From 7e60746005573a06149cdee7acedf428906f3a59 Mon Sep 17 00:00:00 2001 From: Johannes Berg Date: Tue, 24 Sep 2019 09:21:17 +0200 Subject: [PATCH] um: virtio: Keep reading on -EAGAIN When we get an interrupt from the socket getting readable, and start reading, there's a possibility for a race. This depends on the implementation of the device, but e.g. with qemu's libvhost-user, we can see: device virtio_uml --------------------------------------- write header get interrupt read header read body -> returns -EAGAIN write body The -EAGAIN return is because the socket is non-blocking, and then this leads us to abandon this message. In fact, we've already read the header, so when the get another signal/interrupt for the body, we again read it as though it's a new message header, and also abandon it for the same reason (wrong size etc.) This essentially breaks things, and if that message was one that required a response, it leads to a deadlock as the device is waiting for the response but we'll never reply. Fix this by spinning on -EAGAIN as well when we read the message body. We need to handle -EAGAIN as "no message" while reading the header, since we share an interrupt. Note that this situation is highly unlikely to occur in normal usage, since there will be very few messages and only in the startup phase. With the inband call feature this does tend to happen (eventually) though. Signed-off-by: Johannes Berg Signed-off-by: Richard Weinberger --- arch/um/drivers/virtio_uml.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/arch/um/drivers/virtio_uml.c b/arch/um/drivers/virtio_uml.c index ca3067302c15..76b97c2de9a8 100644 --- a/arch/um/drivers/virtio_uml.c +++ b/arch/um/drivers/virtio_uml.c @@ -91,7 +91,7 @@ static int full_sendmsg_fds(int fd, const void *buf, unsigned int len, return 0; } -static int full_read(int fd, void *buf, int len) +static int full_read(int fd, void *buf, int len, bool abortable) { int rc; @@ -101,7 +101,7 @@ static int full_read(int fd, void *buf, int len) buf += rc; len -= rc; } - } while (len && (rc > 0 || rc == -EINTR)); + } while (len && (rc > 0 || rc == -EINTR || (!abortable && rc == -EAGAIN))); if (rc < 0) return rc; @@ -112,7 +112,7 @@ static int full_read(int fd, void *buf, int len) static int vhost_user_recv_header(int fd, struct vhost_user_msg *msg) { - return full_read(fd, msg, sizeof(msg->header)); + return full_read(fd, msg, sizeof(msg->header), true); } static int vhost_user_recv(struct virtio_uml_device *vu_dev, @@ -135,7 +135,7 @@ static int vhost_user_recv(struct virtio_uml_device *vu_dev, size = msg->header.size; if (size > max_payload_size) return -EPROTO; - return full_read(fd, &msg->payload, size); + return full_read(fd, &msg->payload, size, false); } static int vhost_user_recv_resp(struct virtio_uml_device *vu_dev, -- 2.45.2