linux-kernel - Re: [PATCH AUTOSEL 4.19 11/12] gfs2: Refcounting fix in gfs2_thaw

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Date: Tue, 30 Jan 2024 16:16:09 -0500
From: Sasha Levin <sashal@...nel.org>
To: Andreas Gruenbacher <agruenba@...hat.com>
Cc: Pavel Machek <pavel@...x.de>, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org, gfs2@...ts.linux.dev
Subject: Re: [PATCH AUTOSEL 4.19 11/12] gfs2: Refcounting fix in
 gfs2_thaw_super

On Thu, Jan 18, 2024 at 12:50:37PM +0100, Andreas Gruenbacher wrote:
>On Tue, Jan 16, 2024 at 9:53 PM Pavel Machek <pavel@...x.de> wrote:
>> Hi!
>>
>> > From: Andreas Gruenbacher <agruenba@...hat.com>
>> >
>> > [ Upstream commit 4e58543e7da4859c4ba61d15493e3522b6ad71fd ]
>> >
>> > It turns out that the .freeze_super and .thaw_super operations require
>> > the filesystem to manage the superblock refcount itself.  We are using
>> > the freeze_super() and thaw_super() helpers to mostly take care of that
>> > for us, but this means that the superblock may no longer be around by
>> > when thaw_super() returns, and gfs2_thaw_super() will then access freed
>> > memory.  Take an extra superblock reference in gfs2_thaw_super() to fix
>> > that.
>>
>> Patch was broken during backport.
>>
>> > +++ b/fs/gfs2/super.c
>> > @@ -1013,6 +1013,7 @@ static int gfs2_freeze(struct super_block *sb)
>> >               goto out;
>> >       }
>> >
>> > +     atomic_inc(&sb->s_active);
>> >       for (;;) {
>> >               error = gfs2_lock_fs_check_clean(sdp, &sdp->sd_freeze_gh);
>> >               if (!error)
>> > @@ -1034,6 +1035,7 @@ static int gfs2_freeze(struct super_block *sb)
>> >       error = 0;
>> >  out:
>> >       mutex_unlock(&sdp->sd_freeze_mutex);
>> > +     deactivate_super(sb);
>> >       return error;
>> >  }
>>
>> Notice the goto out? That now jumps around the atomic_inc, but we
>> still do decrease. This will break 4.19, please fix or drop.
>
>Thanks, Pavel.
>
>Sasha, you don't want that fix without "gfs2: Rework freeze / thaw
>logic" and the follow-up fixes, and backporting that probably isn't
>going to be worth it.

I'll drop it, thanks!

-- 
Thanks,
Sasha