block/export: Fix graph locking in blk_get_geometry() call

blk_get_geometry() eventually calls bdrv_nb_sectors(), which is a
co_wrapper_mixed_bdrv_rdlock. This means that when it is called from
coroutine context, it already assume to have the graph locked.

However, virtio_blk_sect_range_ok() in block/export/virtio-blk-handler.c
(used by vhost-user-blk and VDUSE exports) runs in a coroutine, but
doesn't take the graph lock - blk_*() functions are generally expected
to do that internally. This causes an assertion failure when accessing
an export for the first time if it runs in an iothread.

This is an example of the crash:

  $ ./storage-daemon/qemu-storage-daemon --object iothread,id=th0 --blockdev file,filename=/home/kwolf/images/hd.img,node-name=disk --export vhost-user-blk,addr.type=unix,addr.path=/tmp/vhost.sock,node-name=disk,id=exp0,iothread=th0
  qemu-storage-daemon: ../block/graph-lock.c:268: void assert_bdrv_graph_readable(void): Assertion `qemu_in_main_thread() || reader_count()' failed.

  (gdb) bt
  #0  0x00007ffff6eafe5c in __pthread_kill_implementation () from /lib64/libc.so.6
  #1  0x00007ffff6e5fa76 in raise () from /lib64/libc.so.6
  #2  0x00007ffff6e497fc in abort () from /lib64/libc.so.6
  #3  0x00007ffff6e4971b in __assert_fail_base.cold () from /lib64/libc.so.6
  #4  0x00007ffff6e58656 in __assert_fail () from /lib64/libc.so.6
  qemu#5  0x00005555556337a3 in assert_bdrv_graph_readable () at ../block/graph-lock.c:268
  qemu#6  0x00005555555fd5a2 in bdrv_co_nb_sectors (bs=0x5555564c5ef0) at ../block.c:5847
  qemu#7  0x00005555555ee949 in bdrv_nb_sectors (bs=0x5555564c5ef0) at block/block-gen.c:256
  qemu#8  0x00005555555fd6b9 in bdrv_get_geometry (bs=0x5555564c5ef0, nb_sectors_ptr=0x7fffef7fedd0) at ../block.c:5884
  qemu#9  0x000055555562ad6d in blk_get_geometry (blk=0x5555564cb200, nb_sectors_ptr=0x7fffef7fedd0) at ../block/block-backend.c:1624
  qemu#10 0x00005555555ddb74 in virtio_blk_sect_range_ok (blk=0x5555564cb200, block_size=512, sector=0, size=512) at ../block/export/virtio-blk-handler.c:44
  qemu#11 0x00005555555dd80d in virtio_blk_process_req (handler=0x5555564cbb98, in_iov=0x7fffe8003830, out_iov=0x7fffe8003860, in_num=1, out_num=0) at ../block/export/virtio-blk-handler.c:189
  qemu#12 0x00005555555dd546 in vu_blk_virtio_process_req (opaque=0x7fffe8003800) at ../block/export/vhost-user-blk-server.c:66
  qemu#13 0x00005555557bf4a1 in coroutine_trampoline (i0=-402635264, i1=32767) at ../util/coroutine-ucontext.c:177
  qemu#14 0x00007ffff6e75c20 in ?? () from /lib64/libc.so.6
  qemu#15 0x00007fffefffa870 in ?? ()
  qemu#16 0x0000000000000000 in ?? ()

Fix this by creating a new blk_co_get_geometry() that takes the lock,
and changing blk_get_geometry() to be a co_wrapper_mixed around it.

To make the resulting code cleaner, virtio-blk-handler.c can directly
call the coroutine version now (though that wouldn't be necessary for
fixing the bug, taking the lock in blk_co_get_geometry() is what fixes
it).

