[PATCH v3 09/36] block: bdrv_refresh_perms: check for parents permissions conflict

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Add additional check that node parents do not interfere with each
other. This should not hurt existing callers and allows in further
patch use bdrv_refresh_perms() to update a subtree of changed
BdrvChild (check that change is correct).

New check will substitute bdrv_check_update_perm() in following
permissions refactoring, so keep error messages the same to avoid
unit test result changes.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 54 insertions(+), 9 deletions(-)

diff --git a/block.c b/block.c
index b951ecb5db..69db01c2ec 100644
--- a/block.c
+++ b/block.c
@@ -1992,6 +1992,57 @@ bool bdrv_is_writable(BlockDriverState *bs)
     return bdrv_is_writable_after_reopen(bs, NULL);
 }
 
+static char *bdrv_child_user_desc(BdrvChild *c)
+{
+    if (c->klass->get_parent_desc) {
+        return c->klass->get_parent_desc(c);
+    }
+
+    return g_strdup("another user");
+}
+
+static bool bdrv_a_allow_b(BdrvChild *a, BdrvChild *b, Error **errp)
+{
+    g_autofree char *user = NULL;
+    g_autofree char *perm_names = NULL;
+
+    if ((b->perm & a->shared_perm) == b->perm) {
+        return true;
+    }
+
+    perm_names = bdrv_perm_names(b->perm & ~a->shared_perm);
+    user = bdrv_child_user_desc(a);
+    error_setg(errp, "Conflicts with use by %s as '%s', which does not "
+               "allow '%s' on %s",
+               user, a->name, perm_names, bdrv_get_node_name(b->bs));
+
+    return false;
+}
+
+static bool bdrv_parent_perms_conflict(BlockDriverState *bs, Error **errp)
+{
+    BdrvChild *a, *b;
+
+    /*
+     * During the loop we'll look at each pair twice. That's correct because
+     * bdrv_a_allow_b() is asymmetric and we should check each pair in both
+     * directions.
+     */
+    QLIST_FOREACH(a, &bs->parents, next_parent) {
+        QLIST_FOREACH(b, &bs->parents, next_parent) {
+            if (a == b) {
+                continue;
+            }
+
+            if (!bdrv_a_allow_b(a, b, errp)) {
+                return true;
+            }
+        }
+    }
+
+    return false;
+}
+
 static void bdrv_child_perm(BlockDriverState *bs, BlockDriverState *child_bs,
                             BdrvChild *c, BdrvChildRole role,
                             BlockReopenQueue *reopen_queue,
@@ -2169,15 +2220,6 @@ void bdrv_get_cumulative_perm(BlockDriverState *bs, uint64_t *perm,
     *shared_perm = cumulative_shared_perms;
 }
 
-static char *bdrv_child_user_desc(BdrvChild *c)
-{
-    if (c->klass->get_parent_desc) {
-        return c->klass->get_parent_desc(c);
-    }
-
-    return g_strdup("another user");
-}
-
 char *bdrv_perm_names(uint64_t perm)
 {
     struct perm_name {
@@ -2321,6 +2363,9 @@ static int bdrv_refresh_perms(BlockDriverState *bs, Error **errp)
     int ret;
     uint64_t perm, shared_perm;
 
+    if (bdrv_parent_perms_conflict(bs, errp)) {
+        return -EPERM;
+    }
     bdrv_get_cumulative_perm(bs, &perm, &shared_perm);
     ret = bdrv_check_perm(bs, NULL, perm, shared_perm, NULL, errp);
     if (ret < 0) {
-- 
2.29.2