Re: [PATCH v2] gpiolib: Fix line event handling in syscall compatible mode

[Arnd Bergmann <arnd@xxxxxxxx>]

On Mon, Sep 14, 2020 at 4:37 PM Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
>
> The introduced line even handling ABI in the commit
>
>   61f922db7221 ("gpio: userspace ABI for reading GPIO line events")
>
> missed the fact that 64-bit kernel may serve for 32-bit applications.
> In such case the very first check in the lineevent_read() will fail
> due to alignment differences.
>
> To workaround this introduce lineeven_to_user() helper which returns actual
> size of the structure and copies its content to user if asked.
>
> Fixes: 61f922db7221 ("gpio: userspace ABI for reading GPIO line events")
> Suggested-by: Arnd Bergmann <arnd@xxxxxxxx>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>

Acked-by: Arnd Bergmann <arnd@xxxxxxxx>

> diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
> index e6c9b78adfc2..95af4a470f1e 100644
> --- a/drivers/gpio/gpiolib-cdev.c
> +++ b/drivers/gpio/gpiolib-cdev.c
> @@ -423,6 +423,21 @@ static __poll_t lineevent_poll(struct file *file,
>         return events;
>  }
>
> +static ssize_t lineevent_get_size(void)
> +{
> +#ifdef __x86_64__
> +       /* i386 has no padding after 'id' */
> +       if (in_ia32_syscall()) {

Christoph Hellwig has recently suggested adding a new macro for this
that would be always available and just evaluate to false on other
architectures.

I'd just merge your version for now and backport to to stable kernels,
but change this instance and a couple of others to use the new
macro in mainline afterwards.

Incidentally that would also address CONFIG_OABI_COMPAT
mode, if anyone cares.

      Arnd