[Libguestfs] [PATCH libnbd 2/3] states: Prevent multi-conn connection unless the server advertizes it.

It is unsafe to connect using multi-conn to a server unless the server advertizes it, so fail if the caller tries to do this. --- generator/generator | 16 ++++++++++++++-- lib/flags.c | 10 ++++++++++ 2 files changed, 24 insertions(+), 2 deletions(-) diff --git a/generator/generator b/generator/generator index a372074..a8e6fb6 100755 --- a/generator/generator +++ b/generator/generator @@ -897,7 +897,8 @@ The C<multi_conn> parameter controls whether this feature is enabled (if E<gt> 1) or disabled (if C<1>). The parameter passed must not be C<0>. Usually small powers of 2 (eg. 2, 4, 8) will provide increments in performance. Some servers do not -support this feature and will return an error on connection. +support this feature and libnbd will fail on connection to +these servers if this setting is E<gt> 1. At present, libnbd assumes that all connections will report the same capabilities; a server that behaves otherwise may trigger @@ -1171,7 +1172,18 @@ connected to and completed the handshake with the server."; Returns true if the server supports multi-conn (see C<nbd_set_multi_conn>). Returns false if the server does not. Can return an error if we have not -connected to and completed the handshake with the server."; +connected to and completed the handshake with the server. + +Note that you I<cannot> set multi-conn on the handle +to E<gt> 1, connect, and then check this setting, because +if the server does not support multi-conn the connection +will fail. Instead you should connect with multi-conn +set to C<1> (the default), check this setting, then if +multi-conn is found to be supported you can call +C<nbd_set_multi_conn> with a larger value to increase +the number of connection objects, then call one of the +C<nbd_connect*> functions again on the handle to connect +the remaining connections."; }; "can_cache", { diff --git a/lib/flags.c b/lib/flags.c index 825b326..1956db6 100644 --- a/lib/flags.c +++ b/lib/flags.c @@ -42,6 +42,16 @@ nbd_internal_set_size_and_flags (struct nbd_handle *h, return -1; } + /* It is unsafe to connect to a server with multi-conn set unless + * the server says it is safe to do so. + */ + if (h->multi_conn > 1 && (eflags & NBD_FLAG_CAN_MULTI_CONN) == 0) { + set_error (EINVAL, "handshake: multi-conn is set on this handle, " + "but the server does not advertize multi-conn support " + "so disconnecting because it is not safe to continue"); + return -1; + } + h->exportsize = exportsize; h->eflags = eflags; return 0; -- 2.21.0