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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>
This commit is contained in:
Clemens Ladisch 2010-05-25 09:01:46 +02:00 committed by Takashi Iwai
parent 57c7ffc941
commit b406e6103b
1 changed files with 3 additions and 1 deletions
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4
sound/core/pcm_lib.c
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@ -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];