4:03 p.m.
I found some more 1-line hacks to nbdkit that can produce a subtly
buggy server that is still easy enough to work around in libnbd.
Eric Blake (3):
states: Refactor where interrupted reply state is stored
states: Skip server replies to unknown command cookies
states: Tolerate simple reply to structured read request
lib/internal.h | 4 +--
generator/states-reply-simple.c | 46 +++++++++++++++++++++-------
generator/states-reply-structured.c | 24 ++++++++-------
generator/states-reply.c | 47 ++++++++++++++---------------
4 files changed, 73 insertions(+), 48 deletions(-)
--
2.37.1
4:03 p.m.
New subject: [libnbd PATCH 1/3] states: Refactor where interrupted reply state is stored
There can be at most one active reply at a time. Storing a resume
state per command is wasteful compared to storing a single resume
state in the overall handle. Furthermore, if we want to be able to
skip bogus server replies to an unexpected cookie, we need a way to
resume reading bytes off the wire even when there is no matching
h->reply_cmd for the current cookie.
---
lib/internal.h | 4 ++--
generator/states-reply.c | 20 +++++++++++---------
2 files changed, 13 insertions(+), 11 deletions(-)
diff --git a/lib/internal.h b/lib/internal.h
index 8dbe2e4..0a61b15 100644
--- a/lib/internal.h
+++ b/lib/internal.h
@@ -303,8 +303,9 @@ struct nbd_handle {
/* length (cmds_to_issue) + length (cmds_in_flight). */
int in_flight;
- /* Current command during a REPLY cycle */
+ /* Current command and POLLIN resumption point during a REPLY cycle */
struct command *reply_cmd;
+ enum state reply_state;
bool disconnect_request; /* True if we've queued NBD_CMD_DISC */
bool tls_shut_writes; /* Used by lib/crypto.c to track disconnect. */
@@ -352,7 +353,6 @@ struct command {
uint32_t count;
void *data; /* Buffer for read/write */
struct command_cb cb;
- enum state state; /* State to resume with on next POLLIN */
bool initialized; /* For read, true if getting a hole may skip memset */
uint32_t data_seen; /* For read, cumulative size of data chunks seen */
uint32_t error; /* Local errno value */
diff --git a/generator/states-reply.c b/generator/states-reply.c
index bff6f4b..1a3a20a 100644
--- a/generator/states-reply.c
+++ b/generator/states-reply.c
@@ -1,5 +1,5 @@
/* nbd client library in userspace: state machine
- * Copyright (C) 2013-2019 Red Hat Inc.
+ * Copyright (C) 2013-2022 Red Hat Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
@@ -25,26 +25,28 @@
* until POLLIN, we instead track where in the state machine we left
* off, then return to READY to actually block. Then, on entry to
* REPLY.START, we can tell if this is the start of a new reply (rlen
- * is 0, stay put), a continuation of the preamble (reply_cmd is NULL,
- * resume with RECV_REPLY), or a continuation from any other location
- * (reply_cmd contains the state to jump to).
+ * is 0, stay put), a continuation of the preamble (reply_state is
+ * STATE_START, resume with RECV_REPLY), or a continuation from any
+ * other location (reply_state contains the state to jump to).
*/
static void
save_reply_state (struct nbd_handle *h)
{
- assert (h->reply_cmd);
assert (h->rlen);
- h->reply_cmd->state = get_next_state (h);
+ assert (h->reply_state == STATE_START);
+ h->reply_state = get_next_state (h);
+ assert (h->reply_state != STATE_START);
}
STATE_MACHINE {
REPLY.START:
/* If rlen is non-zero, we are resuming an earlier reply cycle. */
if (h->rlen > 0) {
- if (h->reply_cmd) {
- assert (nbd_internal_is_state_processing (h->reply_cmd->state));
- SET_NEXT_STATE (h->reply_cmd->state);
+ if (h->reply_state != STATE_START) {
+ assert (nbd_internal_is_state_processing (h->reply_state));
+ SET_NEXT_STATE (h->reply_state);
+ h->reply_state = STATE_START;
}
else
SET_NEXT_STATE (%RECV_REPLY);
--
2.37.1
4:04 p.m.
