linux-kernel - Re: [EXTERNAL] Re: audit: io_uring openat triggers audit reference count underflow in worker thread

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Date:   Mon, 9 Oct 2023 07:40:54 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Dan Clash <Dan.Clash@...rosoft.com>,
        Paul Moore <paul@...l-moore.com>
Cc:     "audit@...r.kernel.org" <audit@...r.kernel.org>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "brauner@...nel.org" <brauner@...nel.org>
Subject: Re: [EXTERNAL] Re: audit: io_uring openat triggers audit reference
 count underflow in worker thread

On 10/8/23 8:38 PM, Dan Clash wrote:
> I retested with the following change as a sanity check:
> 
> -       BUG_ON(name->refcnt <= 0);
> +       BUG_ON(atomic_read(&name->refcnt) <= 0);
> 
> checkpatch.pl suggests using WARN_ON_ONCE rather than BUG.
> 
> devvm ~ $ ~/linux/scripts/checkpatch.pl --patch ~/io_uring_audit_hang_atomic.patch 
> WARNING: Do not crash the kernel unless it is absolutely unavoidable
>   --use WARN_ON_ONCE() plus recovery code (if feasible) instead of BUG() or variants
> #28: FILE: fs/namei.c:262:
> +       BUG_ON(atomic_read(&name->refcnt) <= 0);
> ...
> 
> refcount_t uses WARN_ON_ONCE.
> 
> I can think of three choices:
> 
> 1. Use atomic_t and remove the BUG line.
> 2. Use refcount_t and remove the BUG line. 
> 3. Use atomic_t and partially implement the warn behavior of refcount_t.
> 
> Choice 1 and 2 seem better than choice 3.

I'd probably just make it:

if (WARN_ON_ONCE(!atomic_read(name->refcnt)))
	return;

to make it a straightforward conversion.

-- 
Jens Axboe