Fixes: 8ab8140
Reported-by: Lukáš Doktor <ldoktor@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230327113959.60071-1-kwolf@redhat.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -5879,9 +5879,10 @@ int64_t coroutine_fn bdrv_co_getlength(BlockDriverState *bs)
 }
 
 /* return 0 as number of sectors if no device present or error */
-void bdrv_get_geometry(BlockDriverState *bs, uint64_t *nb_sectors_ptr)
+void coroutine_fn bdrv_co_get_geometry(BlockDriverState *bs,
+                                       uint64_t *nb_sectors_ptr)
 {
-    int64_t nb_sectors = bdrv_nb_sectors(bs);
+    int64_t nb_sectors = bdrv_co_nb_sectors(bs);
     IO_CODE();
 
     *nb_sectors_ptr = nb_sectors < 0 ? 0 : nb_sectors;

block/block-backend.c

@@ -1615,13 +1615,16 @@ int64_t coroutine_fn blk_co_getlength(BlockBackend *blk)
     return bdrv_co_getlength(blk_bs(blk));
 }
 
-void blk_get_geometry(BlockBackend *blk, uint64_t *nb_sectors_ptr)
+void coroutine_fn blk_co_get_geometry(BlockBackend *blk,
+                                      uint64_t *nb_sectors_ptr)
 {
     IO_CODE();
+    GRAPH_RDLOCK_GUARD();
+
     if (!blk_bs(blk)) {
         *nb_sectors_ptr = 0;
     } else {
-        bdrv_get_geometry(blk_bs(blk), nb_sectors_ptr);
+        bdrv_co_get_geometry(blk_bs(blk), nb_sectors_ptr);
     }
 }
 

block/export/virtio-blk-handler.c

@@ -22,8 +22,9 @@ struct virtio_blk_inhdr {
     unsigned char status;
 };
 
-static bool virtio_blk_sect_range_ok(BlockBackend *blk, uint32_t block_size,
-                                     uint64_t sector, size_t size)
+static bool coroutine_fn
+virtio_blk_sect_range_ok(BlockBackend *blk, uint32_t block_size,
+                         uint64_t sector, size_t size)
 {
     uint64_t nb_sectors;
     uint64_t total_sectors;
@@ -41,7 +42,7 @@ static bool virtio_blk_sect_range_ok(BlockBackend *blk, uint32_t block_size,
     if ((sector << VIRTIO_BLK_SECTOR_BITS) % block_size) {
         return false;
     }
-    blk_get_geometry(blk, &total_sectors);
+    blk_co_get_geometry(blk, &total_sectors);
     if (sector > total_sectors || nb_sectors > total_sectors - sector) {
         return false;
     }

include/block/block-io.h

@@ -89,7 +89,9 @@ int64_t co_wrapper bdrv_get_allocated_file_size(BlockDriverState *bs);
 
 BlockMeasureInfo *bdrv_measure(BlockDriver *drv, QemuOpts *opts,
                                BlockDriverState *in_bs, Error **errp);
-void bdrv_get_geometry(BlockDriverState *bs, uint64_t *nb_sectors_ptr);
+
+void coroutine_fn GRAPH_RDLOCK
+bdrv_co_get_geometry(BlockDriverState *bs, uint64_t *nb_sectors_ptr);
 
 int coroutine_fn GRAPH_RDLOCK
 bdrv_co_delete_file(BlockDriverState *bs, Error **errp);

include/sysemu/block-backend-io.h

@@ -70,7 +70,10 @@ void co_wrapper blk_eject(BlockBackend *blk, bool eject_flag);
 int64_t coroutine_fn blk_co_getlength(BlockBackend *blk);
 int64_t co_wrapper_mixed blk_getlength(BlockBackend *blk);
 
-void blk_get_geometry(BlockBackend *blk, uint64_t *nb_sectors_ptr);
+void coroutine_fn blk_co_get_geometry(BlockBackend *blk,
+                                      uint64_t *nb_sectors_ptr);
+void co_wrapper_mixed blk_get_geometry(BlockBackend *blk,
+                                       uint64_t *nb_sectors_ptr);
 
 int64_t coroutine_fn blk_co_nb_sectors(BlockBackend *blk);
 int64_t co_wrapper_mixed blk_nb_sectors(BlockBackend *blk);