = left;					\
-		first_start = first;					\
 		if (element_name[0] == '?') {				\
 			element_name++;					\
 			ignorenotfound = 1;				\
-		} else							\
+		 } else							\
 			ignorenotfound = 0;				\
-		claimed = 0;						\
+		savedlen = sbuf_len(&sb);				\
 		if (first) {						\
-			len = snprintf(curptr, left, "%s/",		\
-			    element_name);				\
+			error = sbuf_printf(&sb, "%s/", element_name);	\
 			first = 0;					\
 		} else							\
-			len = snprintf(curptr, left, ",%s/",		\
-			    element_name);				\
-		if (len >= left) {					\
-			error = EINVAL;		/* XXXMAC: E2BIG */	\
+			error = sbuf_printf(&sb, ",%s/", element_name);	\
+		if (error == -1) {					\
+			error = EINVAL;	/* XXX: E2BIG? */		\
 			break;						\
 		}							\
-		curptr += len;						\
-		left -= len;						\
-									\
+		claimed = 0;						\
 		MAC_CHECK(externalize_ ## type, label, element_name,	\
-		    curptr, left, &len, &claimed);			\
+		    &sb, &claimed);					\
 		if (error)						\
 			break;						\
-		if (claimed == 1) {					\
-			if (len >= outbuflen) {				\
-				error = EINVAL;	/* XXXMAC: E2BIG */	\
-				break;					\
-			}						\
-			curptr += len;					\
-			left -= len;					\
-		} else if (claimed == 0 && ignorenotfound) {		\
-			/*						\
-			 * Revert addition of the label element		\
-			 * name.					\
-			 */						\
-			curptr = curptr_start;				\
-			*curptr = '\0';					\
-			left = left_start;				\
-			first = first_start;				\
-		} else {						\
-			error = EINVAL;		/* XXXMAC: ENOLABEL */	\
+		if (claimed == 0 && ignorenotfound) {			\
+			/* Revert last label name. */			\
+			sbuf_setpos(&sb, savedlen);			\
+		} else if (claimed != 1) {				\
+			error = EINVAL;	/* XXX: ENOLABEL? */		\
 			break;						\
 		}							\
 	}								\
+	sbuf_finish(&sb);						\
 } while (0)
 
 #define	MAC_INTERNALIZE(type, label, instring) do {			\

sys/security/mac/mac_system.c

@@ -56,6 +56,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/mac.h>
 #include <sys/module.h>
 #include <sys/proc.h>
+#include <sys/sbuf.h>
 #include <sys/systm.h>
 #include <sys/sysproto.h>
 #include <sys/sysent.h>
@@ -398,65 +399,44 @@ mac_policy_list_unbusy(void)
 