New subject: [libnbd PATCH 2/3] states: Skip server replies to unknown command cookies
Instead of killing the connection when the server gives us an
unexpected structured or error reply for a cookie we don't recognize,
we can log the issue and otherwise ignore that packet (an unexpected
successful simple reply is harder if we did not negotiate structured
replies; see the lengthy comment in states-reply-simple.c). A
compliant server will never do this to us, but a transmission error
might. Here's a quick patch to nbdkit to demonstrate such a scenario:
| diff --git i/server/protocol.c w/server/protocol.c
| index 2ac77055..55b27e2e 100644
| --- i/server/protocol.c
| +++ w/server/protocol.c
| @@ -742,7 +742,7 @@ protocol_recv_request_send_reply (void)
| (cmd == NBD_CMD_READ || cmd == NBD_CMD_BLOCK_STATUS)) {
| if (!error) {
| if (cmd == NBD_CMD_READ)
| - return send_structured_reply_read (request.handle, cmd,
| + return send_structured_reply_read (0x100^request.handle, cmd,
| buf, count, offset);
| else /* NBD_CMD_BLOCK_STATUS */
| return send_structured_reply_block_status (request.handle,
Note that if a server bit-flips a cookie, our original command waiting
for the right cookie may never get a completion packet from the
server; thus, testing this requires aio commands (unless we someday
add APIs to make it easier for blocking commands that give up when
they hit a timeout). With the following command line and the hack to
nbdkit above:
$ ./run /path/to/nbdkit -U - memory 1M --run 'nbdsh -u "$uri" -c "\
buf=nbd.Buffer(1)
cookie=h.aio_pread(buf, 0)
try:
print(h.poll(1))
except nbd.Error as ex:
print(ex.string)
h.flush()
print(h.get_size())
"'
pre-patch we see that the connection is killed when h.poll()
encounters the garbage reply, before we get to the second command:
nbd_poll: no matching cookie found for server reply, this is probably a bug in the server
nbdkit: memory.1: error: read request: Connection reset by peer
nbdsh: command line script failed: nbd_flush: invalid state: DEAD: the handle must be
connected with the server: Invalid argument
while post-patch the read never retires, but the unexpected cookie is
skipped and we get to the flush:
1
1048576
Watching the LIBNBD_DEBUG=1 stream, we also see:
libnbd: debug: nbdsh: nbd_poll: skipped reply for unexpected cookie 281474976710657, this
is probably a bug in the server
---
generator/states-reply-simple.c | 25 ++++++++++++++++++++++---
generator/states-reply-structured.c | 24 +++++++++++++-----------
generator/states-reply.c | 27 ++++++++++++---------------
3 files changed, 47 insertions(+), 29 deletions(-)
diff --git a/generator/states-reply-simple.c b/generator/states-reply-simple.c
index 2a7b9a9..24f7efa 100644
--- a/generator/states-reply-simple.c
+++ b/generator/states-reply-simple.c
@@ -25,10 +25,29 @@ STATE_MACHINE {
uint64_t cookie;
error = be32toh (h->sbuf.simple_reply.error);
- cookie = be64toh (h->sbuf.simple_reply.handle);
- assert (cmd);
- assert (cmd->cookie == cookie);
+ if (cmd == NULL) {
+ /* Unexpected reply. If error was set or we have structured
+ * replies, we know there should be no payload, so the next byte
+ * on the wire (if any) will be another reply, and we can let
+ * FINISH_COMMAND diagnose/ignore the server bug. If not, we lack
+ * context to know whether the server thinks it was responding to
+ * NBD_CMD_READ, so it is safer to move to DEAD now than to risk
+ * consuming a server's potential data payload as a reply stream
+ * (even though we would be likely to produce a magic number
+ * mismatch on the next pass that would also move us to DEAD).
+ */
+ if (error || h->structured_replies)
+ SET_NEXT_STATE (%^FINISH_COMMAND);
+ else {
+ cookie = be64toh (h->sbuf.simple_reply.handle);
+ SET_NEXT_STATE (%.DEAD);
+ set_error (EPROTO,
+ "no matching cookie %" PRIu64 " found for server reply,
"
+ "this is probably a server bug", cookie);
+ }
+ return 0;
+ }
if (cmd->type == NBD_CMD_READ && h->structured_replies) {
set_error (0, "server sent unexpected simple reply for read");
diff --git a/generator/states-reply-structured.c b/generator/states-reply-structured.c
index 2db2cf6..4fbd62f 100644
--- a/generator/states-reply-structured.c
+++ b/generator/states-reply-structured.c
@@ -69,17 +69,12 @@ STATE_MACHINE {
REPLY.STRUCTURED_REPLY.CHECK:
struct command *cmd = h->reply_cmd;
uint16_t flags, type;
- uint64_t cookie;
uint32_t length;
flags = be16toh (h->sbuf.sr.structured_reply.flags);
type = be16toh (h->sbuf.sr.structured_reply.type);
- cookie = be64toh (h->sbuf.sr.structured_reply.handle);
length = be32toh (h->sbuf.sr.structured_reply.length);
- assert (cmd);
- assert (cmd->cookie == cookie);
-
/* Reject a server that replies with too much information, but don't
* reject a single structured reply to NBD_CMD_READ on the largest
* size we were willing to send. The most likely culprit is a server
@@ -88,12 +83,13 @@ STATE_MACHINE {
* not worth keeping the connection alive.
