linux-kernel - Re: [PATCH 1/2] ipc,shm: move BUG_ON check into shm

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Date:	Thu, 04 Jun 2015 20:35:35 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Davidlohr Bueso <dave@...olabs.net>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 1/2] ipc,shm: move BUG_ON check into shm_lock

Hi Davidlohr,

On 05/30/2015 02:03 AM, Davidlohr Bueso wrote:
> Upon every shm_lock call, we BUG_ON if an error was returned,
> indicating racing either in idr or in RMID. Move this logic
> into the locking.
>
> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
> ---
>   ipc/shm.c | 11 +++++++----
>   1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/ipc/shm.c b/ipc/shm.c
> index 6d76707..3152dea 100644
> --- a/ipc/shm.c
> +++ b/ipc/shm.c
> @@ -155,8 +155,14 @@ static inline struct shmid_kernel *shm_lock(struct ipc_namespace *ns, int id)
>   {
>   	struct kern_ipc_perm *ipcp = ipc_lock(&shm_ids(ns), id);
>   
> -	if (IS_ERR(ipcp))
> +	if (IS_ERR(ipcp)) {
> +		/*
> +		 * We raced in the idr lookup or with RMID,
> +		 * either way, the ID is busted.
The comment is wrong:
"We raced in the idr lookup or with shm_destroy()"

shm is not like msg or sem:
RMID merely marks a shmid as deletable (SHM_DEST), delete (i.e.: 
shm_rmid(), then ipc_rmid()) happens only after the last attached 
segment is detatched.

And: (unrelated to the patch)
For do_shmat(), I'm not 100% sure that the BUG can't be triggered.

--
     Manfred

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