 #define	MAC_EXTERNALIZE(type, label, elementlist, outbuf, 		\
     outbuflen) do {							\
-	char *curptr, *curptr_start, *element_name, *element_temp;	\
-	size_t left, left_start, len;					\
-	int claimed, first, first_start, ignorenotfound;		\
+	int claimed, first, ignorenotfound, savedlen;			\
+	char *element_name, *element_temp;				\
+	struct sbuf sb;							\
 									\
 	error = 0;							\
-	element_temp = elementlist;					\
-	curptr = outbuf;						\
-	curptr[0] = '\0';						\
-	left = outbuflen;						\
 	first = 1;							\
+	sbuf_new(&sb, outbuf, outbuflen, SBUF_FIXEDLEN);		\
+	element_temp = elementlist;					\
 	while ((element_name = strsep(&element_temp, ",")) != NULL) {	\
-		curptr_start = curptr;					\
-		left_start = left;					\
-		first_start = first;					\
 		if (element_name[0] == '?') {				\
 			element_name++;					\
 			ignorenotfound = 1;				\
-		} else							\
+		 } else							\
 			ignorenotfound = 0;				\
-		claimed = 0;						\
+		savedlen = sbuf_len(&sb);				\
 		if (first) {						\
-			len = snprintf(curptr, left, "%s/",		\
-			    element_name);				\
+			error = sbuf_printf(&sb, "%s/", element_name);	\
 			first = 0;					\
 		} else							\
-			len = snprintf(curptr, left, ",%s/",		\
-			    element_name);				\
-		if (len >= left) {					\
-			error = EINVAL;		/* XXXMAC: E2BIG */	\
+			error = sbuf_printf(&sb, ",%s/", element_name);	\
+		if (error == -1) {					\
+			error = EINVAL;	/* XXX: E2BIG? */		\
 			break;						\
 		}							\
-		curptr += len;						\
-		left -= len;						\
-									\
+		claimed = 0;						\
 		MAC_CHECK(externalize_ ## type, label, element_name,	\
-		    curptr, left, &len, &claimed);			\
+		    &sb, &claimed);					\
 		if (error)						\
 			break;						\
-		if (claimed == 1) {					\
-			if (len >= outbuflen) {				\
-				error = EINVAL;	/* XXXMAC: E2BIG */	\
-				break;					\
-			}						\
-			curptr += len;					\
-			left -= len;					\
-		} else if (claimed == 0 && ignorenotfound) {		\
-			/*						\
-			 * Revert addition of the label element		\
-			 * name.					\
-			 */						\
-			curptr = curptr_start;				\
-			*curptr = '\0';					\
-			left = left_start;				\
-			first = first_start;				\
-		} else {						\
-			error = EINVAL;		/* XXXMAC: ENOLABEL */	\
+		if (claimed == 0 && ignorenotfound) {			\
+			/* Revert last label name. */			\
+			sbuf_setpos(&sb, savedlen);			\
+		} else if (claimed != 1) {				\
+			error = EINVAL;	/* XXX: ENOLABEL? */		\
 			break;						\
 		}							\
 	}								\
+	sbuf_finish(&sb);						\
 } while (0)
 
 #define	MAC_INTERNALIZE(type, label, instring) do {			\

sys/security/mac/mac_vfs.c

@@ -56,6 +56,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/mac.h>
 #include <sys/module.h>
 #include <sys/proc.h>
+#include <sys/sbuf.h>
 #include <sys/systm.h>
 #include <sys/sysproto.h>
 #include <sys/sysent.h>
@@ -398,65 +399,44 @@ mac_policy_list_unbusy(void)
 
 #define	MAC_EXTERNALIZE(type, label, elementlist, outbuf, 		\
     outbuflen) do {							\
-	char *curptr, *curptr_start, *element_name, *element_temp;	\
-	size_t left, left_start, len;					\
-	int claimed, first, first_start, ignorenotfound;		\
+	int claimed, first, ignorenotfound, savedlen;			\
+	char *element_name, *element_temp;				\
+	struct sbuf sb;							\
 									\
 	error = 0;							\
-	element_temp = elementlist;					\
-	curptr = outbuf;						\
-	curptr[0] = '\0';						\
-	left = outbuflen;						\
 	first = 1;							\
+	sbuf_new(&sb, outbuf, outbuflen, SBUF_FIXEDLEN);		\
+	element_temp = elementlist;					\
 	while ((element_name = strsep(&element_temp, ",")) != NULL) {	\
-		curptr_start = curptr;					\
-		left_start = left;					\
-		first_start = first;					\
 		if (element_name[0] == '?') {				\
 			element_name++;					\
 			ignorenotfound = 1;				\
-		} else							\
+		 } else							\
 			ignorenotfound = 0;				\
-		claimed = 0;						\
+		savedlen = sbuf_len(&sb);				\
 		if (first) {						\
-			len = snprintf(curptr, left, "%s/",		\
-			    element_name);				\
+			error = sbuf_printf(&sb, "%s/", element_name);	\
 			first = 0;					\
 		} else							\
-			len = snprintf(curptr, left, ",%s/",		\
-			    element_name);				\
-		if (len >= left) {					\
-			error = EINVAL;		/* XXXMAC: E2BIG */	\
+			error = sbuf_printf(&sb, ",%s/", element_name);	\
+		if (error == -1) {					\
+			error = EINVAL;	/* XXX: E2BIG? */		\
 			break;						\
 		}							\
-		curptr += len;						\
-		left -= len;						\
-									\
+		claimed = 0;						\
 		MAC_CHECK(externalize_ ## type, label, element_name,	\
-		    curptr, left, &len, &claimed);			\
+		    &sb, &claimed);					\
 		if (error)						\
 			break;						\
-		if (claimed == 1) {					\
-			if (len >= outbuflen) {				\
-				error = EINVAL;	/* XXXMAC: E2BIG */	\
-				break;					\
-			}						\
-			curptr += len;					\
-			left -= len;					\
-		} else if (claimed == 0 && ignorenotfound) {		\
-			/*						\
-			 * Revert addition of the label element		\
-			 * name.					\
-			 */						\
-			curptr = curptr_start;				\
-			*curptr = '\0';					\
-			left = left_start;				\
-			first = first_start;				\
-		} else {						\
-			error = EINVAL;		/* XXXMAC: ENOLABEL */	\
+		if (claimed == 0 && ignorenotfound) {			\
+			/* Revert last label name. */			\
+			sbuf_setpos(&sb, savedlen);			\
+		} else if (claimed != 1) {				\
+			error = EINVAL;	/* XXX: ENOLABEL? */		\
 			break;						\
 		}							\
 	}								\
+	sbuf_finish(&sb);						\
 } while (0)
 
