[Virtio-fs] [PATCH 1/4] virtiofsd: send reply correctly on read failure

[Liu Bo]

On Thu, Apr 18, 2019 at 01:25:27PM +0100, Dr. David Alan Gilbert wrote:
> * Dr. David Alan Gilbert (dgilbert at redhat.com) wrote:
> > * Liu Bo (bo.liu at linux.alibaba.com) wrote:
> > > From: Eryu Guan <eguan at linux.alibaba.com>
> > > 
> > > Currently when a lo_read() operation fails, we don't send the failure
> > > back to fuse client, and read(2) operation from guest kernel would hang
> > > on waiting for the reply.
> > > 
> > > This is easily triggered by a direct read with non-aligned length.
> > > 
> > > Fix it by detecting preadv(2) error in virtio_send_data_iov(), and
> > > teaching fuse_reply_data() to reply error on error case.
> > 
> > Thank you for spotting this.
> > 
> > > Reviewed-by: Liu Bo <bo.liu at linux.alibaba.com>
> > > Signed-off-by: Eryu Guan <eguan at linux.alibaba.com>
> > > ---
> > >  contrib/virtiofsd/fuse_lowlevel.c |  4 ++--
> > >  contrib/virtiofsd/fuse_virtio.c   | 12 +++++++++---
> > >  2 files changed, 11 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/contrib/virtiofsd/fuse_lowlevel.c b/contrib/virtiofsd/fuse_lowlevel.c
> > > index 111c6e1..aeb5fe2 100644
> > > --- a/contrib/virtiofsd/fuse_lowlevel.c
> > > +++ b/contrib/virtiofsd/fuse_lowlevel.c
> > > @@ -524,11 +524,11 @@ int fuse_reply_data(fuse_req_t req, struct fuse_bufvec *bufv,
> > >  	out.error = 0;
> > >  
> > >  	res = fuse_send_data_iov(req->se, req->ch, iov, 1, bufv, flags);
> > > -	if (res <= 0) {
> > > +	if (res >= 0) {
> > 
> > I need to go and ask the upstream fuse list about this; it's not clear
> > to me what fuse_send_data_iov is supposed to return;  let me clarify
> > that and get back to you.
> 
> Miklos replied to me on the upstream fuse list:
> https://sourceforge.net/p/fuse/mailman/fuse-devel/thread/CAOssrKdVViWewmh3nrfKkq6eaWNJ629AR7w90KG3XT2hV9_D1w%40mail.gmail.com/#msg36642807
> 
> so this comparison is actually already correct because a negative value
> means that something is wrong with the fuse connection (or in our case
> virtio) so we can't send an error reply anyway.
>

I see, that makes sense.

> > Dave
> > 
> > >  		fuse_free_req(req);
> > >  		return res;
> > >  	} else {
> > > -		return fuse_reply_err(req, res);
> > > +		return fuse_reply_err(req, -res);
> > >  	}
> > >  }
> > >  
> > > diff --git a/contrib/virtiofsd/fuse_virtio.c b/contrib/virtiofsd/fuse_virtio.c
> > > index ca988aa..0b5736d 100644
> > > --- a/contrib/virtiofsd/fuse_virtio.c
> > > +++ b/contrib/virtiofsd/fuse_virtio.c
> > > @@ -333,8 +333,13 @@ int virtio_send_data_iov(struct fuse_session *se, struct fuse_chan *ch,
> > >                  ret = preadv(buf->buf[0].fd, in_sg_ptr, in_sg_cpy_count, buf->buf[0].pos);
> > >  
> > >                  if (se->debug)
> > > -                        fprintf(stderr, "%s: preadv_res=%d len=%zd\n",
> > > -                                __func__, ret, len);
> > > +                        fprintf(stderr, "%s: preadv_res=%d(%s) len=%zd\n",
> > > +                                __func__, ret, strerror(errno), len);
> > > +                if (ret == -1) {
> > > +                        ret = -errno;
> 
> So this will need to be      ret = errno    to give a postive error
> response to mean that the error was on the file side on the fuse side.
> (There's also another case below that where I set ret = EIO where it
> should be EIO I think)
>

OK, do you want me to fix both in the same patch?

> > > +                        free(in_sg_cpy);
> > > +                        goto err;
> > > +                }
> > >                  if (ret < len && ret) {
> > >                          if (se->debug)
> > >                                  fprintf(stderr, "%s: ret < len\n", __func__);
> > > @@ -379,7 +384,8 @@ int virtio_send_data_iov(struct fuse_session *se, struct fuse_chan *ch,
> > >          vu_queue_notify(&se->virtio_dev->dev, q);
> > >  
> > >  err:
> > > -        ch->qi->reply_sent = true;
> > > +        if (!ret)
> > > +                ch->qi->reply_sent = true;
> 
> Yes, I think that's OK.

Correct, this is the one causing fuse kernel side's hang.

thanks,
-liubo

> 
> Dave
> 
> > >  
> > >          return ret;
> > >  }
> > > -- 
> > > 1.8.3.1
> > > 
> > > _______________________________________________
> > > Virtio-fs mailing list
> > > Virtio-fs at redhat.com
> > > https://www.redhat.com/mailman/listinfo/virtio-fs
> > --
> > Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK
> > 
> > _______________________________________________
> > Virtio-fs mailing list
> > Virtio-fs at redhat.com
> > https://www.redhat.com/mailman/listinfo/virtio-fs
> --
> Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK