linux-kernel - [024/200] ALSA: pcm: fix delta calculation at boundary wraparound

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Date:	Thu, 01 Jul 2010 10:41:54 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Clemens Ladisch <clemens@...isch.de>,
	Takashi Iwai <tiwai@...e.de>
Subject: [024/200] ALSA: pcm: fix delta calculation at boundary wraparound

2.6.34-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Clemens Ladisch <clemens@...isch.de>

commit b406e6103baa3da85950f22d3d46d21a8da654c5 upstream.

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@...isch.de>
Signed-off-by: Takashi Iwai <tiwai@...e.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 sound/core/pcm_lib.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/sound/core/pcm_lib.c
+++ b/sound/core/pcm_lib.c
@@ -345,7 +345,9 @@ static int snd_pcm_update_hw_ptr0(struct
 		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];

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