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

[Takashi Iwai <tiwai@suse.de>]

On Thu, 07 Oct 2021 13:48:44 +0200,
Arnd Bergmann wrote:
> 
> On Thu, Oct 7, 2021 at 12:53 PM Takashi Iwai <tiwai@suse.de> wrote:
> > On Wed, 06 Oct 2021 19:49:17 +0200, Michael Forney wrote:
> > >
> > > Arnd Bergmann <arnd@arndb.de> wrote:
> > > > +#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];
> > > > +#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)];
> > > > +#endif
> > > > +
> > > > +struct __snd_pcm_mmap_status64 {
> > > > +   __s32 state;                    /* RO: state - SNDRV_PCM_STATE_XXXX */
> > > > +   __u32 pad1;                     /* Needed for 64 bit alignment */
> > > > +   __pad_before_uframe __pad1;
> > > > +   snd_pcm_uframes_t hw_ptr;       /* RO: hw ptr (0...boundary-1) */
> > > > +   __pad_after_uframe __pad2;
> > > > +   struct __snd_timespec64 tstamp; /* Timestamp */
> > > > +   __s32 suspended_state;          /* RO: suspended stream state */
> > > > +   __u32 pad3;                     /* Needed for 64 bit alignment */
> > > > +   struct __snd_timespec64 audio_tstamp; /* sample counter or wall clock */
> > > > +};
> > > > +
> > > > +struct __snd_pcm_mmap_control64 {
> > > > +   __pad_before_uframe __pad1;
> > > > +   snd_pcm_uframes_t appl_ptr;      /* RW: appl ptr (0...boundary-1) */
> > > > +   __pad_before_uframe __pad2;
> > >
> > > I was looking through this header and happened to notice that this
> > > padding is wrong. I believe it should be __pad_after_uframe here.
> > >
> > > I'm not sure of the implications of this typo, but I suspect it
> > > breaks something on 32-bit systems with 64-bit time (regardless of
> > > the endianness, since it changes the offset of avail_min).
> 
> Thanks a lot for the report! Yes, this is definitely broken in some ways.
> 
> > Right, that's the expected breakage.  It seems that the 64bit time on
> > 32bit arch is still rare, so we haven't heard a regression by that, so
> > far...
> 
> It might actually be worse: on a native 32-bit kernel, both user space
> and kernel see the same broken definition with a 64-bit time_t, which
> would end up actually making it work as expected. However, in
> compat mode, the layout seen on the 32-bit user space is now
> different from what the 64-bit kernel has, which would in turn not
> work, in both the SNDRV_PCM_IOCTL_SYNC_PTR ioctl and in
> the mmap() interface.
> 
> Fixing the layout to look like the way we had intended would make
> newly compiled applications work in compat mode, but would break
> applications built against the old header on new kernels and also
> newly built applications on old kernels.
> 
> I still hope I missed something and it's not quite that bad, but I
> fear the best we can do in this case make the broken interface
> the normative one and fixing compat mode to write
> mmap_control64->avail_min in the wrong location for
> SNDRV_PCM_IOCTL_SYNC_PTR, as well as disabling
> the mmap() interface again for compat tasks.
>
> As far as I can tell, the broken interface will always result in
> user space seeing a zero value for "avail_min". Can you
> make a prediction what that would mean for actual
> applications? Will they have no audio output, run into
> a crash, or be able to use recover and appear to work normally
> here?

No, fortunately it's only about control->avail_min, and fiddling this
value can't break severely (otherwise it'd be a security problem ;)

In the buggy condition, it's always zero, and the kernel treated as if
1, i.e. wake up as soon as data is available, which is OK-ish for most
applications.   Apps usually don't care about the wake-up condition so
much.  There are subtle difference and may influence on the stability
of stream processing, but the stability usually depends more strongly
on the hardware and software configurations.

That being said, the impact by this bug (from the application behavior
POV) is likely quite small, but the contamination is large; as you
pointed out, it's much larger than I thought.

The definition in uapi/sound/asound.h is a bit cryptic, but IIUC,
__snd_pcm_mmap_control64 is used for 64bit archs, right?  If so, the
problem rather hits more widely on 64bit archs silently.  Then, the
influence by this bug must be almost negligible, as we've had no bug
report about the behavior change.

We may just fix it in kernel and for new library with hoping that no
one sees the actual problem.  Or, we may provide a complete new set of
mmap offsets and ioctl to cover both broken and fixed interfaces...
The decision depends on how perfectly we'd like to address the bug.
As of now, I'm inclined to go for the former, but I'm open for more
opinions.


thanks,

Takashi