Add support for AFL Persistent Fuzzing Mode based on work by Jonathan Foote <jfoote@fastly.com>

FUZZING

+# Fuzzing
+
+Knot DNS 2.0 includes `tests-fuzz/packet.c`.  This compiles into a
+test harness that is designed to be used with lcamtuf's [American
+Fuzzy Lop (AFL) fuzzer](http://lcamtuf.coredump.cx/afl/).  The test
+harness exercises the packet parsing logic in Knot DNS.
+
+## How it works
+
+AFL 1.83b includes an experimental feature called ["persistent
+mode"](http://lcamtuf.blogspot.com/2015/06/new-in-afl-persistent-mode.html)
+that can be used to control AFL's fork server to fuzz inputs and
+exercise the program without restarting it.  You can use this new
+feature along with the included Knot DNS test harness.
+
+## Using the AFL persistent harness
+
+### Gathering seed inputs
+
+Gathering DNS packets for use in fuzzing is left to the tester, but
+note that the fuzzing shim includes an environment variable to support
+test cases minimization with `afl-cmin`:
+
+```
+$ KNOT_AFL_STDIN=1 KNOT_AFL_CMIN=1 afl-cmin -i ~/knot-seeds -o ~/knot-seeds-cmin -m 1000000 -t 400000 -- tests-fuzz/packet
+```
+
+### Compiling the test harness.
+
+See the AFL [blog
+post](http://lcamtuf.blogspot.com/2015/06/new-in-afl-persistent-mode.html)
+and README for details on how to use LLVM mode and compile binaries
+for use with persistent mode.  For reference, you can use these
+commands to build Knot with the fuzzing harness:
+
+```
+$ CC=afl-clang-fast ./configure --disable-shared
+$ make check
+```
+
+### Fuzz
+
+A basic AFL run can then be kicked off as follows:
+
+```
+AFL_PERSISTENT=1 afl-fuzz -i my_seeds -o my_output_dir -t 10000 -m 100000 -- tests-fuzz/packet
+```
+
+Note that AFL can be scaled up by supplying the `-M` flag and starting
+multiple instances of the fuzzer.

tests-fuzz/packet.c

@@ -17,6 +17,7 @@
 #include <assert.h>
 #include <stdint.h>
 #include <stdio.h>
+#include <signal.h>
 
 #include "libknot/errcode.h"
 #include "libknot/packet/pkt.h"
@@ -25,19 +26,25 @@
 
 int main(void)
 {
-	log("reading packet data from stdin");
-
-	uint8_t buffer[UINT16_MAX + 1] = { 0 };
-	size_t len = fread(buffer, 1, sizeof(buffer), stdin);
-
-	log("parsing packet of size %zu", len);
-
-	knot_pkt_t *pkt = knot_pkt_new(buffer, len, NULL);
-	assert(pkt);
-	int r = knot_pkt_parse(pkt, 0);
-	knot_pkt_free(&pkt);
-
-	log("result %d (%s)", r, r == KNOT_EOK ? "success" : "failure");
-
-	return (r == KNOT_EOK ? 0 : 1);
+	for(;;) {
+		log("reading packet data from stdin");
+
+		uint8_t buffer[UINT16_MAX + 1] = { 0 };
+		size_t len = fread(buffer, 1, sizeof(buffer), stdin);
+
+		log("parsing packet of size %zu", len);
+
+		knot_pkt_t *pkt = knot_pkt_new(buffer, len, NULL);
+		assert(pkt);
+		int r = knot_pkt_parse(pkt, 0);
+		knot_pkt_free(&pkt);
+
+		log("result %d (%s)", r, r == KNOT_EOK ? "success" : "failure");
+		
+		if (getenv("AFL_PERSISTENT")) {
+			raise(SIGSTOP);
+		} else {
+			return (r == KNOT_EOK ? 0 : 1);
+		}
+	}
 }