[musl] Re: [alsa-devel] [PATCH v7 8/9] ALSA: add new 32-bit layout for snd_pcm_mmap_status/control

[Arnd Bergmann]

On Fri, Oct 8, 2021 at 10:43 AM Takashi Iwai <tiwai at suse.de> wrote:
> On Thu, 07 Oct 2021 18:51:58 +0200, Rich Felker wrote:
> > On Thu, Oct 07, 2021 at 06:18:52PM +0200, Takashi Iwai wrote:
>
> @@ -557,11 +558,15 @@ struct __snd_pcm_sync_ptr {
>  #if defined(__BYTE_ORDER) ? __BYTE_ORDER == __BIG_ENDIAN : defined(__BIG_ENDIAN)
>  typedef char __pad_before_uframe[sizeof(__u64) - sizeof(snd_pcm_uframes_t)];
>  typedef char __pad_after_uframe[0];
> +typedef char __pad_before_u32[4];
> +typedef char __pad_after_u32[0];
>  #endif
>
>  #if defined(__BYTE_ORDER) ? __BYTE_ORDER == __LITTLE_ENDIAN : defined(__LITTLE_ENDIAN)
>  typedef char __pad_before_uframe[0];
>  typedef char __pad_after_uframe[sizeof(__u64) - sizeof(snd_pcm_uframes_t)];
> +typedef char __pad_before_u32[0];
> +typedef char __pad_after_u32[4];
>  #endif

I think these should remain unchanged, the complex expression was intentionally
done so the structures are laid out the same way on 64-bit
architectures, so that
the kernel can use the __SND_STRUCT_TIME64 path internally on both 32-bit
and 64-bit architectures.

> @@ -2970,8 +2981,17 @@ static int snd_pcm_sync_ptr(struct snd_pcm_substream *substream,
>         memset(&sync_ptr, 0, sizeof(sync_ptr));
>         if (get_user(sync_ptr.flags, (unsigned __user *)&(_sync_ptr->flags)))
>                 return -EFAULT;
> -       if (copy_from_user(&sync_ptr.c.control, &(_sync_ptr->c.control), sizeof(struct snd_pcm_mmap_control)))
> -               return -EFAULT;
> +       if (buggy_control) {
> +               if (copy_from_user(&sync_ptr.c.control_api_2_0_15,
> +                                  &(_sync_ptr->c.control_api_2_0_15),
> +                                  sizeof(sync_ptr.c.control_api_2_0_15)))
> +                       return -EFAULT;
> +       } else {
> +               if (copy_from_user(&sync_ptr.c.control,
> +                                  &(_sync_ptr->c.control),
> +                                  sizeof(sync_ptr.c.control)))
> +                       return -EFAULT;
> +       }

The problem I see with this is that it might break musl's ability to
emulate the new
interface on top of the old (time32) one for linux-4.x and older
kernels, as the conversion
function is no longer stateless but has to know the negotiated
interface version.

It's probably fine as long as we can be sure that the 2.0.16+ API
version only gets
negotiated if both the kernel and user sides support it, and musl only emulates
the 2.0.15 API version from the current kernels.

I've tried to understand this part of musl's convert_ioctl_struct(), but I just
can't figure out whether it does the conversion based the on the layout that
is currently used in the kernel, or based on the layout we should have been
using, and would use with the above fix. Rich, can you help me here?

       Arnd