 #define	MAC_INTERNALIZE(type, label, instring) do {			\

sys/security/mac_biba/mac_biba.c

@@ -576,57 +576,49 @@ mac_biba_element_to_string(struct sbuf *sb, struct mac_biba_element *element)
 }
 
 /*
- * mac_biba_to_string() converts an Biba label to a string, placing the
- * results in the passed string buffer.  It returns 0 on success,
- * or EINVAL if there isn't room in the buffer.  The size of the
- * string appended, leaving out the nul termination, is returned to
- * the caller via *caller_len.  Eventually, we should expose the
- * sbuf to the caller rather than using C strings at this layer.
+ * mac_biba_to_string() converts a Biba label to a string, and places
+ * the results in the passed sbuf.  It returns 0 on success, or EINVAL
+ * if there isn't room in the sbuf.  Note: the sbuf will be modified
+ * even in a failure case, so the caller may need to revert the sbuf
+ * by restoring the offset if that's undesired.
  */
 static int
-mac_biba_to_string(char *string, size_t size, size_t *caller_len,
-    struct mac_biba *mac_biba)
+mac_biba_to_string(struct sbuf *sb, struct mac_biba *mac_biba)
 {
-	struct sbuf sb;
-
-	sbuf_new(&sb, string, size, SBUF_FIXEDLEN);
 
 	if (mac_biba->mb_flags & MAC_BIBA_FLAG_SINGLE) {
-		if (mac_biba_element_to_string(&sb, &mac_biba->mb_single)
+		if (mac_biba_element_to_string(sb, &mac_biba->mb_single)
 		    == -1)
 			return (EINVAL);
 	}
 
 	if (mac_biba->mb_flags & MAC_BIBA_FLAG_RANGE) {
-		if (sbuf_putc(&sb, '(') == -1)
+		if (sbuf_putc(sb, '(') == -1)
 			return (EINVAL);
 
-		if (mac_biba_element_to_string(&sb, &mac_biba->mb_rangelow)
+		if (mac_biba_element_to_string(sb, &mac_biba->mb_rangelow)
 		    == -1)
 			return (EINVAL);
 
-		if (sbuf_putc(&sb, '-') == -1)
+		if (sbuf_putc(sb, '-') == -1)
 			return (EINVAL);
 
-		if (mac_biba_element_to_string(&sb, &mac_biba->mb_rangehigh)
+		if (mac_biba_element_to_string(sb, &mac_biba->mb_rangehigh)
 		    == -1)
 			return (EINVAL);
 
