ALSA: pcm: fix delta calculation at boundary wraparound

In the cleanup of the hw_ptr update functions in 2.6.33, the calculation
of the delta value was changed to use the modulo operator to protect
against a negative difference due to the pointer wrapping around at the
boundary.

However, the ptr variables are unsigned, so a negative difference would
result in the two complement's value which has no relation to the actual
difference relative to the boundary; the result is typically some value
near LONG_MAX-boundary.  Furthermore, even if the modulo operation would
be done with signed types, the result of a negative dividend could be
negative.

The invalid delta value is then caught by the following checks, but this
means that the pointer update is ignored.

To fix this, use a range check as in the other pointer calculations.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
index 22aff18..e9d98be 100644 (file)
--- a/sound/core/pcm_lib.c
+++ b/sound/core/pcm_lib.c
@@ -345,7 +345,9 @@ static int snd_pcm_update_hw_ptr0(struct snd_pcm_substream *substream,
                new_hw_ptr = hw_base + pos;
        }
       __delta:
-       delta = (new_hw_ptr - old_hw_ptr) % runtime->boundary;
+       delta = new_hw_ptr - old_hw_ptr;
+       if (delta < 0)
+               delta += runtime->boundary;
        if (xrun_debug(substream, in_interrupt ?
                        XRUN_DEBUG_PERIODUPDATE : XRUN_DEBUG_HWPTRUPDATE)) {
                char name[16];