linux-kernel - Re: debugfs_create_dir return value in acer-wmi, intel_ips and ec

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Message-ID: <Pine.LNX.4.64.1011250653370.15506@ask.diku.dk>
Date:	Thu, 25 Nov 2010 07:01:22 +0100 (CET)
From:	Julia Lawall <julia@...u.dk>
To:	Corentin Chary <corentin.chary@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	platform-driver-x86@...r.kernel.org,
	linux acpi <linux-acpi@...r.kernel.org>,
	Carlos Corbacho <carlos@...angeworlds.co.uk>,
	Matthew Garrett <mjg@...hat.com>,
	Axel Lin <axel.lin@...il.com>,
	Thomas Renninger <trenn@...e.de>,
	Thomas Renninger <trenn@...e.de>
Subject: Re: debugfs_create_dir return value in acer-wmi, intel_ips and ec_sys

On Thu, 25 Nov 2010, Corentin Chary wrote:

> Hi,
> 
> I was checking debugfs code in platform/x86, because I want to add
> some files to eeepc-wmi. And I found something disturbing.
> 
> The documentation says:
> 
> > This call, if successful, will make a directory called name underneath the
> > indicated parent directory.  If parent is NULL, the directory will be
> > created in the debugfs root.  On success, the return value is a struct
> > dentry pointer which can be used to create files in the directory (and to
> > clean it up at the end).  A NULL return value indicates that something went
> > wrong.  If ERR_PTR(-ENODEV) is returned, that is an indication that the
> > kernel has been built without debugfs support and none of the functions
> > described below will work.
> 
> But then, here is the code in acer-wmi:
> 
> > static void remove_debugfs(void)
> > {
> >       debugfs_remove(interface->debug.devices);
> >       debugfs_remove(interface->debug.root);
> > }
> >
> > static int create_debugfs(void)
> > {
> >        interface->debug.root = debugfs_create_dir("acer-wmi", NULL);
> >        if (!interface->debug.root) {
> >                printk(ACER_ERR "Failed to create debugfs directory");
> >                return -ENOMEM;
> >        }
> 
> this code is *not* inside #ifndef CONFIG_DEBUG_FS, so debugfs_create_dir
> can return ERR_PTR(-ENODEV) right ?
> 
> Then, remove_debug() will call debugfs_remove(ERR_PTR(-ENODEV)) right ?
> 
> So, acpi-wmi seems to have an issue when debugfs is disabled, that's "ok".
> 
> But then I took a look at intel_ips :
> 
> >        ips->debug_root = debugfs_create_dir("ips", NULL);
> >        if (!ips->debug_root) {
> >                dev_err(&ips->dev->dev,
> >                        "failed to create debugfs entries: %ld\n",
> >                        PTR_ERR(ips->debug_root));
> >                return;
> >        }
> 
> Then PTR_ERR thing is strange, because ips->debug_root can only be NULL 
> here...
> But here, it's ok to only check NULL, because it's inside #ifndef 
> CONFIG_DEBUG_FS.
> 
> So, two drivers checked, to weird error handling code. I did a quick grep and 
> opened
> the first result: ec_sys.c.
> 
> ec_sys.c depends on CONFIG_ACPI_EC_DEBUGFS but doesn't depend on
> CONFIG_DEBUG_FS.
> 
> Here, again, the code only check for != NULL while it could be ERR_PTR(-
> ENODEV):
> 
> >        if (ec_device_count == 0) {
> >                acpi_ec_debugfs_dir = debugfs_create_dir("ec", NULL);
> >                if (!acpi_ec_debugfs_dir)
> >                        return -ENOMEM;
> >        }
> >
> >        sprintf(name, "ec%u", ec_device_count);
> >        dev_dir = debugfs_create_dir(name, acpi_ec_debugfs_dir);
> 
> Here, acpi_ec_debugfs_dir (that can be an invalid pointer) is used as
> a parent dentry, and will be dereferenced without checks.
> 
> I am missing something obvious, or are most of debugfs implementation
> broken when debugfs is disabled ?
> 
> Julia, if I am right, coccinelle could help us right ? Can the tool check
> if the code is between #ifdef CONFIG_DEBUGS_FS ? That would help a lot.

Unfortunately, at the moment, it can't; there is no matching on #ifdefs.  
Perhaps it could be added.

I wonder though if sometimes returning NULL and sometimes returning 
ERR_PTR is something that should be encouraged?  Would one rather convert 
the NULL case to a specific ERR_PTR case?

julia
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