[PULL 09/18] block/export: Fix null pointer dereference in error path

[Kevin Wolf <kwolf@redhat.com>]

There are some error paths in blk_exp_add() that jump to 'fail:' before
'exp' is even created. So we can't just unconditionally access exp->blk.

Add a NULL check, and switch from exp->blk to blk, which is available
earlier, just to be extra sure that we really cover all cases where
BlockDevOps could have been set for it (in practice, this only happens
in drv->create() today, so this part of the change isn't strictly
necessary).

Fixes: Coverity CID 1509238
Fixes: de79b52604e43fdeba6cee4f5af600b62169f2d2
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230510203601.418015-3-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/export/export.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/block/export/export.c b/block/export/export.c
index 62c7c22d45..a5c8f42f53 100644
--- a/block/export/export.c
+++ b/block/export/export.c
@@ -192,8 +192,10 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
     return exp;
 
 fail:
-    blk_set_dev_ops(exp->blk, NULL, NULL);
-    blk_unref(blk);
+    if (blk) {
+        blk_set_dev_ops(blk, NULL, NULL);
+        blk_unref(blk);
+    }
     aio_context_release(ctx);
     if (exp) {
         g_free(exp->id);
-- 
2.40.1