-		if (sbuf_putc(&sb, ')') == -1)
+		if (sbuf_putc(sb, ')') == -1)
 			return (EINVAL);
 	}
 
-	sbuf_finish(&sb);
-	*caller_len = strlen(string);
 	return (0);
 }
 
 static int
 mac_biba_externalize_label(struct label *label, char *element_name,
-    char *element_data, size_t size, size_t *len, int *claimed)
+    struct sbuf *sb, int *claimed)
 {
 	struct mac_biba *mac_biba;
-	int error;
 
 	if (strcmp(MAC_BIBA_LABEL_NAME, element_name) != 0)
 		return (0);
@@ -634,11 +626,7 @@ mac_biba_externalize_label(struct label *label, char *element_name,
 	(*claimed)++;
 
 	mac_biba = SLOT(label);
-	error = mac_biba_to_string(element_data, size, len, mac_biba);
-	if (error)
-		return (error);
-
-	return (0);
+	return (mac_biba_to_string(sb, mac_biba));
 }
 
 static int

sys/security/mac_lomac/mac_lomac.c

@@ -49,6 +49,7 @@
 #include <sys/malloc.h>
 #include <sys/mount.h>
 #include <sys/proc.h>
+#include <sys/sbuf.h>
 #include <sys/systm.h>
 #include <sys/sysproto.h>
 #include <sys/sysent.h>
@@ -483,21 +484,23 @@ mac_lomac_copy(struct mac_lomac *source, struct mac_lomac *dest)
 		mac_lomac_copy_range(source, dest);
 }
 
-static int	mac_lomac_to_string(char *string, size_t size,
-	    size_t *caller_len, struct mac_lomac *mac_lomac);
+static int	mac_lomac_to_string(struct sbuf *sb,
+		    struct mac_lomac *mac_lomac);
 
 static int
 maybe_demote(struct mac_lomac *subjlabel, struct mac_lomac *objlabel,
     const char *actionname, const char *objname, struct vnode *vpq)
 {
+	struct sbuf subjlabel_sb, subjtext_sb, objlabel_sb;
+	char *subjlabeltext, *objlabeltext, *subjtext;
+	struct mac_lomac cached_subjlabel;
+	struct mac_lomac_proc *subj;
 	struct vattr va;
-	static char xxx[] = "<<XXX>>";
-	struct mac_lomac_proc *subj = PSLOT(&curthread->td_proc->p_label);
-	char *subjlabeltext, *objlabeltext, *subjtext, *text;
 	struct proc *p;
-	size_t len;
 	pid_t pgid;
 
