xtables-restore: fix for --noflush and empty lines

Lookahead buffer used for cache requirements estimate in restore
--noflush separates individual lines with nul-chars. Two consecutive
nul-chars are interpreted as end of buffer and remaining buffer content
is skipped.

Sadly, reading an empty line (i.e., one containing a newline character
only) caused double nul-chars to appear in buffer as well, leading to
premature stop when reading cached lines from buffer.

To fix that, make use of xtables_restore_parse_line() skipping empty
lines without calling strtok() and just leave the newline character in
place. A more intuitive approach, namely skipping empty lines while
buffering, is deliberately not chosen as that would cause wrong values
in 'line' variable.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1400
Fixes: 09cb517949e69 ("xtables-restore: Improve performance of --noflush operation")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Arturo Borrero Gonzalez <arturo@netfilter.org>

2 files changed, 21 insertions, 3 deletions

diff --git a/iptables/tests/shell/testcases/ipt-restore/0011-noflush-empty-line_0 b/iptables/tests/shell/testcases/ipt-restore/0011-noflush-empty-line_0
new file mode 100755
index 00000000..bea1a690
--- /dev/null
+++ b/iptables/tests/shell/testcases/ipt-restore/0011-noflush-empty-line_0
@@ -0,0 +1,16 @@
+#!/bin/bash -e
+
+# make sure empty lines won't break --noflush
+
+cat <<EOF | $XT_MULTI iptables-restore --noflush
+# just a comment followed by innocent empty line
+
+*filter
+-A FORWARD -j ACCEPT
+COMMIT
+EOF
+
+EXPECT='Chain FORWARD (policy ACCEPT)
+target     prot opt source               destination         
+ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0           '
+diff -u <(echo "$EXPECT") <($XT_MULTI iptables -n -L FORWARD)
diff --git a/iptables/xtables-restore.c b/iptables/xtables-restore.c
index 63cc15ce..fb2ac8b5 100644
--- a/iptables/xtables-restore.c
+++ b/iptables/xtables-restore.c
@@ -293,11 +293,13 @@ void xtables_restore_parse(struct nft_handle *h,
 		while (fgets(buffer, sizeof(buffer), p->in)) {
 			size_t blen = strlen(buffer);
 
-			/* drop trailing newline; xtables_restore_parse_line()
+			/* Drop trailing newline; xtables_restore_parse_line()
 			 * uses strtok() which replaces them by nul-characters,
 			 * causing unpredictable string delimiting in
-			 * preload_buffer */
-			if (buffer[blen - 1] == '\n')
+			 * preload_buffer.
+			 * Unless this is an empty line which would fold into a
+			 * spurious EoB indicator (double nul-char). */
+			if (buffer[blen - 1] == '\n' && blen > 1)
 				buffer[blen - 1] = '\0';
 			else
 				blen++;