*/
if (length > MAX_REQUEST_SIZE + sizeof h->sbuf.sr.payload.offset_data) {
- set_error (0, "invalid server reply length");
+ set_error (0, "invalid server reply length %" PRIu32, length);
SET_NEXT_STATE (%.DEAD);
return 0;
}
- if (!h->structured_replies) {
+ /* Skip an unexpected structured reply, including to an unknown cookie. */
+ if (cmd == NULL || !h->structured_replies) {
resync:
h->rbuf = NULL;
h->rlen = length;
@@ -487,7 +483,6 @@ STATE_MACHINE {
uint16_t type;
uint32_t length;
- assert (cmd);
assert (h->rbuf == NULL);
switch (recv_into_rbuf (h)) {
case -1: SET_NEXT_STATE (%.DEAD); return 0;
@@ -496,6 +491,15 @@ STATE_MACHINE {
SET_NEXT_STATE (%.READY);
return 0;
case 0:
+ /* If this reply is to an unknown command, FINISH_COMMAND will
+ * diagnose and ignore the server bug. Otherwise, ensure the
+ * pending command sees a failure of EPROTO if it does not already
+ * have an error.
+ */
+ if (cmd == NULL) {
+ SET_NEXT_STATE (%^FINISH_COMMAND);
+ return 0;
+ }
type = be16toh (h->sbuf.sr.structured_reply.type);
length = be32toh (h->sbuf.sr.structured_reply.length);
debug (h, "unexpected reply type %u or payload length %" PRIu32
@@ -505,10 +509,8 @@ STATE_MACHINE {
if (cmd->error == 0)
cmd->error = EPROTO;
SET_NEXT_STATE (%FINISH);
- return 0;
- default:
- abort ();
}
+ return 0;
REPLY.STRUCTURED_REPLY.FINISH:
uint16_t flags;
diff --git a/generator/states-reply.c b/generator/states-reply.c
index 1a3a20a..0736342 100644
--- a/generator/states-reply.c
+++ b/generator/states-reply.c
@@ -124,7 +124,7 @@ STATE_MACHINE {
}
else {
SET_NEXT_STATE (%.DEAD); /* We've probably lost synchronization. */
- set_error (0, "invalid reply magic");
+ set_error (0, "invalid reply magic 0x%" PRIx32, magic);
return 0;
}
@@ -132,22 +132,13 @@ STATE_MACHINE {
* handle (our cookie) is stored at the same offset.
*/
cookie = be64toh (h->sbuf.simple_reply.handle);
- /* Find the command amongst the commands in flight. */
+ /* Find the command amongst the commands in flight. If the server sends
+ * a reply for an unknown cookie, FINISH will diagnose that later.
+ */
for (cmd = h->cmds_in_flight; cmd != NULL; cmd = cmd->next) {
if (cmd->cookie == cookie)
break;
}
- if (cmd == NULL) {
- /* An unexpected structured reply could be skipped, since it
- * includes a length; similarly an unexpected simple reply can be
- * skipped if we assume it was not a read. However, it's more
- * likely we've lost synchronization with the server.
- */
- SET_NEXT_STATE (%.DEAD);
- set_error (0, "no matching cookie found for server reply, "
- "this is probably a bug in the server");
- return 0;
- }
h->reply_cmd = cmd;
return 0;
@@ -167,10 +158,16 @@ STATE_MACHINE {
if (cmd->cookie == cookie)
break;
}
- assert (cmd != NULL);
assert (h->reply_cmd == cmd);
- h->reply_cmd = NULL;
+ if (cmd == NULL) {
+ debug (h, "skipped reply for unexpected cookie %" PRIu64
+ ", this is probably a bug in the server", cookie);
+ SET_NEXT_STATE (%.READY);
+ return 0;
+ }
+
retire = cmd->type == NBD_CMD_DISC;
+ h->reply_cmd = NULL;
/* Notify the user */
if (CALLBACK_IS_NOT_NULL (cmd->cb.completion)) {
--
2.37.1
4:04 p.m.
New subject: [libnbd PATCH 3/3] states: Tolerate simple reply to structured read request
Another patch in the series of being more tolerant to various server
bugs. Although a server is non-compliant for using a simple reply to
NBD_CMD_READ once structured commands are negotiated, it is just as
easy to read the packet and continue on after guaranteeing we report
an error.
