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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