Re: Null dereference in bdrv_unregister_buf() probably memory-backend-file related?

[Jonathan Cameron]

On Mon, 21 Nov 2022 11:47:48 -0500
Stefan Hajnoczi <stefanha@gmail.com> wrote:

> On Mon, 21 Nov 2022 at 11:22, Jonathan Cameron via
> <qemu-devel@nongnu.org> wrote:
> >
> > First CC list is a guess as I haven't managed to root cause where things are
> > going wrong yet.
> >
> > Originally hit this whilst rebasing some CXL patches on v7.2.0-rc1.
> > CXL makes extensive use of memory-backends and most my tests happen
> > to use memory-backend-file
> >
> > Issue seen on arm64 and x86 though helpfully on x86 the crash appears in an 
> > entirely
> > unrelated location (though the 'fix' works).
> >
> > Fairly minimal test command line.
> >
> > qemu-system-aarch64 \
> >     -M virt  \
> >     -drive if=none,file=full.qcow2,format=qcow2,id=hd \
> >     -device virtio-blk,drive=hd \
> >     -object 
> > memory-backend-file,id=cxl-mem1,mem-path=/tmp/cxltest.raw,size=256M,align=256M
> >  \
> >
> > Powerdown the machine or ctrl-c during boot gives a segfault.
> > On arm64 it was in a stable location that made at least some sense in that
> > bs in the below snippet is NULL.
> >
> > I added the follow work around and the segfault goes away...
> >
> >  [PATCH] temp
> >
> > ---
> >  block/io.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/block/io.c b/block/io.c
> > index b9424024f9..750e1366aa 100644
> > --- a/block/io.c
> > +++ b/block/io.c
> > @@ -3324,6 +3324,9 @@ void bdrv_unregister_buf(BlockDriverState *bs, void 
> > *host, size_t size)
> >  {
> >      BdrvChild *child;
> >
> > +    if (!bs) {
> > +        return;
> > +    }
> >      GLOBAL_STATE_CODE();
> >      if (bs->drv && bs->drv->bdrv_unregister_buf) {
> >          bs->drv->bdrv_unregister_buf(bs, host, size);  
> 
> bdrv_*() APIs generally don't accept NULL bs arguments.
> 
> I think blk_unregister_buf() needs to handle the blk_bs() NULL return
> value. Can you confirm that the parent function is
> blk_unregister_buf()?
> 
> This bug may have been introduced by commit baf422684d73 ("virtio-blk:
> use BDRV_REQ_REGISTERED_BUF optimization hint").
Got it in one. I just bisected to exactly that patch

+ using the below change indeed works just as well as the above.
Now I'd send this as a patch, but I don't yet sufficiently understand what that 
change you
referenced did to break things Seems it registered a notifier that is getting
called for all ram blocks, not just the one virtio-blk ones?

Perhaps better if you send a fix with an explanation :)

Thanks for the quick response and correct identification of the problem.

Jonathan


diff --git a/block/block-backend.c b/block/block-backend.c
index b48c91f4e1..e281569137 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -2582,8 +2582,13 @@ bool blk_register_buf(BlockBackend *blk, void *host, 
size_t size, Error **errp)

 void blk_unregister_buf(BlockBackend *blk, void *host, size_t size)
 {
+    BlockDriverState *bs;
     GLOBAL_STATE_CODE();
-    bdrv_unregister_buf(blk_bs(blk), host, size);
+
+    bs = blk_bs(blk);
+    if (bs) {
+        bdrv_unregister_buf(blk_bs(blk), host, size);
+    }
 }

 int coroutine_fn blk_co_copy_range(BlockBackend *blk_in, int64_t off_in,

> 
> Stefan