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Date: Tue, 27 Jan 2009 15:00:21 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Tony Battersby <tonyb@cybernetics.com>
Cc: linux-kernel@vger.kernel.org, olsajiri@gmail.com, jkosina@suse.cz
Subject: Re: [PATCH 2/2] make shm_get_stat() more robust
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On Tue, 27 Jan 2009 17:48:13 -0500
Tony Battersby <tonyb@cybernetics.com> wrote:

> shm_get_stat() assumes idr_find(&shm_ids(ns).ipcs_idr) returns
> "struct shmid_kernel *"; all other callers assume that it returns
> "struct kern_ipc_perm *".  This works because "struct kern_ipc_perm"
> is currently the first member of "struct shmid_kernel", but it would
> be better to use container_of() to prevent future breakage.
> 
> Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
> ---
> --- linux-2.6.29-rc2-git3/ipc/shm.c.orig	2009-01-27 16:23:10.000000000 -0500
> +++ linux-2.6.29-rc2-git3/ipc/shm.c	2009-01-27 16:24:19.000000000 -0500
> @@ -551,12 +551,14 @@ static void shm_get_stat(struct ipc_name
>  	in_use = shm_ids(ns).in_use;
>  
>  	for (total = 0, next_id = 0; total < in_use; next_id++) {
> +		struct kern_ipc_perm *ipc;
>  		struct shmid_kernel *shp;
>  		struct inode *inode;
>  
> -		shp = idr_find(&shm_ids(ns).ipcs_idr, next_id);
> -		if (shp == NULL)
> +		ipc = idr_find(&shm_ids(ns).ipcs_idr, next_id);
> +		if (ipc == NULL)
>  			continue;
> +		shp = container_of(ipc, struct shmid_kernel, shm_perm);
>  
>  		inode = shp->shm_file->f_path.dentry->d_inode;
>  

yup, well spotted.

It would be good to add a little typesafe wrapper:

static inline struct kern_ipc_perm *shm_idr_find(struct ipc_ids *ipc_ids)
{
	return idr_find(&ipc_ids->ipcs_idr);
}

(or similar)

so that this sort of mistake cannot happen again.


As you've found, open-coded use of a bare void*-returning function is a
bit dangerous.