[Libguestfs] [libnbd PATCH v2 06/23] states: Break deadlock if server goofs on extended replies

One of the benefits of extended replies is that we can do a fixed-length read for the entire header of every server reply, which is fewer syscalls than the split-read approach required by structured replies. But one of the drawbacks of doing a large read is that if the server is non-compliant (not a problem for normal servers, but something I hit rather more than I'd like to admit while developing extended header support in servers), nbd_pwrite() and friends will deadlock if the server replies with the wrong header. Add in some code to catch that failure mode and move the state machine to DEAD sooner, to make it easier to diagnose the fault in the server. Unlike in the case of an unexpected simply reply from a structured server (where we never over-read, and can therefore commit b31e7bac can merely fail the command with EPROTO and successfully move on to the next server reply), in this case we really do have to move to DEAD: in addition to having already read the 16 or 20 bytes that the server sent in its (short) reply for this command, we may have already read the initial bytes of the server's next reply, but we have no way to push those extra bytes back onto our read stream for parsing on our next pass through the state machine. --- generator/states-reply.c | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-) diff --git a/generator/states-reply.c b/generator/states-reply.c index dde23b39..e89e9019 100644 --- a/generator/states-reply.c +++ b/generator/states-reply.c @@ -109,7 +109,23 @@ REPLY.START: REPLY.RECV_REPLY: switch (recv_into_rbuf (h)) { case -1: SET_NEXT_STATE (%.DEAD); return 0; - case 1: SET_NEXT_STATE (%.READY); return 0; + case 1: SET_NEXT_STATE (%.READY); + /* Special case: if we have a short read, but got at least far + * enough to decode the magic number, we can check if the server + * is matching our expectations. This lets us avoid deadlocking if + * a buggy server sends only 16 bytes of a simple reply, and is + * waiting for our next command, while we are blocked waiting for + * the server to send 32 bytes of an extended reply. + */ + if (h->extended_headers && + (char *) h->rbuf >= (char *) &h->sbuf.reply.hdr.extended.flags) { + uint32_t magic = be32toh (h->sbuf.reply.hdr.extended.magic); + if (magic != NBD_EXTENDED_REPLY_MAGIC) { + SET_NEXT_STATE (%.DEAD); /* We've probably lost synchronization. */ + set_error (0, "invalid or unexpected reply magic 0x%" PRIx32, magic); + } + } + return 0; case 0: SET_NEXT_STATE (%CHECK_SIMPLE_OR_STRUCTURED_REPLY); } return 0; -- 2.38.1