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

[Takashi Iwai]

On Fri, 08 Oct 2021 14:07:39 +0200,
Rich Felker wrote:
> 
> 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 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.
> > 
> > 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.

OK, I'll submit a proper patch now, to be included in the next PR for
5.15-rc.  For further fixes, let's think carefully.


thanks,

Takashi