libopeniscsiusr: ensure sysfs pathname doesn't overflow.

When instantiating a pathname in sysfs, make sure there's enough
room to prevent possible overflow.

Found by gcc-8, which gave this warning:
> [54s] sysfs.c:326:48: error: '/iscsi_host/' directive output may be
>    truncated writing 12 bytes into a region of size between 1 and 4096
>    [-Werror=format-truncation=]

1 files changed, 13 insertions, 2 deletions

diff --git a/libopeniscsiusr/sysfs.c b/libopeniscsiusr/sysfs.c
index 70298f2..6f590f4 100644
--- a/libopeniscsiusr/sysfs.c
+++ b/libopeniscsiusr/sysfs.c
@@ -311,6 +311,9 @@ int _iscsi_host_id_of_session(struct iscsi_context *ctx, uint32_t sid,
 	int n = 0;
 	const char *host_id_str = NULL;
 	int i = 0;
+	const char iscsi_host_dir_str[] = "/iscsi_host/";
+	const unsigned int iscsi_host_dir_strlen = strlen(iscsi_host_dir_str);
+
 
 	assert(ctx != NULL);
 	assert(sid != 0);
@@ -323,8 +326,16 @@ int _iscsi_host_id_of_session(struct iscsi_context *ctx, uint32_t sid,
 
 	_good(sysfs_get_dev_path(ctx, sys_se_dir_path, sys_dev_path), rc, out);
 
-	snprintf(sys_scsi_host_dir_path, PATH_MAX, "%s/iscsi_host/",
-		 sys_dev_path);
+	if ((strlen(sys_dev_path) + iscsi_host_dir_strlen) >= PATH_MAX) {
+		rc = LIBISCSI_ERR_SYSFS_LOOKUP;
+		_error(ctx, "Pathname too long: %s%s",
+		       sys_dev_path, iscsi_host_dir_str);
+		goto out;
+	}
+
+	strncpy(sys_scsi_host_dir_path, sys_dev_path, PATH_MAX);
+	strncat(sys_scsi_host_dir_path, iscsi_host_dir_str,
+		PATH_MAX - iscsi_host_dir_strlen);
 
 	n = scandir(sys_scsi_host_dir_path, &namelist, _scan_filter_skip_dot,
 		    alphasort);