linux-kernel - Re: [PATCH] s390: kernel: no need to check return value of debugfs

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Message-Id: <20190123093508.GB3424@osiris>
Date:   Wed, 23 Jan 2019 10:35:08 +0100
From:   Heiko Carstens <heiko.carstens@...ibm.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH] s390: kernel: no need to check return value of
 debugfs_create functions

On Tue, Jan 22, 2019 at 05:33:37PM +0100, Greg Kroah-Hartman wrote:
> On Tue, Jan 22, 2019 at 05:24:54PM +0100, Heiko Carstens wrote:
> > On Tue, Jan 22, 2019 at 04:21:02PM +0100, Greg Kroah-Hartman wrote:
> > > When calling debugfs functions, there is no need to ever check the
> > > return value.  The function can work or not, but the code logic should
> > > never do something different based on this.
> > > 
> > > Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
> > > Cc: Heiko Carstens <heiko.carstens@...ibm.com>
> > > Cc: Kees Cook <keescook@...omium.org>
> > > Cc: Christian Borntraeger <borntraeger@...ibm.com>
> > > Cc: Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>
> > > Cc: linux-s390@...r.kernel.org
> > > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > > ---
> > >  arch/s390/kernel/debug.c    | 6 ------
> > >  arch/s390/kernel/kdebugfs.c | 2 --
> > >  arch/s390/kernel/sysinfo.c  | 2 --
> > >  3 files changed, 10 deletions(-)
> > > diff --git a/arch/s390/kernel/sysinfo.c b/arch/s390/kernel/sysinfo.c
> > > index 12f80d1f0415..2ac3c9b56a13 100644
> > > --- a/arch/s390/kernel/sysinfo.c
> > > +++ b/arch/s390/kernel/sysinfo.c
> > > @@ -545,8 +545,6 @@ static __init int stsi_init_debugfs(void)
> > >  	int lvl, i;
> > > 
> > >  	stsi_root = debugfs_create_dir("stsi", arch_debugfs_dir);
> > > -	if (IS_ERR_OR_NULL(stsi_root))
> > > -		return 0;
> > 
> > No objections, however will you also change the odd behaviour that
> > e.g. debugfs_create_file() returns -ENODEV instead of (the expected)
> > NULL pointer if CONFIG_DEBUGFS is disabled?
> 
> Nope.  That is intentional.
> 
> > I do remember this since it caused at least one crash ;)
> 
> Which is why you shouldn't care about the return value of these
> functions :)
> 
> > 19cdd08ba155 ("[S390] qdio: fix broken pointer in case of CONFIG_DEBUG_FS is disabled").
> 
> Odd, what crashes when passed an error pointer?  What was someone trying
> to do with those pointers?  The only thing you can do with a return
> value from a debugfs function is to pass it back into another debugfs
> call.  Sounds like someone wasn't doing that :(

I think it used to be this code that crashed:

static void remove_debugfs_entry(struct qdio_q *q)
{
	int i;

	for (i = 0; i < MAX_DEBUGFS_QUEUES; i++) {
		if (!debugfs_queues[i])
			continue;
----->		if (debugfs_queues[i]->d_inode->i_private == q) {
			debugfs_remove(debugfs_queues[i]);
			debugfs_queues[i] = NULL;
		}
	}
}

Which looks like a layering violation anyway. However this code is
gone, so everything should be fine.