+	subj = PSLOT(&curthread->td_proc->p_label);
+
 	p = curthread->td_proc;
 	mtx_lock(&subj->mtx);
         if (subj->mac_lomac.ml_flags & MAC_LOMAC_FLAG_UPDATE) {
@@ -531,32 +534,29 @@ maybe_demote(struct mac_lomac *subjlabel, struct mac_lomac *objlabel,
 	curthread->td_flags |= TDF_ASTPENDING;
 	curthread->td_proc->p_sflag |= PS_MACPEND;
 	mtx_unlock_spin(&sched_lock);
-	subjtext = subjlabeltext = objlabeltext = xxx;
-	if (mac_lomac_to_string(NULL, 0, &len, &subj->mac_lomac) == 0 &&
-	    (text = malloc(len + 1, M_MACLOMAC, M_NOWAIT)) != NULL) {
-		if (mac_lomac_to_string(text, len + 1, &len,
-		    &subj->mac_lomac) == 0)
-			subjtext = text;
-		else
-			free(text, M_MACLOMAC);
-	}
+
+	/*
+	 * Avoid memory allocation while holding a mutex; cache the
+	 * label.
+	 */
+	mac_lomac_copy_single(&subj->mac_lomac, &cached_subjlabel);
 	mtx_unlock(&subj->mtx);
-	if (mac_lomac_to_string(NULL, 0, &len, subjlabel) == 0 &&
-	    (text = malloc(len + 1, M_MACLOMAC, M_NOWAIT)) != NULL) {
-		if (mac_lomac_to_string(text, len + 1, &len,
-		    subjlabel) == 0)
-			subjlabeltext = text;
-		else
-			free(text, M_MACLOMAC);
-	}
-	if (mac_lomac_to_string(NULL, 0, &len, objlabel) == 0 &&
-	    (text = malloc(len + 1, M_MACLOMAC, M_NOWAIT)) != NULL) {
-		if (mac_lomac_to_string(text, len + 1, &len,
-		    objlabel) == 0)
-			objlabeltext = text;
-		else
-			free(text, M_MACLOMAC);
-	}
+
+	sbuf_new(&subjlabel_sb, NULL, 0, SBUF_AUTOEXTEND);
+	mac_lomac_to_string(&subjlabel_sb, subjlabel);
+	sbuf_finish(&subjlabel_sb);
+	subjlabeltext = sbuf_data(&subjlabel_sb);
+
+	sbuf_new(&subjtext_sb, NULL, 0, SBUF_AUTOEXTEND);
+	mac_lomac_to_string(&subjtext_sb, &subj->mac_lomac);
+	sbuf_finish(&subjtext_sb);
+	subjtext = sbuf_data(&subjtext_sb);
+
+	sbuf_new(&objlabel_sb, NULL, 0, SBUF_AUTOEXTEND);
+	mac_lomac_to_string(&objlabel_sb, objlabel);
+	sbuf_finish(&objlabel_sb);
+	objlabeltext = sbuf_data(&objlabel_sb);
+
 	pgid = p->p_pgrp->pg_id;		/* XXX could be stale? */
 	if (vpq != NULL && VOP_GETATTR(vpq, &va, curthread->td_ucred,
 	    curthread) == 0) {
@@ -572,13 +572,11 @@ maybe_demote(struct mac_lomac *subjlabel, struct mac_lomac *objlabel,
 		    subjlabeltext, p->p_pid, pgid, curthread->td_ucred->cr_uid,
 		    p->p_comm, subjtext, actionname, objlabeltext, objname);
 	}
+
+	sbuf_delete(&subjlabel_sb);
+	sbuf_delete(&subjtext_sb);
+	sbuf_delete(&objlabel_sb);
 		
-	if (subjlabeltext != xxx)
-		free(subjlabeltext, M_MACLOMAC);
-	if (objlabeltext != xxx)
-		free(objlabeltext, M_MACLOMAC);
-	if (subjtext != xxx)
-		free(subjtext, M_MACLOMAC);
 	return (0);
 }
 
@@ -659,28 +657,22 @@ mac_lomac_destroy_proc_label(struct label *label)
 	PSLOT(label) = NULL;
 }
 
-/*
- * mac_lomac_element_to_string() is basically an snprintf wrapper with
- * the same properties as snprintf().  It returns the length it would
- * have added to the string in the event the string is too short.
- */
-static size_t
-mac_lomac_element_to_string(char *string, size_t size,
-    struct mac_lomac_element *element)
+static int
+mac_lomac_element_to_string(struct sbuf *sb, struct mac_lomac_element *element)
 {
 
 	switch (element->mle_type) {
 	case MAC_LOMAC_TYPE_HIGH:
-		return (snprintf(string, size, "high"));
+		return (sbuf_printf(sb, "high"));
 
 	case MAC_LOMAC_TYPE_LOW:
-		return (snprintf(string, size, "low"));
+		return (sbuf_printf(sb, "low"));
 
 	case MAC_LOMAC_TYPE_EQUAL:
-		return (snprintf(string, size, "equal"));
+		return (sbuf_printf(sb, "equal"));
 
 	case MAC_LOMAC_TYPE_GRADE:
-		return (snprintf(string, size, "%d", element->mle_grade));
+		return (sbuf_printf(sb, "%d", element->mle_grade));
 
 	default:
 		panic("mac_lomac_element_to_string: invalid type (%d)",
@@ -689,81 +681,54 @@ mac_lomac_element_to_string(char *string, size_t size,
 }
 
 static int
-mac_lomac_to_string(char *string, size_t size, size_t *caller_len,
-    struct mac_lomac *mac_lomac)
+mac_lomac_to_string(struct sbuf *sb, struct mac_lomac *mac_lomac)
 {
-	size_t left, len, curlen;
-	char *curptr;
 
