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Date: Fri, 8 Jun 2018 14:16:59 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Takashi Iwai <tiwai@suse.de>
Cc: Davidlohr Bueso <dbueso@suse.de>, linux-kernel@vger.kernel.org,
        Waiman Long <longman@redhat.com>
Subject: Re: [PATCH] ipc: Limit sysctl value to IPCMNI
Message-Id: <20180608141659.8a517b128c756b4d0b813c9e@linux-foundation.org>
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On Fri,  8 Jun 2018 15:49:49 +0200 Takashi Iwai <tiwai@suse.de> wrote:

> Currently shmmni proc entry accepts all entered integer values, but
> the practical limit is IPCMNI (32768).  This confuses user as if a
> bigger value were accepted but not applied correctly.
> 
> This patch changes the proc entry to use *_minmax variant to limit the
> accepted values accordingly.

Waiman Long was working on a (vastly more complicated) patchset to
address this.

> --- a/ipc/ipc_sysctl.c
> +++ b/ipc/ipc_sysctl.c
> @@ -99,6 +99,7 @@ static int proc_ipc_auto_msgmni(struct ctl_table *table, int write,
>  static int zero;
>  static int one = 1;
>  static int int_max = INT_MAX;
> +static int ipcmni = IPCMNI;
>  
>  static struct ctl_table ipc_kern_table[] = {
>  	{
> @@ -120,7 +121,9 @@ static struct ctl_table ipc_kern_table[] = {
>  		.data		= &init_ipc_ns.shm_ctlmni,
>  		.maxlen		= sizeof(init_ipc_ns.shm_ctlmni),
>  		.mode		= 0644,
> -		.proc_handler	= proc_ipc_dointvec,
> +		.proc_handler	= proc_ipc_dointvec_minmax,
> +		.extra1		= &zero,
> +		.extra2		= &ipcmni,
>  	},
>  	{
>  		.procname	= "shm_rmid_forced",

What is the back-compatibility situation here?