linux-kernel - Re: [PATCH 2/3] erofs: convert to use kobject_is

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Date:   Thu, 6 Apr 2023 13:19:00 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Gao Xiang <hsiangkao@...ux.alibaba.com>
Cc:     Yangtao Li <frank.li@...o.com>, xiang@...nel.org, chao@...nel.org,
        huyue2@...lpad.com, jefflexu@...ux.alibaba.com,
        damien.lemoal@...nsource.wdc.com, naohiro.aota@....com,
        jth@...nel.org, rafael@...nel.org, linux-erofs@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/3] erofs: convert to use kobject_is_added()

On Thu, Apr 06, 2023 at 06:55:40PM +0800, Gao Xiang wrote:
> 
> 
> On 2023/4/6 18:27, Greg KH wrote:
> > On Thu, Apr 06, 2023 at 06:13:05PM +0800, Gao Xiang wrote:
> > > Hi Greg,
> > > 
> > > On 2023/4/6 18:03, Greg KH wrote:
> > > > On Thu, Apr 06, 2023 at 05:30:55PM +0800, Yangtao Li wrote:
> > > > > Use kobject_is_added() instead of directly accessing the internal
> > > > > variables of kobject. BTW kill kobject_del() directly, because
> > > > > kobject_put() actually covers kobject removal automatically.
> > > > > 
> > > > > Signed-off-by: Yangtao Li <frank.li@...o.com>
> > > > > ---
> > > > >    fs/erofs/sysfs.c | 3 +--
> > > > >    1 file changed, 1 insertion(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/fs/erofs/sysfs.c b/fs/erofs/sysfs.c
> > > > > index 435e515c0792..daac23e32026 100644
> > > > > --- a/fs/erofs/sysfs.c
> > > > > +++ b/fs/erofs/sysfs.c
> > > > > @@ -240,8 +240,7 @@ void erofs_unregister_sysfs(struct super_block *sb)
> > > > >    {
> > > > >    	struct erofs_sb_info *sbi = EROFS_SB(sb);
> > > > > -	if (sbi->s_kobj.state_in_sysfs) {
> > > > > -		kobject_del(&sbi->s_kobj);
> > > > > +	if (kobject_is_added(&sbi->s_kobj)) {
> > > > 
> > > > I do not understand why this check is even needed, I do not think it
> > > > should be there at all as obviously the kobject was registered if it now
> > > > needs to not be registered.
> > > 
> > > I think Yangtao sent a new patchset which missed the whole previous
> > > background discussions as below:
> > > https://lore.kernel.org/r/028a1b56-72c9-75f6-fb68-1dc5181bf2e8@linux.alibaba.com
> > > 
> > > It's needed because once a syzbot complaint as below:
> > > https://lore.kernel.org/r/CAD-N9QXNx=p3-QoWzk6pCznF32CZy8kM3vvo8mamfZZ9CpUKdw@mail.gmail.com
> > > 
> > > I'd suggest including the previous backgrounds at least in the newer patchset,
> > > otherwise it makes me explain again and again...
> > 
> > That would be good, as I do not think this is correct, it should be
> > fixed in a different way, see my response to the zonefs patch in this
> > series as a much simpler method to use.
> 
> Yes, but here (sbi->s_kobj) is not a kobject pointer (also at a quick
> glance it seems that zonefs has similar code), and also we couldn't
> just check the sbi is NULL or not here only, since sbi is already
> non-NULL in this path and there are some others in sbi to free in
> other functions.
> 
> s_kobj could be changed into a pointer if needed.  I'm all fine with
> either way since as you said, it's a boilerplate filesystem kobject
> logic duplicated from somewhere.  Hopefully Yangtao could help take
> this task since he sent me patches about this multiple times.

I made the same mistake with the zonefs code.  If the kobject in this
structure controls the lifespan of it (which makes it not a pointer, my
mistake), then that whole memory chunk can't be valid anymore if the
kobject registering function failed so you need to get rid of it then,
not later.

thanks,

greg k-h