In the process, I noticed that in the corner case of a server that
starts with a structured packet and follows with a simple reply, we
can no longer assume that the simple reply is the only receipt of
data, so we have to be more careful with assertions and increments to
data_seen.
As in earlier patches, the following temporary one-line tweak to
nbdkit will produce a server that demonstrates the scenario:
| diff --git i/server/protocol.c w/server/protocol.c
| index 2ac77055..ac359ca1 100644
| --- i/server/protocol.c
| +++ w/server/protocol.c
| @@ -738,7 +738,7 @@ protocol_recv_request_send_reply (void)
| * us from sending human-readable error messages to the client, so
| * we should reconsider this in future.
| */
| - if (conn->structured_replies &&
| + if (conn->structured_replies && !error &&
| (cmd == NBD_CMD_READ || cmd == NBD_CMD_BLOCK_STATUS)) {
| if (!error) {
| if (cmd == NBD_CMD_READ)
The following command line:
$ ./run /path/to/nbdkit -U - memory 1M --filter=error error-pread-rate=100% \
--run 'nbdsh -u "$uri" -c "\
try:
h.pread(1,0)
except nbd.Error as ex:
print(ex.string)
h.flush()
print(h.get_size())
"'
demonstrates the following issue pre-patch:
nbdkit: memory.0: error: injecting EIO error into pread
nbd_pread: server sent unexpected simple reply for read
nbdsh: command line script failed: nbd_flush: invalid state: DEAD: the handle must be
connected with the server: Invalid argument
and the nicer flow post-patch:
nbdkit: memory.0: error: injecting EIO error into pread
nbd_pread: read: command failed: Protocol error
1048576
where we report EPROTO rather than the server's EIO.
---
generator/states-reply-simple.c | 21 +++++++++++++--------
1 file changed, 13 insertions(+), 8 deletions(-)
diff --git a/generator/states-reply-simple.c b/generator/states-reply-simple.c
index 24f7efa..ac58823 100644
--- a/generator/states-reply-simple.c
+++ b/generator/states-reply-simple.c
@@ -49,17 +49,23 @@ STATE_MACHINE {
return 0;
}
+ /* Although a server with structured replies negotiated is in error
+ * for using a simple reply to NBD_CMD_READ, we can cope with the
+ * packet, but diagnose it by failing the read with EPROTO.
+ */
if (cmd->type == NBD_CMD_READ && h->structured_replies) {
- set_error (0, "server sent unexpected simple reply for read");
- SET_NEXT_STATE(%.DEAD);
- return 0;
+ debug (h, "server sent unexpected simple reply for read");
+ if (cmd->error == 0)
+ cmd->error = EPROTO;
}
- cmd->error = nbd_internal_errno_of_nbd_error (error);
- if (cmd->error == 0 && cmd->type == NBD_CMD_READ) {
+ error = nbd_internal_errno_of_nbd_error (error);
+ if (cmd->error == 0)
+ cmd->error = error;
+ if (error == 0 && cmd->type == NBD_CMD_READ) {
h->rbuf = cmd->data;
h->rlen = cmd->count;
- cmd->data_seen = cmd->count;
+ cmd->data_seen += cmd->count;
SET_NEXT_STATE (%RECV_READ_PAYLOAD);
}
else {
@@ -80,9 +86,8 @@ STATE_MACHINE {
/* guaranteed by START */
assert (cmd);
if (CALLBACK_IS_NOT_NULL (cmd->cb.fn.chunk)) {
- int error = 0;
+ int error = cmd->error;
- assert (cmd->error == 0);
if (CALL_CALLBACK (cmd->cb.fn.chunk,
cmd->data, cmd->count,
cmd->offset, LIBNBD_READ_DATA,
--
2.37.1
8:23 p.m.
On Fri, Aug 12, 2022 at 04:03:58PM -0500, Eric Blake wrote:
I found some more 1-line hacks to nbdkit that can produce a subtly
buggy server that is still easy enough to work around in libnbd.
Eric Blake (3):
states: Refactor where interrupted reply state is stored
states: Skip server replies to unknown command cookies
states: Tolerate simple reply to structured read request
I've now pushed this series as 32af175..b31e7ba
I'm still thinking about adding a strictness knob to let us decide
whether to try and cope with servers or fail as fast as possible on
the first non-compliance; but whether that belongs as a new bit to the
existing nbd_set_strict_mode (which to date has controlled client
strictness, not our response to the server) or a new API, is still
worth thinking about.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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Eric Blake