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Subject: Re: btrfs: qgroup: Try our best to delete qgroup relations (bug
 report)
From:   Colin Ian King <colin.king@canonical.com>
To:     Qu Wenruo <wqu@suse.com>, David Sterba <dsterba@suse.com>,
        Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
        linux-btrfs@vger.kernel.org
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <ea7f88e7-108e-ad2a-232f-b18715607bf3@canonical.com>
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Message-ID: <ca0946de-5343-aaf9-10f2-007e341cb8a4@canonical.com>
Date:   Tue, 6 Aug 2019 14:49:52 +0100
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On 06/08/2019 14:48, Colin Ian King wrote:
> 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;

oops, I mean:

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

> 
> Colin
>