linux-kernel - Re: [PATCH] devpts: If initialization failed, don't crash when opening /dev/ptmx

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Message-Id: <20150507155919.16ab7177e4956d8f47803750@linux-foundation.org>
Date:	Thu, 7 May 2015 15:59:19 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Iulia Manda <iulia.manda21@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Fabian Frederick <fabf@...net.be>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] devpts: If initialization failed, don't crash when
 opening /dev/ptmx

On Wed, 6 May 2015 17:35:47 -0700 Josh Triplett <josh@...htriplett.org> wrote:

> If devpts failed to initialize, it would store an ERR_PTR in the global
> devpts_mnt.  A subsequent open of /dev/ptmx would call devpts_new_index,
> which would dereference devpts_mnt and crash.
> 
> Avoid storing invalid values in devpts_mnt; leave it NULL instead.
> Make both devpts_new_index and devpts_pty_new fail gracefully with
> ENODEV in that case, which then becomes the return value to the
> userspace open call on /dev/ptmx.

It looks like the system is pretty crippled if init_devptr_fs() fails. 
Can the user actually get access to consoles and do useful things in
this situation?  Maybe it would be better to just give up and panic?

> @@ -676,12 +689,15 @@ static int __init init_devpts_fs(void)
>  	struct ctl_table_header *table;
>  
>  	if (!err) {
> +		static struct vfsmount *mnt;

static is weird.  I assume this was a braino?

>  		table = register_sysctl_table(pty_root_table);
> -		devpts_mnt = kern_mount(&devpts_fs_type);
> -		if (IS_ERR(devpts_mnt)) {
> -			err = PTR_ERR(devpts_mnt);
> +		mnt = kern_mount(&devpts_fs_type);
> +		if (IS_ERR(mnt)) {
> +			err = PTR_ERR(mnt);
>  			unregister_filesystem(&devpts_fs_type);
>  			unregister_sysctl_table(table);
> +		} else {
> +			devpts_mnt = mnt;
>  		}

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