pam_motd: do not rely on all filesystems providing a filetype

When using scandir() to look for MOTD files to display, we wrongly
relied on all filesystems providing a filetype.  This is a fix to divert
to lstat() when we have no filetype.  To maintain MT safety, it isn't
possible to use lstat() in the scandir() filter function, so all of the
filtering has been moved to an additional loop after scanning all the
motd dirs.
Also, remove superfluous alphasort from scandir(), we are doing
a qsort() later.

Resolves: #455

modules/pam_motd/pam_motd.c

@@ -166,11 +166,6 @@ static int compare_strings(const void *a, const void *b)
     }
 }
 
-static int filter_dirents(const struct dirent *d)
-{
-    return (d->d_type == DT_REG || d->d_type == DT_LNK);
-}
-
 static void try_to_display_directories_with_overrides(pam_handle_t *pamh,
 	char **motd_dir_path_split, unsigned int num_motd_dirs, int report_missing)
 {
@@ -199,8 +194,7 @@ static void try_to_display_directories_with_overrides(pam_handle_t *pamh,
 
     for (i = 0; i < num_motd_dirs; i++) {
 	int rv;
-	rv = scandir(motd_dir_path_split[i], &(dirscans[i]),
-		filter_dirents, alphasort);
+	rv = scandir(motd_dir_path_split[i], &(dirscans[i]), NULL, NULL);
 	if (rv < 0) {
 	    if (errno != ENOENT || report_missing) {
 		pam_syslog(pamh, LOG_ERR, "error scanning directory %s: %m",
@@ -215,6 +209,41 @@ static void try_to_display_directories_with_overrides(pam_handle_t *pamh,
     if (dirscans_size_total == 0)
         goto out;
 
+    /* filter out unwanted names, directories, and complement data with lstat() */
+    for (i = 0; i < num_motd_dirs; i++) {
+	struct dirent **d = dirscans[i];
+	for (unsigned int j = 0; j < dirscans_sizes[i]; j++) {
+	    int rc;
+	    char *fullpath;
+	    struct stat s;
+
+	    switch(d[j]->d_type) {    /* the filetype determines how to proceed */
+	    case DT_REG:              /* regular files and     */
+	    case DT_LNK:              /* symlinks              */
+		continue;             /* are good.             */
+	    case DT_UNKNOWN:   /* for file systems that do not provide */
+			       /* a filetype, we use lstat()           */
+		if (join_dir_strings(&fullpath, motd_dir_path_split[i],
+				     d[j]->d_name) <= 0)
+		    break;
+		rc = lstat(fullpath, &s);
+		_pam_drop(fullpath);  /* free the memory alloc'ed by join_dir_strings */
+		if (rc != 0)          /* if the lstat() somehow failed */
+		    break;
+
+		if (S_ISREG(s.st_mode) ||          /* regular files and  */
+		    S_ISLNK(s.st_mode)) continue;  /* symlinks are good  */
+		break;
+	    case DT_DIR:          /* We don't want directories     */
+	    default:              /* nor anything else             */
+		break;
+	    }
+	    _pam_drop(d[j]);  /* free memory                   */
+	    d[j] = NULL;      /* indicate this one was dropped */
+	    dirscans_size_total--;
+	}
+    }
+
     /* Allocate space for all file names found in the directories, including duplicates. */
     if ((dirnames_all = calloc(dirscans_size_total, sizeof(*dirnames_all))) == NULL) {
 	pam_syslog(pamh, LOG_CRIT, "failed to allocate dirname array");
@@ -225,8 +254,10 @@ static void try_to_display_directories_with_overrides(pam_handle_t *pamh,
 	unsigned int j;
 
 	for (j = 0; j < dirscans_sizes[i]; j++) {
-	    dirnames_all[i_dirnames] = dirscans[i][j]->d_name;
-	    i_dirnames++;
+	    if (NULL != dirscans[i][j]) {
+	        dirnames_all[i_dirnames] = dirscans[i][j]->d_name;
+	        i_dirnames++;
+	    }
 	}
     }