linux-kernel - Re: [PATCH] kcsan, debugfs: Move debugfs file creation out of early init

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Date:   Wed, 3 Mar 2021 11:27:16 +0100
From:   Marco Elver <elver@...gle.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     rafael@...nel.org, "Paul E. McKenney" <paulmck@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] kcsan, debugfs: Move debugfs file creation out of early init

On Wed, 3 Mar 2021 at 11:24, Greg KH <gregkh@...uxfoundation.org> wrote:
> On Wed, Mar 03, 2021 at 11:18:06AM +0100, Marco Elver wrote:
> > On Wed, 3 Mar 2021 at 10:57, Greg KH <gregkh@...uxfoundation.org> wrote:
> > >
> > > On Wed, Mar 03, 2021 at 10:38:45AM +0100, Marco Elver wrote:
> > > > Commit 56348560d495 ("debugfs: do not attempt to create a new file
> > > > before the filesystem is initalized") forbids creating new debugfs files
> > > > until debugfs is fully initialized. This breaks KCSAN's debugfs file
> > > > creation, which happened at the end of __init().
> > >
> > > How did it "break" it?  The files shouldn't have actually been created,
> > > right?
> >
> > Right, with 56348560d495 the debugfs file isn't created anymore, which
> > is the problem. Before 56348560d495 the file exists (syzbot wants the
> > file to exist.)
> >
> > > > There is no reason to create the debugfs file during early
> > > > initialization. Therefore, move it into a late_initcall() callback.
> > > >
> > > > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > > > Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> > > > Cc: stable <stable@...r.kernel.org>
> > > > Fixes: 56348560d495 ("debugfs: do not attempt to create a new file before the filesystem is initalized")
> > > > Signed-off-by: Marco Elver <elver@...gle.com>
> > > > ---
> > > > I've marked this for 'stable', since 56348560d495 is also intended for
> > > > stable, and would subsequently break KCSAN in all stable kernels where
> > > > KCSAN is available (since 5.8).
> > >
> > > No objection from me, just odd that this actually fixes anything :)
> >
> > 56348560d495 causes the file to just not be created if we try to
> > create at the end of __init(). Having it created as late as
> > late_initcall() gets us the file back.
> >
> > When you say "fixes anything", should the file be created even though
> > it's at the end of __init()? Perhaps I misunderstood what 56348560d495
> > changes, but I verified it to be the problem by reverting (upon which
> > the file exists as expected).
>
> All my change did is explicitly not allow you to create a file if
> debugfs had not been initialized.  If you tried to do that before, you
> should have gotten an error from the vfs layer that the file was not
> created, as otherwise how would it have succeeded?
>
> I just moved the check up higher in the "stack" to the debugfs code, and
> not relied on the vfs layer to do a lot of work only to reject things
> later on.
>
> So there "should" not have been any functional change with this patch.
> If there was, then something is really odd as how can the vfs layer
> create a file for a filesystem _before_ that filesystem has been
> registered with the vfs layer?

Ah, I see. I do confirm that the file has been created until
56348560d495, without any errors.

Thanks,
-- Marco