According to the 9P spec [*], when a client wants to cancel a pending I/O request identified by a given tag (uint16), it must send a Tflush message and wait for the server to respond with a Rflush message before reusing this tag for another I/O. The server may still send a completion message for the I/O if it wasn't actually cancelled but the Rflush message must arrive after that.
QEMU hence waits for the flushed PDU to complete before sending the Rflush message back to the client. If a client sends 'Tflush tag oldtag' and tag == oldtag, QEMU will then allocate a PDU identified by tag, find it in the PDU list and wait for this same PDU to complete... i.e. wait for a completion that will never happen. This causes a tag and ring slot leak in the guest, and a PDU leak in QEMU, all of them limited by the maximal number of PDUs (128). But, worse, this causes QEMU to hang on device reset since v9fs_reset() wants to drain all pending I/O. This insane behavior is likely to denote a bug in the client, and it would deserve an Rerror message to be sent back. Unfortunately, the protocol allows it and requires all flush requests to suceed (only a Tflush response is expected). The only option is to detect when we have to handle a self-referencing flush request and report success to the client right away. [*] http://man.cat-v.org/plan_9/5/flush Reported-by: Al Viro <v...@zeniv.linux.org.uk> Signed-off-by: Greg Kurz <gr...@kaod.org> --- v2: print out a warning --- hw/9pfs/9p.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-) diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c index 76c9247c777d..b8c0b993580c 100644 --- a/hw/9pfs/9p.c +++ b/hw/9pfs/9p.c @@ -2353,7 +2353,7 @@ static void coroutine_fn v9fs_flush(void *opaque) ssize_t err; int16_t tag; size_t offset = 7; - V9fsPDU *cancel_pdu; + V9fsPDU *cancel_pdu = NULL; V9fsPDU *pdu = opaque; V9fsState *s = pdu->s; @@ -2364,9 +2364,13 @@ static void coroutine_fn v9fs_flush(void *opaque) } trace_v9fs_flush(pdu->tag, pdu->id, tag); - QLIST_FOREACH(cancel_pdu, &s->active_list, next) { - if (cancel_pdu->tag == tag) { - break; + if (pdu->tag == tag) { + error_report("Warning: the guest sent a self-referencing 9P flush request"); + } else { + QLIST_FOREACH(cancel_pdu, &s->active_list, next) { + if (cancel_pdu->tag == tag) { + break; + } } } if (cancel_pdu) {