linux-kernel - Re: [PATCH] fs: Detect API misuse of i

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Message-ID: <20150125221643.GK29656@ZenIV.linux.org.uk>
Date:	Sun, 25 Jan 2015 22:16:43 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Richard Weinberger <richard@....at>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: Detect API misuse of i_op->lookup

On Sun, Jan 25, 2015 at 10:57:40PM +0100, Richard Weinberger wrote:
> Hunting down a refcount issue in an out-of-tree filesystem uncovered
> that its lookup function directly returned the dentry which was passed to it.
> Add a BUG_ON() to detect such misuse in future.

In principle, it's not a bug.  You *must* grab a reference to dentry before
returning it, but you can very well decide to return the one you've got -
any place where have ->lookup() return NULL has every right to become
return dget(dentry).
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