-	/*
-	 * Also accept NULL string to allow for predetermination of total
-	 * string length.
-	 */
-	if (string != NULL)
-		bzero(string, size);
-	else if (size != 0)
-		return (EINVAL);
-	curptr = string;
-	left = size;
-	curlen = 0;
-
-#define	INCLEN(length, leftover) do {					\
-	if (string != NULL) {						\
-		if (length >= leftover)					\
-			return (EINVAL);				\
-		leftover -= length;					\
-		curptr += length;					\
-	}								\
-	curlen += length;						\
-} while (0)
 	if (mac_lomac->ml_flags & MAC_LOMAC_FLAG_SINGLE) {
-		len = mac_lomac_element_to_string(curptr, left,
-		    &mac_lomac->ml_single);
-		INCLEN(len, left);
+		if (mac_lomac_element_to_string(sb, &mac_lomac->ml_single)
+		    == -1)
+			return (EINVAL);
 	}
 
 	if (mac_lomac->ml_flags & MAC_LOMAC_FLAG_AUX) {
-		len = snprintf(curptr, left, "[");
-		INCLEN(len, left);
+		if (sbuf_putc(sb, '[') == -1)
+			return (EINVAL);
 
-		len = mac_lomac_element_to_string(curptr, left,
-		    &mac_lomac->ml_auxsingle);
-		INCLEN(len, left);
+		if (mac_lomac_element_to_string(sb, &mac_lomac->ml_auxsingle)
+		    == -1)
+			return (EINVAL);
 
-		len = snprintf(curptr, left, "]");
-		INCLEN(len, left);
+		if (sbuf_putc(sb, ']') == -1)
+			return (EINVAL);
 	}
 
 	if (mac_lomac->ml_flags & MAC_LOMAC_FLAG_RANGE) {
-		len = snprintf(curptr, left, "(");
-		INCLEN(len, left);
+		if (sbuf_putc(sb, '(') == -1)
+			return (EINVAL);
 
-		len = mac_lomac_element_to_string(curptr, left,
-		    &mac_lomac->ml_rangelow);
-		INCLEN(len, left);
+		if (mac_lomac_element_to_string(sb, &mac_lomac->ml_rangelow)
+		    == -1)
+			return (EINVAL);
 
-		len = snprintf(curptr, left, "-");
-		INCLEN(len, left);
+		if (sbuf_putc(sb, '-') == -1)
+			return (EINVAL);
 
-		len = mac_lomac_element_to_string(curptr, left,
-		    &mac_lomac->ml_rangehigh);
-		INCLEN(len, left);
+		if (mac_lomac_element_to_string(sb, &mac_lomac->ml_rangehigh)
+		    == -1)
+			return (EINVAL);
 
-		len = snprintf(curptr, left, ")");
-		INCLEN(len, left);
+		if (sbuf_putc(sb, '-') == -1)
+			return (EINVAL);
 	}
-#undef INCLEN
 
-	*caller_len = curlen;
 	return (0);
 }
 
 static int
 mac_lomac_externalize_label(struct label *label, char *element_name,
-    char *element_data, size_t size, size_t *len, int *claimed)
+    struct sbuf *sb, int *claimed)
 {
 	struct mac_lomac *mac_lomac;
-	int error;
 
 	if (strcmp(MAC_LOMAC_LABEL_NAME, element_name) != 0)
 		return (0);
@@ -771,12 +736,8 @@ mac_lomac_externalize_label(struct label *label, char *element_name,
 	(*claimed)++;
 
 	mac_lomac = SLOT(label);
-	error = mac_lomac_to_string(element_data, size, len, mac_lomac);
-	if (error)
-		return (error);
 
