linux-kernel - Re: [PATCH] pidfs: detect refcount bugs

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Message-ID: <bsap2vh4o7h3c5kwmtbgrcjuzldic2m33tlierxx6eqxz7uuqy@p3v3ipakqv3y>
Date: Tue, 6 May 2025 16:43:56 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Christian Brauner <brauner@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pidfs: detect refcount bugs

On Tue, May 06, 2025 at 01:55:54PM +0200, Christian Brauner wrote:
> Now that we have pidfs_{get,register}_pid() that needs to be paired with
> pidfs_put_pid() it's possible that someone pairs them with put_pid().
> Thus freeing struct pid while it's still used by pidfs. Notice when that
> happens. I'll also add a scheme to detect invalid uses of
> pidfs_get_pid() and pidfs_put_pid() later.
> 
> Signed-off-by: Christian Brauner <brauner@...nel.org>
> ---
>  kernel/pid.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 26f1e136f017..8317bcbc7cf7 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -100,6 +100,7 @@ void put_pid(struct pid *pid)
>  
>  	ns = pid->numbers[pid->level].ns;
>  	if (refcount_dec_and_test(&pid->count)) {
> +		WARN_ON_ONCE(pid->stashed);
>  		kmem_cache_free(ns->pid_cachep, pid);
>  		put_pid_ns(ns);
>  	}
> -- 
> 2.47.2
> 

With the patch as proposed you are only catching the misuse if this is
the last ref though.

iow, the check should be hoisted above unrefing?