gossipd: handle overflowing query properly (avoid slow 100% CPU reports)

Don't do this: (gdb) bt #0 0x00007f37ae667c40 in ?? () from /lib/x86_64-linux-gnu/libz.so.1 #1 0x00007f37ae668b38 in ?? () from /lib/x86_64-linux-gnu/libz.so.1 #2 0x00007f37ae669907 in deflate () from /lib/x86_64-linux-gnu/libz.so.1 #3 0x00007f37ae674c65 in compress2 () from /lib/x86_64-linux-gnu/libz.so.1 #4 0x000000000040cfe3 in zencode_scids (ctx=0xc1f118, scids=0x2599bc49 "\a\325{", len=176320) at gossipd/gossipd.c:218 #5 0x000000000040d0b3 in encode_short_channel_ids_end (encoded=0x7fff8f98d9f0, max_bytes=65490) at gossipd/gossipd.c:236 #6 0x000000000040dd28 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17290511, number_of_blocks=8) at gossipd/gossipd.c:576 ElementsProject#7 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17290511, number_of_blocks=16) at gossipd/gossipd.c:595 ElementsProject#8 0x000000000040ddee in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17290495, number_of_blocks=32) at gossipd/gossipd.c:596 ElementsProject#9 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17290495, number_of_blocks=64) at gossipd/gossipd.c:595 ElementsProject#10 0x000000000040ddee in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17290431, number_of_blocks=128) at gossipd/gossipd.c:596 ElementsProject#11 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17290431, number_of_blocks=256) at gossipd/gossipd.c:595 ElementsProject#12 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17290431, number_of_blocks=512) at gossipd/gossipd.c:595 ElementsProject#13 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17290431, number_of_blocks=1024) at gossipd/gossipd.c:595 ElementsProject#14 0x000000000040ddee in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17289408, number_of_blocks=2047) at gossipd/gossipd.c:596 ElementsProject#15 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17289408, number_of_blocks=4095) at gossipd/gossipd.c:595 ElementsProject#16 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17289408, number_of_blocks=8191) at gossipd/gossipd.c:595 ElementsProject#17 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17289408, number_of_blocks=16382) at gossipd/gossipd.c:595 ElementsProject#18 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17289408, number_of_blocks=32764) at gossipd/gossipd.c:595 ElementsProject#19 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17289408, number_of_blocks=65528) at gossipd/gossipd.c:595 ElementsProject#20 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17289408, number_of_blocks=131056) at gossipd/gossipd.c:595 ElementsProject#21 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17289408, number_of_blocks=262112) at gossipd/gossipd.c:595 ElementsProject#22 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17289408, number_of_blocks=524225) at gossipd/gossipd.c:595 ElementsProject#23 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17289408, number_of_blocks=1048450) at gossipd/gossipd.c:595 ElementsProject#24 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17289408, number_of_blocks=2096900) at gossipd/gossipd.c:595 ElementsProject#25 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17289408, number_of_blocks=4193801) at gossipd/gossipd.c:595 ElementsProject#26 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17289408, number_of_blocks=8387603) at gossipd/gossipd.c:595 ElementsProject#27 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=17289408, number_of_blocks=16775207) at gossipd/gossipd.c:595 ElementsProject#28 0x000000000040ddee in queue_channel_ranges (peer=0x3868fc8, first_blocknum=514201, number_of_blocks=33550414) at gossipd/gossipd.c:596 ElementsProject#29 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=514201, number_of_blocks=67100829) at gossipd/gossipd.c:595 ElementsProject#30 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=514201, number_of_blocks=134201659) at gossipd/gossipd.c:595 ElementsProject#31 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=514201, number_of_blocks=268403318) at gossipd/gossipd.c:595 ElementsProject#32 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=514201, number_of_blocks=536806636) at gossipd/gossipd.c:595 ElementsProject#33 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=514201, number_of_blocks=1073613273) at gossipd/gossipd.c:595 ElementsProject#34 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=514201, number_of_blocks=2147226547) at gossipd/gossipd.c:595 ElementsProject#35 0x000000000040ddc6 in queue_channel_ranges (peer=0x3868fc8, first_blocknum=514201, number_of_blocks=4294453094) at gossipd/gossipd.c:595 ElementsProject#36 0x000000000040df26 in handle_query_channel_range (peer=0x3868fc8, msg=0x37e0678 "\001\ao\342\214\n\266\361\263r\301\246\242F\256c\367O\223\036\203e\341Z\b\234h\326\031") at gossipd/gossipd.c:625 The cause was that converting a block number to an scid truncates it at 24 bits. When we look through the index from (truncated number) to (real end number) we get every channel, which is too large to encode, so we iterate again. This fixes both that problem, and also the issue that we'd end up dividing into many empty sections until we get to the highest block number. Instead, we just tack the empty blocks on to then end of the final query. (My initial version requested 0xFFFFFFFE blocks, but the dev code which records what blocks were returned can't make a bitmap that big on 32 bit). Reported-by: George Vaccaro Signed-off-by: Rusty Russell <[email protected]>
cdecker · Jan 15, 2019 · 2ffaa46 · 2ffaa46
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commit 2ffaa46
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Showing 3 changed files with 50 additions and 22 deletions.
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -25,6 +25,8 @@ changes.
 
