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

[Guenter Roeck]

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@xxxxxxxxxxxx> 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