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

[Rich Felker <dalias@libc.org>]

On Fri, Oct 08, 2021 at 01:11:34PM +0200, Takashi Iwai wrote:
> On Fri, 08 Oct 2021 11:24:39 +0200,
> Arnd Bergmann wrote:
> > 
> > On Fri, Oct 8, 2021 at 10:43 AM Takashi Iwai <tiwai@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.
> 
> That was explicitly defined, but OK, this isn't necessarily defined
> here.
> 
> > > @@ -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?
> 
> So, at this moment, I'm not sure whether we should correct the struct
> at all.  This will lead to yet more breakage, and basically the struct
> itself *works* -- the only bug is in 32bit compat handling in the
> kernel (again).
> 
> The below is a revised kernel patch (again untested), just correcting
> the behavior of 32bit compat mode.  32bit apps on 32bit kernel work
> fine as is, as well as 64bit apps on 64bit kernel.

I'm perfectly okay with this if Arnd is! It's probably the least
invasive and has the least long-term maintenance cost and fallout on
other projects.

Rich