See commit b317006a3f1 ("docs/secure-coding-practices: Describe how
to use 'null-co' block driver") for rationale.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 block/null.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/block/null.c b/block/null.c
index cc9b1d4ea72..11e428f3cc2 100644
--- a/block/null.c
+++ b/block/null.c
@@ -99,6 +99,13 @@ static int null_file_open(BlockDriverState *bs, QDict *options, int flags,
     return ret;
 }
 
+static bool null_taints_security_policy(BlockDriverState *bs)
+{
+    BDRVNullState *s = bs->opaque;
+
+    return !s->read_zeroes;
+}
+
 static int64_t null_getlength(BlockDriverState *bs)
 {
     BDRVNullState *s = bs->opaque;
@@ -283,6 +290,7 @@ static BlockDriver bdrv_null_co = {
     .bdrv_parse_filename    = null_co_parse_filename,
     .bdrv_getlength         = null_getlength,
     .bdrv_get_allocated_file_size = null_allocated_file_size,
+    .bdrv_taints_security_policy = null_taints_security_policy,
 
     .bdrv_co_preadv         = null_co_preadv,
     .bdrv_co_pwritev        = null_co_pwritev,
-- 
2.31.1