On 03/10/14 23:44, Max Reitz wrote: > Before dereferencing bs->drv for a call to its member bdrv_co_readv(), > copy_sectors() should check whether that pointer is indeed valid, since > it may have been set to NULL by e.g. a concurrent write triggering the > corruption prevention mechanism. > > Signed-off-by: Max Reitz <mre...@redhat.com> > --- > To be precise, this still is a race condition. If bs->drv is set to NULL > after the check and before the call to bdrv_co_readv(), QEMU will > obviously still crash. However, in order to circumvent this behavior, we > would probably have to re-lock s->lock, check bs->drv, take the function > pointer to bdrv_co_readv() and then unlock s->lock before the function > is called. I found this rather ugly and therefore this still has a very > small chance of running into a race condition. > Therefore, I'm asking for your opinion on this, whether we can really > take this chance or should rather "do it right". In fact, if I were a > reviewer, I'd probably reject this patch and request the solution with > the function pointer (if there is no better solution), but I was afraid > to send such an ugly patch. > --- > block/qcow2-cluster.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c > index 36c1bed..9499df9 100644 > --- a/block/qcow2-cluster.c > +++ b/block/qcow2-cluster.c > @@ -380,6 +380,10 @@ static int coroutine_fn copy_sectors(BlockDriverState > *bs, > > BLKDBG_EVENT(bs->file, BLKDBG_COW_READ); > > + if (!bs->drv) { > + return -ENOMEDIUM; > + } > + > /* Call .bdrv_co_readv() directly instead of using the public block-layer > * interface. This avoids double I/O throttling and request tracking, > * which can lead to deadlock when block layer copy-on-read is enabled. >
I can't answer your question nor review this patch -- instead, I have a question of my own: when you say "set to NULL by [...] the corruption prevention mechanism", do you mean qcow2_pre_write_overlap_check(): bs->drv = NULL; /* make BDS unusable */ If so: I thought that it was quite a bold move, but also that we'd find the SIGSEGVs sooner or later... :) Thanks Laszlo