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Date:	Fri, 26 Nov 2010 17:53:30 +0100
From:	Reinhard Tartler <Reinhard.Tartler@...ormatik.uni-erlangen.de>
To:	Corentin Chary <corentin.chary@...il.com>,
	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>,
	daniel.lohmann@...ormatik.uni-erlangen.de,
	Julio.Sincero@...ormatik.uni-erlangen.de
Cc:	Julia Lawall <julia@...u.dk>
Subject: Re: debugfs_create_dir return value in acer-wmi, intel_ips and ec_sys

On Thu, Nov 25, 2010 at 11:21:20 (CET), Julia Lawall wrote:

> On Thu, 25 Nov 2010, Corentin Chary wrote:
>
>> On Thu, Nov 25, 2010 at 7:01 AM, Julia Lawall <julia@...u.dk> wrote:
>> > 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 ?
>> 
>> Answer to myself, when debugfs is disabled, it's ok to give broken
>> dentry pointers to debugfs functions since they won't do anything.
>> 
>> >> 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.
>> 
>> Or better, something to check if a macro is defined in a particular contact ?
>
> Actually, Daniel Lohmann's group has been working on analyzing #ifdef's.   
> Perhaps they have a solution to this problem?  I have added them to the CC 
> list.

Thanks for bringing this thread to our attention, Julia.

We indeed do have a tool that is able to calculate the conditions under
which a line of code is activated or not, taking the constraints from
Kconfig into account. This allows us e.g. to find nested/broken ifdefs
like

#ifndef CONFIG_DEBUG_FS
...
#ifdef CONFIG_DEBUG_FS
#else
#endif
...
#endif

because we are taking kconfig into account, the inner CPP item can also
be some other kconfig item on which CONFIG_DEBUG_FS depends and we would
still find it.

I'm not sure yet how to turn this technique into a tool that would be
helpful to solve this particular problem. Maybe we can integrate this
somehow in coccinelle?

regards,
        Reinhard.

-- 
Gruesse/greetings,
Reinhard Tartler, KeyID 945348A4
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