Re: [PATCH] ipc/mqueue: fix dentry refcount imbalance in prepare_open()

[Amir Goldstein]

On Sun, Nov 30, 2025 at 10:27 AM Deepanshu Kartikey
<kartikey406@xxxxxxxxx> wrote:
>
> When opening an existing message queue, prepare_open() does not increment
> the dentry refcount, but end_creating() always calls dput(). This causes
> a refcount imbalance that triggers a WARN_ON_ONCE in fast_dput() when the
> file is later closed.
>
> The creation path via vfs_mkobj() correctly increments the refcount, but
> the "already exists" path was missing the corresponding dget().
>
> Add the missing dget() call when opening an existing queue to balance the
> dput() in end_creating().

Sorry but this analysis looks wrong.

AFAIS, the bug was that end_creating() should have been before the out_putfd
label just as path_put() was before the commit.

>
> Reported-by: syzbot+b74150fd2ef40e716ca2@xxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=b74150fd2ef40e716ca2

Fix should have
Fixes: c9ba789dad15b ("VFS: introduce start_creating_noperm() and
start_removing_noperm()")

But I see this is already fixed by the FD_ADD() work and  mqueue_file_open()
helper by Christian.

Specifically, the bug was fixed by the merge conflict resolution
d6ea5537c1a66 Merge tag 'vfs-6.19-rc1.fd_prepare' of
gitolite.kernel.org:pub/scm/linux/kernel/git/vfs/vfs into vfs.all

Which should be in linux-next from yesterday.

Christian,

Do you think we need a fix mid-way before the merge of FD_ADD()?

Thanks,
Amir.