-	*len = strlen(element_data);
-	return (0);
+	return (mac_lomac_to_string(sb, mac_lomac));
 }
 
 static int

sys/security/mac_mls/mac_mls.c

@@ -541,57 +541,49 @@ mac_mls_element_to_string(struct sbuf *sb, struct mac_mls_element *element)
 }
 
 /*
- * mac_mls_to_string() converts an MLS label to a string, placing the
- * results in the passed string buffer.  It returns 0 on success,
- * or EINVAL if there isn't room in the buffer.  The size of the
- * string appended, leaving out the nul termination, is returned to
- * the caller via *caller_len.  Eventually, we should expose the
- * sbuf to the caller rather than using C strings at this layer.
+ * mac_mls_to_string() converts an MLS label to a string, and places
+ * the results in the passed sbuf.  It returns 0 on success, or EINVAL
+ * if there isn't room in the sbuf.  Note: the sbuf will be modified
+ * even in a failure case, so the caller may need to revert the sbuf
+ * by restoring the offset if that's undesired.
  */
 static int
-mac_mls_to_string(char *string, size_t size, size_t *caller_len,
-    struct mac_mls *mac_mls)
+mac_mls_to_string(struct sbuf *sb, struct mac_mls *mac_mls)
 {
-	struct sbuf sb;
-
-	sbuf_new(&sb, string, size, SBUF_FIXEDLEN);
 
 	if (mac_mls->mm_flags & MAC_MLS_FLAG_SINGLE) {
-		if (mac_mls_element_to_string(&sb, &mac_mls->mm_single)
+		if (mac_mls_element_to_string(sb, &mac_mls->mm_single)
 		    == -1)
 			return (EINVAL);
 	}
 
 	if (mac_mls->mm_flags & MAC_MLS_FLAG_RANGE) {
-		if (sbuf_putc(&sb, '(') == -1)
+		if (sbuf_putc(sb, '(') == -1)
 			return (EINVAL);
 
-		if (mac_mls_element_to_string(&sb, &mac_mls->mm_rangelow)
+		if (mac_mls_element_to_string(sb, &mac_mls->mm_rangelow)
 		    == -1)
 			return (EINVAL);
 
-		if (sbuf_putc(&sb, '-') == -1)
+		if (sbuf_putc(sb, '-') == -1)
 			return (EINVAL);
 
-		if (mac_mls_element_to_string(&sb, &mac_mls->mm_rangehigh)
+		if (mac_mls_element_to_string(sb, &mac_mls->mm_rangehigh)
 		    == -1)
 			return (EINVAL);
 
-		if (sbuf_putc(&sb, ')') == -1)
+		if (sbuf_putc(sb, ')') == -1)
 			return (EINVAL);
 	}
 
-	sbuf_finish(&sb);
-	*caller_len = strlen(string);
 	return (0);
 }
 
 static int
 mac_mls_externalize_label(struct label *label, char *element_name,
-    char *element_data, size_t size, size_t *len, int *claimed)
+    struct sbuf *sb, int *claimed)
 {
 	struct mac_mls *mac_mls;
-	int error;
 
 	if (strcmp(MAC_MLS_LABEL_NAME, element_name) != 0)
 		return (0);
@@ -600,11 +592,7 @@ mac_mls_externalize_label(struct label *label, char *element_name,
 
 	mac_mls = SLOT(label);
 
