linux-kernel - Re: [PATCH] fuse: add missing iput() in fuse

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Message-ID: <87sed12rte.fsf@wotan.olymp>
Date: Tue, 23 Dec 2025 14:31:09 +0000
From: Luis Henriques <luis@...lia.com>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Miklos Szeredi <miklos@...redi.hu>,  linux-fsdevel@...r.kernel.org,
  linux-kernel@...r.kernel.org,  kernel-dev@...lia.com
Subject: Re: [PATCH] fuse: add missing iput() in fuse_lookup() error path

On Fri, Dec 19 2025, Al Viro wrote:

> On Fri, Dec 19, 2025 at 05:43:09PM +0000, Luis Henriques wrote:
>> The inode use count needs to be dropped in the fuse_lookup() error path,
>> when there's an error returned by d_splice_alias().
>> 
>> (While there, remove extra white spaces before labels.)
>> 
>> Fixes: 5835f3390e35 ("fuse: use d_materialise_unique()")
>> Signed-off-by: Luis Henriques <luis@...lia.com>
>
> Have you actually looked at d_splice_alias()?
>
> It does consume inode reference in all cases, success or error.  On success
> it gets transferred to dentry; on failure it is dropped.  That's quite
> deliberate, since it makes life much simplier for failure handling in the
> callers.
>
> If you can reproduce a leak there, I would like to see a reproducer.
> If not, I would say that your patch introduces a double-iput.
>
> NAK.

Totally deserved :-(

To be honest, while investigating this I remember finding the following in
d_obtain_alias() documentation:

  On successful return, the reference to the inode has been transferred
  to the dentry.  In case of an error the reference on the inode is released.

Since d_splice_alias() didn't included that note explicitly, I (wrongly)
assumed it would *not* consume the reference.  Sure, I did had a look at
the function, but clearly not close enough.

Sorry for wasting your time, and thank you for the explanation.

Cheers,
-- 
Luís