Re: [Qemu-devel] [Qemu-block] [PATCH] virtio-blk: check for NULL BlockDriverState

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Jan 29, 2018 at 04:41:07PM +0100, Kevin Wolf wrote:
> Am 24.01.2018 um 12:31 hat Stefan Hajnoczi geschrieben:
> > On Mon, Jan 22, 2018 at 09:01:49AM -0600, Mark Kanda wrote:
> > > Add a BlockDriverState NULL check to virtio_blk_handle_request()
> > > to prevent a segfault if the drive is forcibly removed using HMP
> > > 'drive_del' (without performing a hotplug 'device_del' first).
> > > 
> > > Signed-off-by: Mark Kanda <mark.kanda@oracle.com>
> > > Reviewed-by: Karl Heubaum <karl.heubaum@oracle.com>
> > > Reviewed-by: Ameya More <ameya.more@oracle.com>
> > > ---
> > >  hw/block/virtio-blk.c | 7 +++++++
> > >  1 file changed, 7 insertions(+)
> > > 
> > > diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> > > index b1532e4..76ddbbf 100644
> > > --- a/hw/block/virtio-blk.c
> > > +++ b/hw/block/virtio-blk.c
> > > @@ -507,6 +507,13 @@ static int virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
> > >          return -1;
> > >      }
> > >  
> > > +    /* If the drive was forcibly removed (e.g. HMP 'drive_del'), the block
> > > +     * driver state may be NULL and there is nothing left to do. */
> > > +    if (!blk_bs(req->dev->blk)) {
> > 
> > Adding Markus Armbruster to check my understanding of drive_del:
> > 
> > 1. If id is a node name (e.g. created via blockdev-add) then attempting
> >    to remove the root node produces the "Node %s is in use" error.  In
> >    that case this patch isn't needed.
> > 
> > 2. If id is a BlockBackend (e.g. created via -drive) then removing the
> >    root node is allowed.  The BlockBackend stays in place but blk->root
> >    becomes NULL, hence this patch is needed.
> > 
> > Markus: What are the valid use cases for #2?  If blk->bs becomes NULL I
> > would think a lot more code beyond virtio-blk can segfault.
> 
> blk->root = NULL is completely normal, it is what happens with removable
> media when the drive is empty.
> 
> The problem, which was first reported during the 2.10 RC phase and was
> worked around in IDE code then, is that Paolo's commit 99723548561 added
> unconditional bdrv_inc/dec_in_flight() calls. I am pretty sure that any
> segfaults that Mark is seeing have the same cause.
> 
> We do need an in-flight counter even for those requests so that
> blk_drain() works correctly, so just making the calls condition wouldn't
> be right. However, this needs to become a separate counter in
> BlockBackend, and the drain functions must be changed to make use of it.
> 
> I did post rough patches back then, but they weren't quite ready, and
> since then they have fallen through the cracks.

Will you send a new version of that patch series?

Stefan

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