 ### Fixed
 
+- Protocol: handling `query_channel_range` for large numbers of blocks
+  (eg. 4 billion) was slow due to a bug.
 
 ### Security
 

diff --git a/gossipd/gossipd.c b/gossipd/gossipd.c
@@ -659,9 +659,14 @@ static void reply_channel_range(struct peer *peer,
 /*~ When we need to send an array of channels, it might go over our 64k packet
  * size.  If it doesn't, we recurse, splitting in two, etc.  Each message
  * indicates what blocks it contains, so the recipient knows when we're
- * finished. */
+ * finished.
+ *
+ * tail_blocks is the empty blocks at the end, in case they asked for all
+ * blocks to 4 billion.
+ */
 static void queue_channel_ranges(struct peer *peer,
-				 u32 first_blocknum, u32 number_of_blocks)
+				 u32 first_blocknum, u32 number_of_blocks,
+				 u32 tail_blocks)
 {
 	struct routing_state *rstate = peer->daemon->rstate;
 	u8 *encoded = encode_short_channel_ids_start(tmpctx);
@@ -704,7 +709,8 @@ static void queue_channel_ranges(struct peer *peer,
 
 	/* If we can encode that, fine: send it */
 	if (encode_short_channel_ids_end(&encoded, max_encoded_bytes)) {
-		reply_channel_range(peer, first_blocknum, number_of_blocks,
+		reply_channel_range(peer, first_blocknum,
+				    number_of_blocks + tail_blocks,
 				    encoded);
 		return;
 	}
@@ -717,22 +723,26 @@ static void queue_channel_ranges(struct peer *peer,
 			      first_blocknum);
 		return;
 	}
-	status_debug("queue_channel_ranges full: splitting %u+%u and %u+%u",
+	status_debug("queue_channel_ranges full: splitting %u+%u and %u+%u(+%u)",
 		     first_blocknum,
 		     number_of_blocks / 2,
 		     first_blocknum + number_of_blocks / 2,
-		     number_of_blocks - number_of_blocks / 2);
-	queue_channel_ranges(peer, first_blocknum, number_of_blocks / 2);
+		     number_of_blocks - number_of_blocks / 2,
+		     tail_blocks);
+	queue_channel_ranges(peer, first_blocknum, number_of_blocks / 2, 0);
 	queue_channel_ranges(peer, first_blocknum + number_of_blocks / 2,
-			     number_of_blocks - number_of_blocks / 2);
+			     number_of_blocks - number_of_blocks / 2,
+			     tail_blocks);
 }
 
 /*~ The peer can ask for all channels is a series of blocks.  We reply with one
  * or more messages containing the short_channel_ids. */
 static u8 *handle_query_channel_range(struct peer *peer, const u8 *msg)
 {
+	struct routing_state *rstate = peer->daemon->rstate;
 	struct bitcoin_blkid chain_hash;
-	u32 first_blocknum, number_of_blocks;
+	u32 first_blocknum, number_of_blocks, tail_blocks;
+	struct short_channel_id last_scid;
 
 	if (!fromwire_query_channel_range(msg, &chain_hash,
 					  &first_blocknum, &number_of_blocks)) {
@@ -751,14 +761,25 @@ static u8 *handle_query_channel_range(struct peer *peer, const u8 *msg)
 		return NULL;
 	}
 
-	/* This checks for 32-bit overflow! */
-	if (first_blocknum + number_of_blocks < first_blocknum) {
-		return towire_errorfmt(peer, NULL,
-				       "query_channel_range overflow %u+%u",
-				       first_blocknum, number_of_blocks);
-	}
-
-	queue_channel_ranges(peer, first_blocknum, number_of_blocks);
+	/* If they ask for number_of_blocks UINTMAX, and we have to divide
+	 * and conquer, we'll do a lot of unnecessary work.  Cap it at the
+	 * last value we have, then send an empty reply. */
+	if (uintmap_last(&rstate->chanmap, &last_scid.u64)) {
+		u32 last_block = short_channel_id_blocknum(&last_scid);
+
+		/* u64 here avoids overflow on number_of_blocks
+		   UINTMAX for example */
+		if ((u64)first_blocknum + number_of_blocks > last_block) {
+			tail_blocks = first_blocknum + number_of_blocks
+				- last_block - 1;
+			number_of_blocks -= tail_blocks;
+		} else
+			tail_blocks = 0;
+	} else
+		tail_blocks = 0;
+
+	queue_channel_ranges(peer, first_blocknum, number_of_blocks,
+			     tail_blocks);
 	return NULL;
 }
 
@@ -2291,6 +2312,13 @@ static struct io_plan *query_channel_range(struct io_conn *conn,
 		goto fail;
 	}
 
+	/* Check for overflow on 32-bit machines! */
+	if (BITMAP_NWORDS(number_of_blocks) < number_of_blocks / BITMAP_WORD_BITS) {
+		status_broken("query_channel_range: huge number_of_blocks (%u) not supported",
+			number_of_blocks);
+		goto fail;
+	}
+
 	status_debug("sending query_channel_range for blocks %u+%u",
 		     first_blocknum, number_of_blocks);
 	msg = towire_query_channel_range(NULL, &daemon->chain_hash,

diff --git a/tests/test_gossip.py b/tests/test_gossip.py
@@ -4,7 +4,6 @@
 import json
 import logging
 import os
-import pytest
 import struct
 import subprocess
 import time
@@ -548,7 +547,6 @@ def check_gossip(n):
 
 
 @unittest.skipIf(not DEVELOPER, "needs DEVELOPER=1")
-@pytest.mark.xfail(strict=True)
 def test_gossip_query_channel_range(node_factory, bitcoind):
     l1, l2, l3, l4 = node_factory.line_graph(4, opts={'log-level': 'io'},
                                              fundchannel=False)
@@ -662,8 +660,8 @@ def test_gossip_query_channel_range(node_factory, bitcoind):
                                          first=0,
                                          num=1000000)
 
-    # Turns out it sends huge number of empty replies here.
-    l1.daemon.wait_for_logs([r'\[IN\] 0108'] * 21)
+    # Turns out it sends: 0+53, 53+26, 79+13, 92+7, 99+3, 102+2, 104+1, 105+999895
+    l1.daemon.wait_for_logs([r'\[IN\] 0108'] * 8)
 
     # It should definitely have split
     assert ret['final_first_block'] != 0 or ret['final_num_blocks'] != 1000000
@@ -673,11 +671,11 @@ def test_gossip_query_channel_range(node_factory, bitcoind):
     assert ret['short_channel_ids'][1] == scid23
     l2.daemon.wait_for_log('queue_channel_ranges full: splitting')
 
-    # Test overflow case doesn't split forever; should only get 32 for this.
+    # Test overflow case doesn't split forever; should still only get 8 for this
     ret = l1.rpc.dev_query_channel_range(id=l2.info['id'],
                                          first=1,
                                          num=429496000)
-    l1.daemon.wait_for_logs([r'\[IN\] 0108'] * 32)
+    l1.daemon.wait_for_logs([r'\[IN\] 0108'] * 8)
 
     # And no more!
     time.sleep(1)