linux-kernel - Re: [PATCH 14/33] vfs: Implement a filesystem superblock creation/configuration context [ver #11]

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Message-ID: <80eb1ec0-7b61-debe-9e93-0176d3e2c76a@roeck-us.net>
Date:   Tue, 18 Sep 2018 07:06:58 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     David Howells <dhowells@...hat.com>, viro@...iv.linux.org.uk,
        torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 14/33] vfs: Implement a filesystem superblock
 creation/configuration context [ver #11]

On 09/18/2018 02:07 AM, Sergey Senozhatsky wrote:
> Hi,
> 
> On (09/11/18 16:54), Guenter Roeck wrote:
>> On Wed, Sep 12, 2018 at 12:17:35AM +0100, David Howells wrote:
>>> Guenter Roeck <linux@...ck-us.net> wrote:
>>>
>>>> [    8.507672] RIP: 0010:reconfigure_super+0x47/0x210
>>>
>>> Can you tell me the file and line this corresponds to?
>>>
>> I don't know, but some debugging shows that fc->ops == NULL.
> 
> This NULL derefs linux-next.
> 
> Emergency (sysrq remount/reboot):
> 
> emergency_remount()
>   do_emergency_remount()
>    do_emergency_remount_callback()
>     reconfigure_super()
> 
> At fc->ops dereference:
> 
>   981         if (fc->ops->reconfigure) {
> 		^^^^^^^^^
>   982                 retval = fc->ops->reconfigure(fc);
>   983                 if (retval == 0) {
>   984                         security_sb_reconfigure(fc);
> 
> 
> So the check either better be
> 
> 	if (fc->ops && fc->ops->reconfigure)
> 

Since there are multiple instances of fs_context where fc->ops isn't set,
this check would be needed wherever fc->ops is dereferenced.

Guenter

> Or, we need to set ->ops properly. But I'm not sure if invoking
> ->init_fs_context() from emergency-reboot path is going to work
> well all the time.
> 
> ---
> 
>   fs/super.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/fs/super.c b/fs/super.c
> index efb0567c8389..e2e03c47c817 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1017,6 +1017,7 @@ int reconfigure_super(struct fs_context *fc)
>   static void do_emergency_remount_callback(struct super_block *sb)
>   {
>   	struct fs_context fc = {
> +		.ops		= &legacy_fs_context_ops,
>   		.purpose	= FS_CONTEXT_FOR_EMERGENCY_RO,
>   		.fs_type	= sb->s_type,
>   		.root		= sb->s_root,
> 
> ---
> 
> 	-ss
>