dns: avoid truncation of searches list due to 256 char limit in glibc

Before glibc 2.26, glibc's resolver would only honor 6 search entries
and a character limit of 256. This was lifted recently ([1], [2], [3]).

We also lift this limitation in NetworkManager ([4], [5]).

However, older glibc versions would just truncate the string at 255
characters. In particular, it would not only tuncate the list to 6
entries, but the entry which crosses the 256th character boundary would
be mangled. Avoid that, by adding spaces.

[1] https://sourceware.org/ml/libc-alpha/2017-08/msg00010.html
[2] https://sourceware.org/bugzilla/show_bug.cgi?id=19569
[3] https://sourceware.org/bugzilla/show_bug.cgi?id=21475
[4] https://gitlab.freedesktop.org/NetworkManager/NetworkManager/merge_requests/47
[5] https://gitlab.freedesktop.org/NetworkManager/NetworkManager/issues/80

2 files changed, 31 insertions, 2 deletions

diff --git a/src/dns/nm-dns-manager.c b/src/dns/nm-dns-manager.c
index e1c295666b..7542228f54 100644
--- a/src/dns/nm-dns-manager.c
+++ b/src/dns/nm-dns-manager.c
@@ -581,10 +581,39 @@ create_resolv_conf (const char *const*searches,
 	g_string_append (str, "# Generated by NetworkManager\n");
 
 	if (searches && searches[0]) {
+		gsize search_base_idx;
+
 		g_string_append (str, "search");
+		search_base_idx = str->len;
+
 		for (i = 0; searches[i]; i++) {
+			const char *s = searches[i];
+			gsize l = strlen (s);
+
+			if (   l == 0
+			    || NM_STRCHAR_ANY (s, ch, NM_IN_SET (ch, ' ', '\t', '\n'))) {
+				/* there should be no such characters in the search entry. Also,
+				 * because glibc parser would treat them as line/word separator.
+				 *
+				 * Skip the value silently. */
+				continue;
+			}
+
+			if (search_base_idx > 0) {
+				if (str->len - search_base_idx + 1 + l > 254) {
+					/* this entry crosses the 256 character boundery. Older glibc versions
+					 * would truncate the entry at this point.
+					 *
+					 * Fill the line with spaces to cross the 256 char boundary and continue
+					 * afterwards. This way, the truncation happens between two search entries. */
+					while (str->len - search_base_idx < 257)
+						g_string_append_c (str, ' ');
+					search_base_idx = 0;
+				}
+			}
+
 			g_string_append_c (str, ' ');
-			g_string_append (str, searches[i]);
+			g_string_append_len (str, s, l);
 		}
 		g_string_append_c (str, '\n');
 	}
diff --git a/src/tests/test-general.c b/src/tests/test-general.c
index 87ff7f8480..2ab5f8f735 100644
--- a/src/tests/test-general.c
+++ b/src/tests/test-general.c
@@ -1903,7 +1903,7 @@ test_dns_create_resolv_conf (void)
 	          NULL,
 	          NULL,
 	          "# Generated by NetworkManager\n"
-	          "search a2x456789.b2x456789.c2x456789.d2x456789.e2x456789.f2x456789.g2x456789.h2x456789.i2x456789.j2x4567890 a2y456789.b2y456789.c2y456789.d2y456789.e2y456789.f2y456789.g2y456789.h2y456789.i2y456789.j2y4567890 a2z456789.b2z456789.c2z456789.d2z456789.e2z456789.f2z456789.g2z456789.h2z456789.i2z456789.j2z4567890\n"
+	          "search a2x456789.b2x456789.c2x456789.d2x456789.e2x456789.f2x456789.g2x456789.h2x456789.i2x456789.j2x4567890 a2y456789.b2y456789.c2y456789.d2y456789.e2y456789.f2y456789.g2y456789.h2y456789.i2y456789.j2y4567890                                                        a2z456789.b2z456789.c2z456789.d2z456789.e2z456789.f2z456789.g2z456789.h2z456789.i2z456789.j2z4567890\n"
 	          "");
 
 }