re: btrfs: qgroup: Try our best to delete qgroup relations (bug report)

[Colin Ian King <colin.king@canonical.com>]

Hi,

Static analysis with Coverity on linux-next picked up a potential issue
with the following commit:

commit 035087b3c256741be367747eab866505cece31fb
Author: Qu Wenruo <wqu@suse.com>
Date:   Sat Aug 3 14:45:59 2019 +0800

    btrfs: qgroup: Try our best to delete qgroup relations

The static analysis report is as follows:

1334         */

    3. Condition !member, taking true branch.
    4. var_compare_op: Comparing member to null implies that member
might be null.
    5. Condition !parent, taking false branch.

1335        if (!member && !parent)
1336                goto delete_item;
1337
1338        /* check if such qgroup relation exist firstly */


CID 85026 (#1 of 1): Dereference after null check (FORWARD_NULL)

6. var_deref_op: Dereferencing null pointer member.

1339        list_for_each_entry(list, &member->groups, next_group) {
1340                if (list->group == parent) {
1341                        found = true;
1342                        break;
1343                }
1344        }

An example of the issue that if member is NULL and parent is not null
then  the list_for_each_entry loop with dereference the NULL member
pointer.  The changed logic in the patch on line 1335 is the root cause
of this regression.  I believe it should still be:

	if (!member && !parent)
		goto delete_item;

Colin