linux-kernel - Re: [PATCH v2 2/3] platform/x86: wmi: add WMI support to MODULE_DEVICE

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Message-ID: <20190128142701.xj4vwiind4ddbcj6@pali>
Date:   Mon, 28 Jan 2019 15:27:01 +0100
From:   Pali Rohár <pali.rohar@...il.com>
To:     Mattias Jacobsson <2pi@....nu>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        michal.lkml@...kovi.net, Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Platform Driver <platform-driver-x86@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] platform/x86: wmi: add WMI support to
 MODULE_DEVICE_TABLE()

On Monday 28 January 2019 15:09:11 Mattias Jacobsson wrote:
> Hi,
> 
> On 2019-01-27, Andy Shevchenko wrote:
> > On Sun, Jan 27, 2019 at 9:04 PM Mattias Jacobsson <2pi@....nu> wrote:
> > >
> > > The kernel provides the macro MODULE_DEVICE_TABLE() where driver authors
> > > can specify their device type and their array of device_ids and thereby
> > > trigger the generation of the appropriate MODULE_ALIAS() output. This is
> > > opposed to having to specify one MODULE_ALIAS() for each device. The WMI
> > > device type is currently not supported.
> > >
> > > While using MODULE_DEVICE_TABLE() does increase the complexity as well
> > > as spreading out the implementation across the kernel, it does come with
> > > some benefits too;
> > > * It makes different drivers look more similar; if you can specify the
> > >   array of device_ids any device type specific input to MODULE_ALIAS()
> > >   will automatically be generated for you.
> > > * It helps each driver avoid keeping multiple versions of the same
> > >   information in sync. That is, both the array of device_ids and the
> > >   potential multitude of MODULE_ALIAS()'s.
> > >
> > > Add WMI support to MODULE_DEVICE_TABLE() by adding info about struct
> > > wmi_device_id in devicetable-offsets.c and add a WMI entry point in
> > > file2alias.c.
> > >
> > > The type argument for MODULE_DEVICE_TABLE(type, name) is wmi.
> > 
> > > +/* Looks like: wmi:guid */
> > > +static int do_wmi_entry(const char *filename, void *symval, char *alias)
> > > +{
> > > +       DEF_FIELD_ADDR(symval, wmi_device_id, guid_string);
> > > +       if (strlen(*guid_string) != WMI_GUID_STRING_LEN) {
> > > +               warn("Invalid WMI device id 'wmi:%s' in '%s'\n",
> > > +                               *guid_string, filename);
> > > +               return 0;
> > > +       }
> > 
> > > +       if (snprintf(alias, 500, WMI_MODULE_PREFIX "%s", *guid_string) < 0) {
> > 
> > What the point to use snprintf here with arbitrary buffer size if we
> > exactly know 2 facts:
> > 1. UUID string is  36 characters
> > 2. buffer is long enough
> > 
> > ?
> 
> As long as no one changes the code, not much.

At least instead of hardcoded number 500, you should use pass size of alias:

  static int do_wmi_entry(const char *filename, void *symval, char *alias, size_t alias_size)

  if (snprintf(alias, alias_size, WMI_MODULE_PREFIX "%s", *guid_string) < 0) {

Or pass buffer of constant size and then you do not need to use snprintf:

  #define ALIAS_SIZE (sizeof(WMI_MODULE_PREFIX)+WMI_GUID_STRING_LEN)

  static int do_wmi_entry(const char *filename, void *symval, char alias[ALIAS_SIZE])

This should not break even when code around changes.

> > 
> > > +               warn("Could not generate all MODULE_ALIAS's in '%s'\n",
> > > +                               filename);
> > > +               return 0;
> > > +       }
> > > +       return 1;
> > > +}
> > 
> > -- 
> > With Best Regards,
> > Andy Shevchenko
> 
> Thanks,
> Mattias

-- 
Pali Rohár
pali.rohar@...il.com