linux-kernel - Re: [PATCH] btrfs: fix warning in create_pending

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Date:   Sat, 11 Nov 2023 06:20:59 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Edward Adam Davis <eadavis@...com>
Cc:     syzbot+4d81015bc10889fd12ea@...kaller.appspotmail.com,
        boris@....io, clm@...com, dsterba@...e.com, josef@...icpanda.com,
        linux-btrfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] btrfs: fix warning in create_pending_snapshot

On Sat, Nov 11, 2023 at 01:06:01PM +0800, Edward Adam Davis wrote:
> +++ b/fs/btrfs/disk-io.c
> @@ -4931,7 +4931,8 @@ int btrfs_get_free_objectid(struct btrfs_root *root, u64 *objectid)
>  		goto out;
>  	}
>  
> -	*objectid = root->free_objectid++;
> +	while (find_qgroup_rb(root->fs_info, root->free_objectid++));
> +	*objectid = root->free_objectid;

This looks buggy to me.  Let's say that free_objectid is currently 3.

Before, it would assign 3 to *objectid, and increment free_objectid to
4.  After (assuming the loop terminates on first iteration), it will
increment free_objectid to 4, then assign 4 to *objectid.

I think you meant to write:

	while (find_qgroup_rb(root->fs_info, root->free_objectid))
		root->free_objectid++;
	*objectid = root->free_objectid++;

And the lesson here is that more compact code is not necessarily more
correct code.

(I'm not making any judgement about whether this is the correct fix;
I don't understand btrfs well enough to have an opinion.  Just that
this is not an equivalent transformation)