ALSA: pcm: Workaround for a wrong offset in SYNC_PTR compat ioctl

ALSA: pcm: Workaround for a wrong offset in SYNC_PTR compat ioctl

Michael Forney reported an incorrect padding type that was defined in
the commit 80fe7430c708 ("ALSA: add new 32-bit layout for
snd_pcm_mmap_status/control") for PCM control mmap data.
His analysis is correct, and this caused the misplacements of PCM
control data on 32bit arch and 32bit compat mode.

The bug is that the __pad2 definition in __snd_pcm_mmap_control64
struct was wrongly with __pad_before_uframe, which should have been
__pad_after_uframe instead.  This struct is used in SYNC_PTR ioctl and
control mmap.  Basically this bug leads to two problems:

- The offset of avail_min field becomes wrong, it's placed right after
  appl_ptr without padding on little-endian

- When appl_ptr and avail_min are read as 64bit values in kernel side,
  the values become either zero or corrupted (mixed up)

One good news is that, because both user-space and kernel
misunderstand the wrong offset, at least, 32bit application running on
32bit kernel works as is.  Also, 64bit applications are unaffected
because the padding size is zero.  The remaining problem is the 32bit
compat mode; as mentioned in the above, avail_min is placed right
after appl_ptr on little-endian archs, 64bit kernel reads bogus values
for appl_ptr updates, which may lead to streaming bugs like jumping,
XRUN or whatever unexpected.
(However, we haven't heard any serious bug reports due to this over
years, so practically seen, it's fairly safe to assume that the impact
by this bug is limited.)

Ideally speaking, we should correct the wrong mmap status control
definition.  But this would cause again incompatibility with the
existing binaries, and fixing it (e.g. by renumbering ioctls) would be
really messy.

So, as of this patch, we only correct the behavior of 32bit compat
mode and keep the rest as is.  Namely, the SYNC_PTR ioctl is now
handled differently in compat mode to read/write the 32bit values at
the right offsets.  The control mmap of 32bit apps on 64bit kernels
has been already disabled (which is likely rather an overlook, but
this worked fine at this time :), so covering SYNC_PTR ioctl should
suffice as a fallback.

Fixes: 80fe7430c708 ("ALSA: add new 32-bit layout for snd_pcm_mmap_status/control")
Reported-by: Michael Forney <mforney@mforney.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Cc: <stable@vger.kernel.org>
Cc: Rich Felker <dalias@libc.org>
Link: https://lore.kernel.org/r/29QBMJU8DE71E.2YZSH8IHT5HMH@mforney.org
Link: https://lore.kernel.org/r/20211010075546.23220-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>