-	error = mac_mls_to_string(element_data, size, len, mac_mls);
-	if (error)
-		return (error);
-
-	return (0);
+	return (mac_mls_to_string(sb, mac_mls));
 }
 
 static int

sys/security/mac_none/mac_none.c

@@ -127,7 +127,7 @@ mac_none_destroy_label(struct label *label)
 
 static int
 mac_none_externalize_label(struct label *label, char *element_name,
-    char *element_data, size_t size, size_t *len, int *claimed)
+    struct sbuf *sb, int *claimed)
 {
 
 	return (0);

sys/security/mac_partition/mac_partition.c

@@ -46,6 +46,7 @@
 #include <sys/mac.h>
 #include <sys/mount.h>
 #include <sys/proc.h>
+#include <sys/sbuf.h>
 #include <sys/systm.h>
 #include <sys/sysproto.h>
 #include <sys/sysent.h>
@@ -102,15 +103,18 @@ mac_partition_destroy_label(struct label *label)
 
 static int
 mac_partition_externalize_label(struct label *label, char *element_name,
-    char *element_data, size_t size, size_t *len, int *claimed)
+    struct sbuf *sb, int *claimed)
 {
 
 	if (strcmp(MAC_PARTITION_LABEL_NAME, element_name) != 0)
 		return (0);
 
 	(*claimed)++;
-	*len = snprintf(element_data, size, "%ld", SLOT(label));
-	return (0);
+
+	if (sbuf_printf(sb, "%ld", SLOT(label)) == -1)
+		return (EINVAL);
+	else
+		return (0);
 }
 
 static int

sys/security/mac_stub/mac_stub.c

@@ -127,7 +127,7 @@ mac_none_destroy_label(struct label *label)
 
 static int
 mac_none_externalize_label(struct label *label, char *element_name,
-    char *element_data, size_t size, size_t *len, int *claimed)
+    struct sbuf *sb, int *claimed)
 {
 
 	return (0);

sys/security/mac_test/mac_test.c

@@ -531,7 +531,7 @@ mac_test_destroy_vnode_label(struct label *label)
 
 static int
 mac_test_externalize_label(struct label *label, char *element_name,
-    char *element_data, size_t size, size_t *len, int *claimed)
+    struct sbuf *sb, int *claimed)
 {
 
 	atomic_add_int(&externalize_count, 1);

sys/sys/mac_policy.h

@@ -60,6 +60,7 @@ struct mac_policy_conf;
 struct mbuf;
 struct mount;
 struct pipe;
+struct sbuf;
 struct socket;
 struct ucred;
 struct uio;
@@ -114,23 +115,17 @@ struct mac_policy_ops {
 	void	(*mpo_copy_vnode_label)(struct label *src,
 		    struct label *dest);
 	int	(*mpo_externalize_cred_label)(struct label *label,
-		    char *element_name, char *buffer, size_t buflen,
-		    size_t *len, int *claimed);
+		    char *element_name, struct sbuf *sb, int *claimed);
 	int	(*mpo_externalize_ifnet_label)(struct label *label,
-		    char *element_name, char *buffer, size_t buflen,
-		    size_t *len, int *claimed);
+		    char *element_name, struct sbuf *sb, int *claimed);
 	int	(*mpo_externalize_pipe_label)(struct label *label,
-		    char *element_name, char *buffer, size_t buflen,
-		    size_t *len, int *claimed);
+		    char *element_name, struct sbuf *sb, int *claimed);
 	int	(*mpo_externalize_socket_label)(struct label *label,
-		    char *element_name, char *buffer, size_t buflen,
-		    size_t *len, int *claimed);
+		    char *element_name, struct sbuf *sb, int *claimed);
 	int	(*mpo_externalize_socket_peer_label)(struct label *label,
-		    char *element_name, char *buffer, size_t buflen,
-		    size_t *len, int *claimed);
+		    char *element_name, struct sbuf *sb, int *claimed);
 	int	(*mpo_externalize_vnode_label)(struct label *label,
-		    char *element_name, char *buffer, size_t buflen,
-		    size_t *len, int *claimed);
+		    char *element_name, struct sbuf *sb, int *claimed);
 	int	(*mpo_internalize_cred_label)(struct label *label,
 		    char *element_name, char *element_data, int *claimed);
 	int	(*mpo_internalize_ifnet_label)(struct label *label,