diff --git a/sys/kern/imgact_binmisc.c b/sys/kern/imgact_binmisc.c
index 9a78d24fe8b..266cf15f6e3 100644
--- a/sys/kern/imgact_binmisc.c
+++ b/sys/kern/imgact_binmisc.c
@@ -63,8 +63,10 @@ typedef struct imgact_binmisc_entry {
 	uint8_t				 *ibe_magic;
 	uint8_t				 *ibe_mask;
 	uint8_t				 *ibe_interpreter;
+	ssize_t				  ibe_interp_offset;
 	uint32_t			  ibe_interp_argcnt;
 	uint32_t			  ibe_interp_length;
+	uint32_t			  ibe_argv0_cnt;
 	uint32_t			  ibe_flags;
 	uint32_t			  ibe_moffset;
 	uint32_t			  ibe_msize;
@@ -154,7 +156,8 @@ imgact_binmisc_populate_interp(char *str, imgact_binmisc_entry_t *ibe)
  * Allocate memory and populate a new entry for the interpreter table.
  */
 static imgact_binmisc_entry_t *
-imgact_binmisc_new_entry(ximgact_binmisc_entry_t *xbe)
+imgact_binmisc_new_entry(ximgact_binmisc_entry_t *xbe, ssize_t interp_offset,
+    int argv0_cnt)
 {
 	imgact_binmisc_entry_t *ibe = NULL;
 	size_t namesz = min(strlen(xbe->xbe_name) + 1, IBE_NAME_MAX);
@@ -175,7 +178,8 @@ imgact_binmisc_new_entry(ximgact_binmisc_entry_t *xbe)
 	ibe->ibe_moffset = xbe->xbe_moffset;
 	ibe->ibe_msize = xbe->xbe_msize;
 	ibe->ibe_flags = xbe->xbe_flags;
-
+	ibe->ibe_interp_offset = interp_offset;
+	ibe->ibe_argv0_cnt = argv0_cnt;
 	return (ibe);
 }
 
@@ -227,7 +231,8 @@ imgact_binmisc_add_entry(ximgact_binmisc_entry_t *xbe)
 {
 	imgact_binmisc_entry_t *ibe;
 	char *p;
-	int cnt;
+	ssize_t interp_offset;
+	int argv0_cnt, cnt;
 
 	if (xbe->xbe_msize > IBE_MAGIC_MAX)
 		return (EINVAL);
@@ -242,23 +247,21 @@ imgact_binmisc_add_entry(ximgact_binmisc_entry_t *xbe)
 
 	/* Make sure we don't have any invalid #'s. */
 	p = xbe->xbe_interpreter;
-	while (1) {
-		p = strchr(p, '#');
-		if (!p)
-			break;
-
+	interp_offset = 0;
+	argv0_cnt = 0;
+	while ((p = strchr(p, '#')) != NULL) {
 		p++;
 		switch(*p) {
 		case ISM_POUND:
 			/* "##" */
 			p++;
+			interp_offset--;
 			break;
-
 		case ISM_OLD_ARGV0:
 			/* "#a" */
 			p++;
+			argv0_cnt++;
 			break;
-
 		case 0:
 		default:
 			/* Anything besides the above is invalid. */
@@ -273,7 +276,7 @@ imgact_binmisc_add_entry(ximgact_binmisc_entry_t *xbe)
 	}
 
 	/* Preallocate a new entry. */
-	ibe = imgact_binmisc_new_entry(xbe);
+	ibe = imgact_binmisc_new_entry(xbe, interp_offset, argv0_cnt);
 
 	SLIST_INSERT_HEAD(&interpreter_list, ibe, link);
 	interp_list_entry_count++;
@@ -586,12 +589,16 @@ imgact_binmisc_exec(struct image_params *imgp)
 	const char *image_header = imgp->image_header;
 	const char *fname = NULL;
 	int error = 0;
-	size_t offset, l;
+#ifdef INVARIANTS
+	int argv0_cnt = 0;
+#endif
+	size_t namelen, offset;
 	imgact_binmisc_entry_t *ibe;
 	struct sbuf *sname;
 	char *s, *d;
 
 	sname = NULL;
+	namelen = 0;
 	/* Do we have an interpreter for the given image header? */
 	INTERP_LIST_RLOCK();
 	if ((ibe = imgact_binmisc_find_interpreter(image_header)) == NULL) {
@@ -607,14 +614,22 @@ imgact_binmisc_exec(struct image_params *imgp)
 
 	imgp->interpreted |= IMGACT_BINMISC;
 
-	if (imgp->args->fname != NULL) {
-		fname = imgp->args->fname;
-	} else {
-		/* Use the fdescfs(5) path for fexecve(2). */
-		sname = sbuf_new_auto();
-		sbuf_printf(sname, "/dev/fd/%d", imgp->args->fd);
-		sbuf_finish(sname);
-		fname = sbuf_data(sname);
+	/*
+	 * Don't bother with the overhead of putting fname together if we're not
+	 * using #a.
+	 */
+	if (ibe->ibe_argv0_cnt != 0) {
+		if (imgp->args->fname != NULL) {
+			fname = imgp->args->fname;
+		} else {
+			/* Use the fdescfs(5) path for fexecve(2). */
+			sname = sbuf_new_auto();
+			sbuf_printf(sname, "/dev/fd/%d", imgp->args->fd);
+			sbuf_finish(sname);
+			fname = sbuf_data(sname);
+		}
+
+		namelen = strlen(fname);
 	}
 
 	/*
@@ -622,38 +637,14 @@ imgact_binmisc_exec(struct image_params *imgp)
 	 * we first shift all the other values in the `begin_argv' area to
 	 * provide the exact amount of room for the values added.  Set up
 	 * `offset' as the number of bytes to be added to the `begin_argv'
-	 * area.
+	 * area.  ibe_interp_offset is the fixed offset from macros present in
+	 * the interpreter string.
 	 */
-	offset = ibe->ibe_interp_length;
+	offset = ibe->ibe_interp_length + ibe->ibe_interp_offset;
 
-	/* Adjust the offset for #'s. */
-	s = ibe->ibe_interpreter;
-	while (1) {
-		s = strchr(s, '#');
-		if (!s)
-			break;
-
-		s++;
-		switch(*s) {
-		case ISM_POUND:
-			/* "##" -> "#": reduce offset by one. */
-			offset--;
-			break;
-
-		case ISM_OLD_ARGV0:
-			/* "#a" -> (old argv0): increase offset to fit fname */
-			offset += strlen(fname) - 2;
-			break;
-
-		default:
-			/* Hmm... This shouldn't happen. */
-			printf("%s: Unknown macro #%c sequence in "
-			    "interpreter string\n", KMOD_NAME, *(s + 1));
-			error = EINVAL;
-			goto done;
-		}
-		s++;
-	}
+	/* Variable offset to be added from macros to the interpreter string. */
+	MPASS(ibe->ibe_argv0_cnt == 0 || namelen > 0);
+	offset += ibe->ibe_argv0_cnt * (namelen - 2);
 
 	/* Make room for the interpreter */
 	error = exec_args_adjust_args(imgp->args, 0, offset);
@@ -680,26 +671,20 @@ imgact_binmisc_exec(struct image_params *imgp)
 				/* "##": Replace with a single '#' */
 				*d++ = '#';
 				break;
-
 			case ISM_OLD_ARGV0:
 				/* "#a": Replace with old arg0 (fname). */
-				if ((l = strlen(fname)) != 0) {
-					memcpy(d, fname, l);
-					d += l;
-				}
+				MPASS(ibe->ibe_argv0_cnt >= ++argv0_cnt);
+				memcpy(d, fname, namelen);
+				d += namelen;
 				break;
-
 			default:
-				/* Shouldn't happen but skip it if it does. */
-				break;
+				__assert_unreachable();
 			}
 			break;
-
 		case ' ':
 			/* Replace space with NUL to separate arguments. */
 			*d++ = '\0';
 			break;
-
 		default:
 			*d++ = *s;
 			break;
@@ -708,6 +693,8 @@ imgact_binmisc_exec(struct image_params *imgp)
 	}
 	*d = '\0';
 
+	/* Catch ibe->ibe_argv0_cnt counting more #a than we did. */
+	MPASS(ibe->ibe_argv0_cnt == argv0_cnt);
 	imgp->interpreter_name = imgp->args->begin_argv;
 
 done: