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

[Jonathan Cameron]

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);
--
2.37.2

Not present on v7.1.0 . I'll start a bisection shortly but may take a while.