Re: [Qemu-devel] [PATCH v2 04/37] hw/usb-storage: Check whether BB is inserted

[Kevin Wolf <kwolf@redhat.com>]

Am 04.03.2015 um 15:41 hat Max Reitz geschrieben:
> On 2015-03-04 at 09:39, Kevin Wolf wrote:
> >Am 04.03.2015 um 15:24 hat Max Reitz geschrieben:
> >>On 2015-03-04 at 09:20, Kevin Wolf wrote:
> >>>Am 04.03.2015 um 15:07 hat Max Reitz geschrieben:
> >>>>On 2015-03-04 at 09:02, Kevin Wolf wrote:
> >>>>>Am 09.02.2015 um 18:11 hat Max Reitz geschrieben:
> >>>>>>Only call bdrv_key_required() on the BlockDriverState if the
> >>>>>>BlockBackend has an inserted medium.
> >>>>>>
> >>>>>>Signed-off-by: Max Reitz <mreitz@redhat.com>
> >>>>>>Reviewed-by: Eric Blake <eblake@redhat.com>
> >>>>>>---
> >>>>>>  hw/usb/dev-storage.c | 2 +-
> >>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>>>
> >>>>>>diff --git a/hw/usb/dev-storage.c b/hw/usb/dev-storage.c
> >>>>>>index 4539733..3123baf 100644
> >>>>>>--- a/hw/usb/dev-storage.c
> >>>>>>+++ b/hw/usb/dev-storage.c
> >>>>>>@@ -638,7 +638,7 @@ static void usb_msd_realize_storage(USBDevice *dev, Error **errp)
> >>>>>>      usb_msd_handle_reset(dev);
> >>>>>>      s->scsi_dev = scsi_dev;
> >>>>>>-    if (bdrv_key_required(blk_bs(blk))) {
> >>>>>>+    if (blk_is_inserted(blk) && bdrv_key_required(blk_bs(blk))) {
> >>>>>>          if (cur_mon) {
> >>>>>>              monitor_read_bdrv_key_start(cur_mon, blk_bs(blk),
> >>>>>>                                          usb_msd_password_cb, s);
> >>>>>Why would bdrv_key_required() ever return true when no medium is
> >>>>>inserted? Sounds like a bug to me, like not resetting state correctly on
> >>>>>bdrv_close() of an encrypted image.
> >>>>The point is that blk_bs(blk) might be NULL.
> >>>This is not what blk_is_inserted() is checking. It happens to protect
> >>>you against segfaults because it's robust against using NULL, but with
> >>>an existing BDS, checking whether there is a medium inserted (in the
> >>>physical device for passthrough drivers) doesn't make sense.
> >>Not right now it's not. See patch 6.
> >Patch 6 looks unrelated, at least in v2. But if you're trying to say
> >that I looked at the wrong version, you're right: It doesn't protect you
> >against segfaults at this point yet (which is okay, because blk->bs
> >can't be NULL yet), it only performs the misguided inserted check.
> 
> Oops, yes, I meant patch 7.
> 
> >Doesn't answer my initial question or make that check any better.
> 
> The answer to your initial question is: bdrv_key_required() assumes
> a non-NULL BDS pointer is passed (which is reasonable). Therefore,
> it crashes when "no medium is inserted" in the sense of
> !blk_bs(blk).

That's a great argument in favour of checking blk_bs(bs), but I can't
see how it's one for the completely unrelated blk_inserted(blk).

Kevin