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

[Stefan Hajnoczi]

On Mon, Jan 29, 2018 at 10:13:02AM -0600, Mark Kanda wrote:
> 
> 
> On 1/29/2018 9:41 AM, 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 <address@hidden>
> > > > Reviewed-by: Karl Heubaum <address@hidden>
> > > > Reviewed-by: Ameya More <address@hidden>
> > > > ---
> > > >   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.
> 
> That's correct. The segfault I encountered was the bdrv_inc_in_flight() call
> in blk_aio_prwv().

I think we should aim to fix the root cause for QEMU 2.12.  blk_*() APIs
are supposed to return an error when blk->root is NULL but the
blk_aio_*() ones currently crash.

If that's not possible then this patch can be merged as a workaround for
2.12 close to the release date.

Stefan

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