autofs-5.1.9 - fix crash in make_options_string()

From: Ian Kent <raven@themaw.net>

glibc reports a memory overflow when make_options_string() in snprintf()
As described by Andreas Hasenack on the autofs mailing list this is due
to my incorrect use of max_len in snprintf(), it should in fact be
max_len - <length of buffer already used>.

Anyway looking at the calculated maximum options string length there's
no actual overflow possible.

To fix this use strcat(3) instead of snprintf(), in this case there's
probably less overhead anyway. While we are at it drop the useless error
checks because we know it won't overflow.

Signed-off-by: Ian Kent <raven@themaw.net>
---
 lib/mounts.c |   35 +++++++++--------------------------
 1 file changed, 9 insertions(+), 26 deletions(-)

diff --git a/lib/mounts.c b/lib/mounts.c
index 05f18dbcf..7680c59c1 100644
--- a/lib/mounts.c
+++ b/lib/mounts.c
@@ -695,10 +695,11 @@ static int cacl_max_options_len(unsigned int flags)
 	unsigned int kver_minor = get_kver_minor();
 	int max_len;
 
-	/* %d and %u are maximum lenght of 10 and mount type is maximum
-	 * length of 9 (e. ",indirect").
+	/* %d and %u are maximum length of 10 and mount type is maximum
+	 * length of 9 (ie. ",indirect").
 	 * The base temaplate is "fd=%d,pgrp=%u,minproto=5,maxproto=%d"
-	 * plus the length of mount type plus 1 for the NULL.
+	 * plus the length of mount type plus 1 for the NULL (and an
+	 * additional 10 characters for good measure!).
 	 */
 	max_len = 79 + 1;
 
@@ -728,7 +729,7 @@ char *make_options_string(char *path, int pipefd,
 	unsigned int kver_major = get_kver_major();
 	unsigned int kver_minor = get_kver_minor();
 	char *options;
-	int max_len, len, new;
+	int max_len, len;
 
 	max_len = cacl_max_options_len(flags);
 
@@ -751,21 +752,13 @@ char *make_options_string(char *path, int pipefd,
 	if (len < 0)
 		goto error_out;
 
-	if (len >= max_len)
-		goto truncated;
-
 	if (kver_major < 5 || (kver_major == 5 && kver_minor < 4))
 		goto out;
 
 	/* maybe add ",strictexpire" */
 	if (flags & MOUNT_FLAG_STRICTEXPIRE) {
-		new = snprintf(options + len,
-			       max_len, "%s", ",strictexpire");
-		if (new < 0)
-		       goto error_out;
-		len += new;
-		if (len >= max_len)
-			goto truncated;
+		strcat(options, ",strictexpire");
+		len += 13;
 	}
 
 	if (kver_major == 5 && kver_minor < 5)
@@ -773,23 +766,13 @@ char *make_options_string(char *path, int pipefd,
 
 	/* maybe add ",ignore" */
 	if (flags & MOUNT_FLAG_IGNORE) {
-		new = snprintf(options + len,
-			       max_len, "%s", ",ignore");
-		if (new < 0)
-		       goto error_out;
-		len += new;
-		if (len >= max_len)
-			goto truncated;
+		strcat(options, ",ignore");
+		len += 7;
 	}
 out:
 	options[len] = '\0';
 	return options;
 
-truncated:
-	logerr("buffer to small for options - truncated");
-	len = max_len -1;
-	goto out;
-
 error_out:
 	logerr("error constructing mount options string for %s", path);
 	free(options);